Past LASSP Seminars | Cornell Lab of Atomic and Solid State Physics

Dec/04/2012
4:00 pm, 700 Clark Hall
Professor Li Baowen 李保文 (Dr.rer.nat) Department Physics and Director, Centre for Computational Science & Engineering Nation University of Singapore
Phononics: manipulating heat flow with electronic analogs and beyond

The form of energy termed heat that typically derives from latticevibrations, i.e. the phonons, is usually considered as waste energy and,moreover, deleterious to information processing. However, in this talk, weattempt to rebut this common view: By use of tailored models we demonstratethat phonons can be manipulated like electrons and photons can, thus enablingcontrolled heat transport. Moreover, we explain that phonons can be put tobeneficial use to carry and process information.

In a first part we present ways to control heat transport and how toprocess information for physical systems which are driven by a temperaturebias. Particularly, we put forward the toolkit of familiar electronic analogsfor exercising phononics; i.e. phononic devices which act as thermal diodes,thermal transistors, thermal logic gates and thermal memories, etc.. Theseconcepts are then put to work to transport, control and rectify heat in physicalrealistic nanosystems by devising practical designs of hybrid nanostructuresthat permit the operation of functional phononic devices and, as well, reportfirst experimental realizations.

Next, we discuss yet richer possibilities to manipulate heat flow by useof time varying thermal bath temperatures or various other external fields.These give rise to a plenty of intriguing phononic nonequilibrium phenomena asfor example the directed shuttling of heat, a geometrical phase induced heatpumping, or the phonon Hall effect, that all may find its way into operationwith electronic analogs.

References

[1] N. B Li, J. Ren, G Zhang, L Wang, P Hanggi, and B Li,Rev. Mod. Phys. 84 (3),1045 (2012)

Nov/27/2012
4:00 pm, 700 Clark Hall
Professor Lenoid Rokhinson, Purdue University
Observation of fractional ac Josephson effect: the signature of Majorana particles

I will report the observation of the fractional acJosephson effect in a hybrid semiconductor/superconductor InSb/Nb nanowirejunction, a hallmark of topological matter. When the junction is irradiatedwith rf frequency f quantized voltage steps (Shapiro steps) with a height hf/2eare observed, as is expected for conventional superconductor junctions wherethe supercurrent is carried by charge-2e Cooper pairs. At high magnetic fieldsthe height of the first Shapiro step is doubled to hf/e, suggesting that thesupercurrent is carried by charge-e quasiparticles.

This is a unique signature of Majorana fermions, elusiveparticles predicted ca. 80 years ago.

Nov/20/2012
4:00 pm, 700 Clark Hall
Johnpierre Paglione, University of Maryland
Topological Superconductors and Insulators: New and Improved!
Recently a new class of materials, the topological insulators, have caught the attention of the world with promises of demonstrating exotic physical phenomena and new spin-based technologies. With the help of the more well-known phenomenon of superconductivity, recent observations of the long-sought Majorana fermion have surfaced and led to a new direction of interest: topological superconductivity. In this talk I will review our efforts in synthesis and investigation of the most well-known topological insulator material to date, Bi2Se3, and present our observations of surface state quantum oscillations at high magnetic fields and pressure-induced superconductivity, as well as review our efforts to synthesize true “insulating” materials. In addition, I will review our observations of superconductivity in a new family of topological insulator materials.

Nov/13/2012
4:00 pm, 700 Clark Hall
CANCELLED – Liang Fu, MIT
SEMINAR CANCELLATION
Prof Liang Fu had to cancel due to travel problems. Wehope to possibly arrange another visit in the future.

Nov/06/2012
4:00 pm, 700 Clark Hall
Michel Gingras, University of Waterloo
The Tb2Ti2O7 Pyrochlore Antiferromagnet: the Platypus of Highly Frustrated Magnetic Materials

The Tb2Ti2O7 pyrochlore antiferromagnet was first found to lacklong range order down to 50 mK at about the same time as spin ice behavior wasreported in the Ho2Ti2O7 compound. Thirteen years later, despite numerousexperimental studies and theoretical attempts, we still do not have a clearunderstanding of what is the real nature of the low temperature state of thismaterial. In this talk, I will first review our current understanding ofclassical spin ices, magnetic materials that are analogue of common water ice. I will then discuss the salients facts, some agreed upon by mostresearchers and some not, regarding the nature of the low-temperature state ofTb2Ti2O7, commenting on the possibility that Tb2Ti2O7 may be viewed as aquantum spin ice, a novel state of matter topic that is currently attractinggrowing interest in the field of highly frustrated magnetism.

Oct/30/2012
4:00 pm, 700 Clark Hall
Jeff Gore, MIT
Anticipating sudden transitions in biological populations: Cooperation, cheating, and collapse
Natural populationscan suffer catastrophic collapse in response to smallchanges in environmental conditions, and recovery after such a collapsecan be exceedingly difficult. We have used laboratory microbial ecosystems tostudy early warning signals of impending population collapse. Yeastcooperatively breakdown the sugar sucrose, meaning that below a critical sizethe population cannot sustain itself. We have demonstrated experimentally thatchanges in the fluctuations of the population size can serve as an earlywarning signal that the population is close to collapse.The cooperative nature of yeast growth on sucrose suggests that the populationmay be susceptible to cheater cells, which do not contribute to the public goodand instead merely take advantage of the cooperative cells. We confirm thispossibility experimentally and explore how such social parasitism can lead topopulation extinction.

Oct/23/2012
4:00 pm, 700 Clark Hall
Ashvin Vishwanath, University of California, Berkeley
To Be Announced

Oct/16/2012
4:00 pm, 700 Clark Hall
N/A
NO SEMINAR THIS WEEK
Seminar has been cancelled this week.

Oct/02/2012
4:00 pm, 700 Clark Hall
Nam-Jung Kim, Seoul National University
Graphene-based hybrid Nanomaterial for Photonic and Photocatalytic Applications

Graphene has been of great interest owing to itsexcellent electrical and thermal properties as well as mechanical stability andflexibility. While the electrical and optical properties have been heavily investigatedsince the historical debut in 2004, the mechanical and structural properties ofgraphene have drawn increasing attention of late from scientists and engineers.Due to graphene’s atomically flat single layer with few defects, it can be usedas a flexible, robust substrate to create various organic-inorganic hybridnanostructures. In the first part of this talk, I will introduce the recentefforts in making transferrable photonic devices based on oxide nanostructuresgrown on graphene substrates, especially realizing semiconductorheterostructure LEDs. The graphene-semiconductor hybrid nanostructures are alsofound to be useful for photocatalytic applications. Finally, it will bedemonstrated that graphene oxide/metal nanoparticles composites are a verypromising photocatalytic material and convenient platform to study thephoto-generated charge carriers. Because of the relatively easy manufacturingprocess and hydrophilic property of the graphene oxide, it may provide a viablesolution for the wide range of environment and energy-related applications.

Sep/25/2012
4:00 pm, 700 Clark Hall
Nandini Trivedi, The Ohio State University
Prediction of a novel insulator across the disorder-driven superconductor-insulator transition
I will show how emergent mesa-scale granularity in a disordered s-wave superconductor eventually destroys superconductivity and produces an insulator of Cooper pairs with a hard gap in the single particle spectral function. This insulator sustains large diamagnetic currents and has a Josephson energy scale that vanishes as the transition is approached from the insulator.

Reference: K. Bouadim, Y.-L. Loh, M. Randeria and N. Trivedi, “Single- and two-particle energy gaps across the disorder-driven superconductor–insulator transition”, Nature Physics 7, 884-889 (2011).

Sep/18/2012
4:00 pm, 700 Clark Hall
Poul B. Peterson, Cornell University
New spectroscopic techniques for measuring ultrafast dynamics at surfaces and in the bulk
Ultrafast infrared spectroscopy is technically challenging and limited by the available light sources. In my group we are developing two new methods for probing the ultrafast dynamics in bulk and at surfaces. The first technique takes advantage of our newly developed ultrafast continuum mid-IR light source in bulk pump-probe spectroscopy. In this experiment we can excite any wavelength from the UV to the mid-IR and probe the resulting spectral changes over the whole 2-20 µm range (corresponding to 500-5000 cm-1) with ~150 fs time resolution. We are using this method to examine (1) proton transfer in the condensed phase and (2) intraband transitions and trap states in nanocrystals. In the second method we are combining bulk 2D-IR spectroscopy with surface-specific spectroscopy in the form of sum-frequency generation (SFG) to develop surface-specific 2D-IR spectroscopy. This fourth order technique will be able to probe ultrafast spectral diffusion dynamics and couplings in interfacial systems. On the road to surface-specific 2D-IR, we have currently set up transient pump-SFG probe spectroscopy to measure ultrafast vibrational relaxation at surfaces. We will apply this method to study (3) the ultrafast dynamics of interfacial water and (4) interfacial electron transfer for alternative solar energy applications. In this talk I will provide an overview of the experimental techniques and our initial efforts within our four research directions.

Sep/11/2012
4:00 pm, 700 Clark Hall
Salvatore Torquato, Center for Theoretical Physics, Princeton University
Unusual Low-Temperature States of Matter: Challenging Orthodoxy
We use inverse statistical-mechanical techniques to challenge conventional wisdom about the nature of classical low-temperature states of matter. We pose a number of unusual questions that have surprising answers. Can graphene or diamond crystal structures be the ground states of many-particle interactions that involve only non-directional (isotropic) pair potentials? Are there single-component many-particle systems characterized by isotropic pair potentials that possess exotic low-temperature bulk properties, such as negative thermal expansion or negative Poisson’s ratio? Can ground states ever be disordered? In other words, can cooling a liquid to absolute zero result in a completely disordered many-particle configuration (as opposed to the usual crystal ground state)? I will show that the answers to all of these questions are in the affirmative, and have fundamental and practical implications. I then will discuss the problem of determining the densest packings of spheres in high dimensions, which is directly related to the optimal way of sending digital signals over noisy channels. We provide strong theoretical evidence for the remarkable possibility that disordered (rather than ordered) sphere packings are the densest for sufficiently large dimensions, implying the existence of disordered ground states for some continuous potentials. This is done by providing the putative exponential improvement on Minkowski’s 100-year-old lower bound on the maximal packing density of spheres in high dimensions.

Host: Veit Elser

Sep/05/2012
11:00 am, 700 Clark Hall
Brian Josephson, University of Cambridge
Creative Mind and Physical Reality
Traditionally in science, specific physical laws are the basis for understanding all phenomena. Wheeler however argued (in an article entitled Law Without Law) that the laws of physics are the consequence of the action of minds (observers) themselves. As will be discussed, proper development of this idea requires consideration of mechanisms of cognitive development. This kind of picture offers the possibility of explanations for certain phenomena, such as the origin of life, and the very specific details of musical compositions, that are hard to understand on the basis of conventional thinking.

Host: N.W. Ashcroft, 255-8613

Sep/04/2012
4:00 pm, 700 Clark Hall
Dominik Schneble, Stony Brook University
Exploring Ultracold Atomic Mixtures in Optical Lattices
Quantum gases in optical lattices allow for fundamental studies in atomic and condensed-matter physics, including strongly-correlated many-body systems. In our group, we have studied 1D atomic mixtures (derived from a BEC) in lattices whose depth can be independently controlled for each component. In my talk, I will discuss three recent experiments:

The first experiment focused on the effects of randomly localized impurities on transport and excitations in a lattice-modulated 1D Bose gas. We observed an insulating gapless phase consistent with the formation of a Bose glass,and found that the correlation properties of a disorder potential can play an important role.

The second experiment explored the scattering of atomic matter waves from atomic crystals held in an optical lattice. We observed inelastic excitations as well as Bragg processes that revealed the spatial ordering and localization of atoms on individual sites. Matter-wave Bragg diffraction is an in-situ, non-destuctive interrogation technique that also precludes limitations on spatial resolution since the atomic de Boglie wavelength can be tuned freely.

The third experiment, based on a single component only, investigated the dynamical response of matter waves to a periodically pulsed, incommensurate optical lattice, realizing a system of two coupled kicked quantum rotors. Depending on the relative kicking strengths, we observed that the coupling can destroy dynamical localization and induce a diffusive spreading of momentum, revealing a transition from quantum to classical dynamics.

Aug/06/2012
11:00 am, 609 Clark Hall
J. L. Tallon, Victoria University of Wellington
Magnetic versus dielectric pairing in cuprate superconductors
This is a talk in two parts. In the first, I will review where we (in New Zealand) are presently positioned in the applications and manufacturing of a broad range of high-Tc systems. Then I will address the topic of the title.

A quarter of a century after their discovery the mechanism that pairs electrons in cuprate superconductors remains uncertain, though it is widely believed to be magnetic. If so, the energy scale for the pairing boson and hence for T_c should be governed to leading order by J, the antiferromagnetic exchange interaction. We can vary J (as measured by two-magnon Raman scattering) by more than 160% by changing ion sizes in LnA₂Cu₃O_7-δ where A=(Ba,Sr) and Ln=(La, … Lu). Such changes are often referred to as originating with “internal” pressure. Surprisingly, we find T_c^max anticorrelates with J. This is opposite to the effect of external pressure, suggesting that J is not the dominant energy scale governing T_c^max. In contrast, we show there is a clear correlation between T_c^max and the sum of polarizabilities of the constituent ions, these weighted by their densities (the molar refractivities). An immediate consequence is the possibility of a pairing mechanism having its origin with quantized waves of electronic polarization.

Jun/21/2012
4:00 pm, 700 Clark Hall
Yanming Ma, State Key Laboratory of Superhard Materials, Jilin University
Crystal Structural Prediction via CALYPSO Methodology: Principles and Applications

May/01/2012
4:00 p.m., 700 Clark Hall
Richard Hennig, Cornell University
Creatio Ex Nihilo or the Ab-Initio Discovery of Crystal Structure and Phase Diagrams

Predictions of structureformation by computational methods have the potential to accelerate materialsdiscovery and design. Many materials properties are controlled by compositionand crystal structure. Finding the ground state crystal structure of a materialis a very difficult problem because of the number of minima in the potentialenergy surface increases exponentially with the number of atoms. Here we presentan approach based on evolutionary algorithms coupled to ab-initio relaxations that accurately predicts how atoms arrangeinto crystal structure and what the composition is of the ground state phases ofmaterials without any prior information about the system. I will describeour genetic algorithm for structure prediction (GASP, http://gasp.mse.cornell.edu) and discusswhy these algorithms succeed in finding the ground state for many systems. Iwill present results for the Li-Be and Eu systems under pressure and for thebattery material Li-Si. For Li-Be we discover several stable phases underpressures and observe a rather unexpected quasi-1D and 2D electronic structurein some of these compounds [1]. For Eu we identify three phase transformations[2] Following the well-known bcc-to-hcp transition at 12 GPa, a mixed phaseregion is observed from 18 to 66 GPa until finally a single orthorhombic phasepersists from 66 to 92 GPa. The orthorhombic phase becomessuperconducting above 84 GPa. For Li-Si, a promising battery electrodematerial, we discover a large number of low-energy metastable phases that canaffect the charging characteristics of this material.

[1] J. Feng, R. G. Hennig, N. W.Ashcroft and Roald Hoffmann. Nature 451, 445 (2008).

[2] W. Bi, Y. Meng,R. S. Kumar, A. L. Cornelius, W. W. Tipton,R. G. Hennig, Y. Zhang, C. Chen, andJ. S. Schilling. Phys. Rev. B 83, 104106 (2011).

Apr/24/2012
4:00 p.m., 700 Clark Hall
Ned Wingreen, Princeton University
Why Are Chemotaxis Receptors Clustered But Other Receptors Aren’t?
The chemotaxis network of bacteria such as E. coli is remarkable for its sensitivity to minute relative changes in chemical concentrations in the environment. Indeed, E. coli cells can detect concentration changes corresponding to only ~3 molecules in the volume of a cell. Much of this acute sensitivity can be traced to the collective behavior of teams of chemoreceptors on the cell surface. Instead of receptors switching individually between active and inactive configurations, teams of 6-18 receptors switch on and off, and bind or unbind ligand, collectively. Similar to the binding and unbinding of oxygen molecules by tetramers of hemoglobin, the result is a sigmoidal binding curve. Coupled with a system for adaptation that tunes the operating point to the steep region of this sigmoidal curve, the advantage for chemotaxis is gain i.e., small relative changes in chemical concentrations are transduced into large relative changes in signaling activity (specifically, the rate of phosphorylation of the response regulator CheY). However, something is troubling about this simple explanation: in addition to providing gain, the coupling of receptors into teams also increases noise, and the net result is a decrease in the signal-to-noise ratio of the network. Why then are chemoreceptors observed to form cooperative teams? We present a novel hypothesis that the run-and-tumble chemotactic strategy of bacteria leads to a “noise threshold”, below which noise does not significantly decrease chemotactic velocity, but above which noise dramatically decreases this velocity.

Apr/17/2012
4:00 p.m., 700 Clark Hall
Eduardo Fradkin, University of Illinois, Urbana-Champaign
Intertwined Orders in High Temperature Superconductors
I will argue that the orders that are present in high temperature superconductors naturally arise with the same strength and are better regarded as intertwined rather than competing. I illustrate this concept in the context of the orders that are present in the pair-density-wave state and the phase diagrams that result from this analysis. I will also briefly discuss some recent progress in the microscopic origin of this phenomenon.

Apr/10/2012
4:00 p.m., 700 Clark Hall
Vladan Vuletic, MIT
Squeezing Quantum Noise: Atomic Clock Below the Standard Quantum Limit
The performance of the best atomic clocks and otherprecision atom interferometers is limited by the quantum noise in the final readoutmeasurement, a situation referred to as the standard quantum limit. This limitarises from the projection postulate when applied to an ensemble of independentparticles, i.e. it arises from single-particle quantum mechanics. I willdiscuss how quantum mechanically correlated (entangled) states of the many-bodysystem can be used to overcome the standard quantum limit, and how to generatesuch states in an ensemble of distant atoms using light. We demonstrate a clockoperated with a phase-squeezed input state that achieves a given precisionalmost three times faster than a clock operating at the standard quantum limit.

Apr/03/2012
4:00 pm., 700 Clark Hall
Jonathan Simon, Harvard University
Building Synthetic Materials from Ultracold Atoms: Quantum Magnetism in an Optical Lattice
Ultracold atoms in optical lattices are a unique testbed for quantum many-body physics. Using these systems it has recently become possible to engineer strongly correlated materials from the ground up and probe them with single-atom resolution. I will present experiments in which we have synthesized the first magnetic material composed of ultracold atoms, and watched in situ as quantum fluctuations induce a phase transition from a paramagnet to an antiferromagnet. I will then introduce a new algorithmic cooling scheme that we employ to drive atomic quantum gases to yet-lower entropies, paving the way for even more exotic quantum matter. I will conclude by discussing ways to combine these techniques to address outstanding questions in many-body dynamics, topological materials and beyond.

Mar/29/2012
10:00 am, 416 Physical Sciences Building
Kai Sun, University of Maryland
Zero-Energy Edge Modes, Emergent Conformal Symmetry, and Elastic Holography
Strongly Correlated Informal Thursday Seminar (SCITS)

Rigidity transitions in elastic systems have been the focus of intensive studies. In the theoretical understanding of these transitions, the idea of isostaticity plays an important role and captures the essential physics in many different systems as diverse as architectural structures, network glasses, randomly packed spheres, and biopolymer networks. In this talk, I will present our recent studies on a class of newly discovered 2D isostatic lattices, which contain a large number of zero-energy vibrational modes localized at the edges. In the phenomenological level, the emergence of these edge modes is a classical analogy to some of the quantum topological states of matter. However, in contrast to quantum systems, the edge modes here are protected by an emergent conformal symmetry in the low-energy effective theory, instead of a nontrivial topology. At long distance, this emergent symmetry results in universal behaviors for all 2D isostatic lattices with vanishing bulk moduli. However, in the short-distance limit, subleading corrections from dangerous irrelevant terms break this emergent conformal symmetry and leads to nonuniversal behaviors sensitive to microscopic details. The crossover between different length scales, as well as the holographic property of these isostatic systems, will also be addressed.

Reference: K. Sun, A. Souslov, X. Mao, and T. C. Lubensky, arXiv:1112.1109 (2011)

Mar/27/2012
4:00 p.m., 700 Clark Hall
Milan Allan, Cornell University
Discovery of the Nematic Parent State and the Anisotropic EnergyGap Structure of Iron-based High-Tc Superconductors
Iron-based superconductors reveal new routes towards understanding high-temperature superconductivity. I will discuss our discovery of strong electronic nematicity in the parent state of iron-based superconductors [1], and the recent realization of how dopant-atom induced unidirectional impurity states in this nematic environment could explain the mysterious anisotropic transport characteristics in this material [2]. Then I will describe our exploration of the superconducting energy gaps in the canonical Fe-based superconductor LiFeAs [3]. If strong electron-electron interactions between neighboring Fe atoms mediate the Cooper pairing in iron-pnictide superconductors, then specific and distinct anisotropic superconducting energy gaps Di(k) should appear on the different electronic bands i. We introduced intra-band Bogoliubov quasiparticle scattering interference (QPI) techniques for determination of Di(k) in such materials. We identify three hole-like bands and determine the anisotropy, magnitude and relative orientations of their Di(k). Such data could play a key role in identifying the Cooper pairing mechanism of iron-based superconductivity.

[1] Nematic electronic structure in the ‘parent’ state of the iron-based superconductor Ca(Fe1-xCox)2As2. T.-M. Chuang & M.P. Allan, et al., Science 327, 181 (2010).
[2] Nanoscale electronic-dimer scattering and the mechanism of electronic anisotropy in underdoped iron-arsenides. M. P. Allan & T.-M. Chuang et al., Submitted (2011).
[3] Anisotropic energy-gaps of iron-based superconductivity from intra-band quasiparticle interference in LiFeAs. M. P. Allan & A.W. Rost et al., to appear in Science (2012).

Mar/06/2012
4:00 p.m., 700 Clark Hall
Matteo Mariantoni, University of California, Santa Barbara
Superconducting Circuits in Context: The Photon Shell Game, the Quantum von Neumann Architecture, and the Surface Code
Superconducting quantum circuits have made significant advances over the past decade, allowing more complex and integrated circuits that perform with good fidelity. We have recently implemented a machine comprising seven quantum channels, with three superconducting resonators, two phase qubits, and two zeroing registers.

After introducing the main concepts on superconducting quantum circuits, I will explain the design and operation of our quantum machine. I will first show how a single microwave photon |1> can be prepared in one resonator and coherently transferred between the three resonators of the machine, thus allowing us to realize a quantum register on a chip [+]. I will then demonstrate how our machine can be used as the quantum-mechanical analog of the von Neumann computer architecture, which for a classical computer comprises a central processing unit and a memory holding both instructions and data. The quantum version comprises a quantum central processing unit (quCPU) that exchanges data with a quantum random-access memory (quRAM) integrated on one chip, with instructions stored on a classical computer. I will also present a proof-of-concept implementation of a code that involves all seven quantum elements: (1), Preparing an entangled state in the quCPU, (2), writing it to the quRAM, (3), preparing a second state in the quCPU, (4), zeroing it, and, (5), reading out the first state stored in the quRAM [*].

Finally, I will demonstrate that the quantum von Neumann machine provides one unit cell of a two-dimensional qubit-resonator lattice that can be used for surface code quantum computing. This will allow, (1), the realization of a scalable, fault-tolerant quantum processor with the most forgiving error rates to date and, (2), the possibility to emulate nontrivial many-body systems.

[+] M. Mariantoni et al., Nature Physics 7, 287-293 (2011).
[*] M. Mariantoni et al., Science 334, 61-65 (2011).

Matteo Mariantoni was supported in this work by an Elings Prize Fellowship in Experimental Science from UCSB’s California NanoSystems Institute. The work was supported by the Intelligence Advanced Research Projects Activity (IARPA) under the Army Research Office (ARO) award W911NF-08-1-0336 and W911NF-09-1-0375. Devices were made at the UCSB Nanofabrication Facility, a part of the NSF-funded National Nanotechnology Infrastructure Network.

Feb/21/2012
4:00 p.m., 700 Clark Hall
Wesley Campbell, University of Maryland and NIST (hold)
Coherent Manipulations with Ultrafast Pulses and Trapped Ions
Quantum many-body systems with tens or hundreds of particles are often intractable to simulate on classical computers due to the exponential growth of the Hilbert space with the number of particles. We can nonetheless hope to simulate them efficiently by using another quantum system—one that also shows this exponential scaling but is initializable, well controlled, and easy to manipulate and probe. We use a collection of trapped atomic ions as a platform for the quantum simulation of lattice spin models. The use of mode-locked lasers to create the state-dependent potentials that simulate a variety of spin Hamiltonians allows operation in a spectral region that is relatively free of laser-induced decoherence. We also realize ultrafast gates where a single laser pulse can drive a high-fidelity single-qubit gate in ~50 ps. Single laser pulses may also be used for rapid deceleration of (neutral) molecular beams, which may provide us with a bridge over the “temperature gap” for direct cooling of molecules to ultracold temperatures.

Feb/14/2012
4:00 p.m., 700 Clark Hall
Ni Ni, Princeton University
Manipulating Tc in Fe-based Superconductors through Doping and Interlayer Coupling
Searching for new superconductors and determining the key factors impacting Tc are at the core of research in superconductivity. Recently, Fe-based superconductors, the second high temperature superconductor family besides the cuprates, have been discovered to show Tcs up to 55 K. The interplay of the magnetism, superconductivity and structure in Fe-based superconductors makes them a great platform for understanding unconventional superconductivity. First, the temperature-dopant concentration (T-x), temperature-extra electrons (T-e), and temperature-pressure (T-P) phase diagrams of the Ba(Fe_1-xTM_x)₂As₂ series will be presented. Quantitative analysis shows that there exists a limited range of electron counts for which superconductivity can be stabilized if the structural and magnetic phase transitions of the parent compound BaFe₂As₂ are sufficiently suppressed. Furthermore, the Tc on the underdoped side can be related to the suppression of the structural / magnetic phase transition, while Tcmax on the overdoped side is determined by the electron concentration. Second, the crystal structures and properties of two structurally and chemically similar Fe based superconductors, Pt doped Ca₁₀(Pt₃As₈)(Fe₂As₂)₅ (the “10-3-8 phase”) with highest Tc around 11 K, and Ca₁₀(Pt₄As₈)(Fe₂As₂)₅ (the “10-4-8 phase”) with the highest Tc around 38 K, will be shown. The structural and chemical analysis of these compounds emphasizes the importance of strong interlayer coupling in enhancing Tc in Fe based superconductors.

Feb/09/2012
10:30 a.m., 609 Clark Hall
David Hawthorn, University of Waterloo
Electronic Structure Modulations in the Stripe Phase of High-Tc Cuprates
Special LASSP Seminar

A prevailing description of the stripe phase in underdoped cuprate superconductors is that the charge carriers (holes) phase segregate into hole rich regions that form anti-phase boundaries between regions of anti-ferromagnetic order. I will present resonant elastic x-ray scattering measurements in stripe ordered La_1.475Nd_0.4Sr_0.125CuO₄ (LNSCO) at the Cu L and O K absorption edges that point to an alternate interpretation of the stripe phase. Analysis of the energy dependence of the scattering intensity reveal that the dominant feature of the charge density wave (CDW) state is a spatial modulation in the energies of Cu 3d and O 2p states rather than the large modulation of the valence (charge density) envisioned in the common stripe paradigm. These energy shifts are interpreted as a spatial modulation in the electronic structure possibly involving a modulation of the Cu 3d – O 2p hybridization tpd, the onsite Coulomb repulsion, Udd, and other local electronic structure parameters. New results from the REIXS beamline at the Canadian Light Source will be presented.

Feb/07/2012
4:00 p.m., 700 Clark Hall
James Williams, Stanford University
Signatures of Majorana Fermions in Hybrid Superconductor-Topological Insulator Devices
The ability to measure and manipulate complex particles in the solid state is a cornerstone of modern condensed-matter physics. Typically, excitations of a sea of electrons, called quasiparticles, have properties very similar to those of free electrons. However, excitations with properties very different from electrons have been created in designer quantum materials: for example, Dirac quasiparticles in graphene and fractionally-charged quasiparticles in fractional quantum Hall systems. Here we report signatures of a new quasiparticle — a Majorana fermion — seen in a device in which a 3D topological insulator couples two conventional superconducting leads, forming a Josephson junction. We observe two striking departures from the common transport properties of Josephson junctions: a characteristic energy that scales inversely with the width of the junction, and an unexpectedly low characteristic magnetic field for suppressing critical current. We propose an explanation for these effects based on a model where a one-dimensional wire of Majorana fermions is present along the width of a junction, similar to the original theoretical prediction for these systems. These results should open a path for investigation of Majorana fermions in the solid state.

Feb/02/2012
4:00 p.m., 700 Clark Hall
Eva-Maria Schoetz, Princeton University
Body Sculpting: Collective Phenomena in Development and Regeneration
Embryogenesis and regeneration are among the most striking and beautiful phenomena in nature. For a physicist, this brings together many major themes—pattern formation, information processing, the mechanics of complex fluid-like materials—that are essential for our understanding of life more broadly. Connecting macroscopic observables which we can quantify to their microscopic origins is one of the major challenges toward an understanding of these complex processes. In my talk I will give two examples that try to make this connection.

First, I will discuss how tissue surface tension is connected to the mechanical properties of the constituent cells, such as cortical tension and adhesion. I will directly compare theoretical predictions with experimental data using primarily zebrafish embryonic tissues as the experimental system.

In the second part of my talk, I’ll switch gears to discuss asexual reproduction in planarians. Asexual reproduction and the ability to regenerate are intrinsically connected, but despite this important link, little is known about asexual reproduction in planarians due to experimental challenges. I will discuss our current understanding of the asexual population dynamics based on a large-scale experiment in which we have been tracking >10,000 reproductive events over the course of ~3 years and up to 55 generations using a custom-built Scan-Add-Print database system. Statistical analysis of the reproduction dynamics reveals a reproductive memory whose molecular basis we have now begun to elucidate.

Jan/31/2012
4:00 p.m., 700 Clark Hall
Jacob Ruff, Argonne National Laboratory
Shining Light on Materials in High Magnetic Fields
Recent advances in resistive mini-coil pulsed magnet design have enabled, for the first time, synchrotron x-ray scattering experiments in high magnetic fields. Two pulsed magnet instruments under development at the Advanced Photon Source offer a new microscopic perspective on the magnetoelastic properties of materials, which we are now learning to exploit. In this seminar, I will review some results from the first four years of pulsed magnet scattering at the APS. Specific examples will include magnetic detwinning of an iron arsenide superconductor, our attempt at suppressing the volume collapse transition in a cerium alloy, and our exploration of giant magnetostrictive effects in a frustrated pyrochlore antiferromagnet. In each of these cases, I will highlight the unique information provided by pulsed-field diffraction, which could not otherwise be obtained by bulk measurements at dedicated high-field laboratories. Future directions for high-field scattering at x-ray and neutron facilities will be discussed.

[1] Z. Islam, J.P.C. Ruff, H. Nojiri, et al. “A portable high-field pulsed-magnet system for single-crystal x-ray scattering studies.” Rev. Sci. Instrum. 80, 113902 (2009)

[2] Z. Islam, D. Capatina, J.P.C. Ruff, et al. “A high-field pulsed magnet system for x-ray scattering studies in Voigt geometry.” arXiv:1109.6713v1

[3] J.P.C. Ruff, Z. Islam, J.P. Clancy, et al. “Magnetoelastics of a Spin Liquid: X-Ray Diffraction Studies of Tb2Ti2O7 in Pulsed Magnetic Fields.” Phys. Rev. Letters 105, 077203 (2010)

[4] J.P.C. Ruff, Z. Islam, R.K. Das, et al. “A robust but disordered collapsed-volume phase in a cerium alloy under the application of pulsed magnetic fields.” arXiv:1109.5986v1

Jan/26/2012
4:00 p.m., 700 Clark Hall
Cheng Chin, University of Chicago
Having your cake and seeing it too — In situ Observation of Quantum Phase Transition in Ultracold Atomic Gases
Atoms at ultralow temperatures are fascinating quantumobjects, which can tunnel through barriers, repel or attract each other, andinterfere like electromagnetic waves. This wavy behavior of ultracold atomsevidently illustrates the particle-wave duality as discussed in quantumphysics. By loading repulsively interacting atoms into an array of opticalcells (or optical lattices), the wavy nature of the atoms can be completelysuppressed even at zero temperature. At the same time, the atomic sampledevelops an interesting multi-layer structure with density plateaus, resemblinga multi-tier wedding cake. Our observation of the cake structure in 2D opticallattices [1] raises new prospects to investigate the dynamics and transportacross the phase boundary [2] and to identify universal scaling laws [3] andquantum critical behaviors [4]. Surprising findings in these experiments willbe presented and discussed.

References:
[1] In situ Observation of Incompressible Mott-insulating Domains in Atomic Gases, Nathan Gemelke, XiboZhang, Chen-Lung Hung, Cheng Chin, Nature 460, 995 (2009)

[2] Slow Mass Transport and Statistical Evolution of AnAtomic Gas Across the Superfluid-Mott Insulator Transition, Chen-Lung Hung, Xibo Zhang, Nathan Gemelke, Cheng Chin, Phys. Rev. Lett.104, 160403 (2010)

[3] Observation of Scale Invariance and Universality in Two-DimensionalBose Gases, Chen-Lung Hung, Xibo Zhang, Nathan Gemelke, Cheng Chin,Nature 470, 236 (2011)

[4] Quantum Critical Behavior of Ultracold Atoms in Two-DimensionalOptical Lattices, Xibo Zhang, Chen-Lung Hung, Shih-Kuang Tung, Cheng Chin,arXive:1109.0344 (submitted to Science)

Jan/24/2012
4:00 p.m., 700 Clark Hall
Jonathan Simon, Harvard University/MIT Center for Ultracold Atoms
Building Synthetic Materials from Ultracold Atoms: Quantum Magnetism in an Optical Lattice
Ultracold atoms in optical lattices are a unique testbed for quantum many-body physics. Using these systems it has recently become possible to engineer strongly correlated materials from the ground up and probe them with single-atom resolution. I will present experiments in which we have synthesized the first magnetic material composed of ultracold atoms, and watched in situ as quantum fluctuations induce a phase transition from a paramagnet to an antiferromagnet. I will then introduce a new algorithmic cooling scheme that we have demonstrated, for driving atomic quantum gases to yet lower entropies, paving the way for even more exotic quantum matter. I will conclude by discussing ways to combine these techniques to address outstanding questions in many-body dynamics, topological materials and beyond.

Jan/18/2012
4:30 p.m., 120 Physical Sciences Building
James Analytis, SLAC National Accelerator Lab
Coupled Order Parameters and Quantum Criticality in Iron-Based Superconductors
LASSP & AEP Faculty Candidate Seminar

Condensed matter systems are a playground for exploring new states of quantum matter. The nature of ordered states and their associated critical fluctuations connects many problems in condensed matter physics, particularly unconventional superconductivity in the cuprate, organo-metallic and heavy-fermion materials. The recently discovered iron-based superconductors could provide a simpler route into understanding this broad problem because of their simple (and relatively uncontroversial) phase structure. As these materials are doped or the temperature is lowered, the different thermodynamic phases that are stabilized include magnetism and superconductivity. However recent measurements have also suggested that nematic order plays an important role by coupling to magneto-elastic or superconducting-elastic degrees of freedom. I will present evidence from transport measurements of underdoped (orthorhombic) and overdoped (tetragonal) compounds that magneto-structural fluctuations are closely linked with quantum critical fluctuations at optimal doping, suggesting a strong connection to superconductivity.

Dec/01/2011
3:00 p.m., 416 Physical Sciences Building
Roger Melko, University of Waterloo
Spin Liquid and Deconfined Criticality in a Kagome Lattice Bose-Hubbard Model
We present large-scale quantum Monte Carlo simulations on a sign-problem free Bose-Hubbard model on the kagome lattice. This model supports a quantum Z2 spin liquid phase with fractional excitations and topological order, which can be characterized definitively through calculation of the topological entanglement entropy. I will outline how the entanglement entropy can be measured in general using a direct implementation of the familiar “replica trick,” which allows for the study of entanglement scaling in a variety of other models amenable to study by GMC. Finally, I will examine the kagome model’s superfluid/spin-liquid transition, which is an example of an exotic deconfirmed quantum critical point called XY*, mediated by the fractional charges. This fact is demonstrated in several universal quantities that we measure, and may also be reflected in the scaling of entanglement entropy at the critical point.

Nov/29/2011
4:00 pm, 700 Clark Hall
Jay Fineberg, Racah Institute of Physics, Hebrew University of Jerusalem
The Onset of Frictional Motion: “Fracture” or “Friction” or How Things Slide
The onset of frictional motion has for hundreds of yearsbeen described by the concept of the static friction coefficient μ_S, that reflects proportionality between appliedshear and normal forces when frictional motion initiates. The onset offrictional slip is, however, mediated by the rapid rupture of the discreteensemble of contacts that forms a rough frictional interface. We’ll firstdescribe the dynamics of these rupture modes and show that they are intimatelycoupled to the local values of the shear and normal stresses along theinterface. Moreover, for “simple” external loading conditions the local ratioof this stresses can not only vary considerably but can, locally, surpass μ_S by hundreds of percent. We will go on todemonstrate that μ_S is not a constantmaterial property; μ_S varies systematically withthe external loading configuration. These results suggest a picture of frictionthat is closely linked to the rapid fracture dynamics of the interfacialcontacts—with implications that range from the physics of materials to ourfundamental understanding of earthquake dynamics.

Nov/22/2011
4:00 pm, 700 Clark Hall
David K. Lubensky, University of Michigan
Self-Organization in Animal Development
Developmental biology presents some of the most amazing examples of self-organization found in nature: Starting from a single-celled egg, an animal is able, largely without outside help, to construct an entire, incredibly intricate organism, with cells of different types taking on myriad shapes and arranged in complex patterns to create a viable adult. Many of the individual steps in this maturation bear at least a superficial resemblance to examples of pattern formation or self-assembly in simpler physical systems, and there is a long tradition of using physically-inspired models to try to understand developmental processes. Only relatively recently, however, have experimental techniques reached the point where it is possible to subject these models to stringent tests. While confirming some classical ideas, such experiments have also begun to reveal how specifically biological controls interact with more generic physical effects to lead to reliable and robust development. Using examples from my own research on eye development in fruit flies and in fish, I will show how it is becoming possible to dissect the mechanisms of self-organization during development and illustrate some of the surprising new physical ideas that can result from such investigations.

Nov/15/2011
4:00 pm, 700 Clark Hall
Keith Nugent, The University of Melbourne
Imaging with Coherent X-rays

There has been considerable interest overthe last decade in the development of coherence imaging methods. Major goals ofthis research program are the development of molecular imaging methods suitablefor use with the X-ray free electron lasers (XFEL) now operating at both SLACin the USA and SPring8 in Japan, and high-resolution X-ray imaging withsynchrotron and other short-wavelength sources.

In this talk I will present the work of mygroup in Australia on the development of novel approaches to coherent imaging.We have paid a particular emphasis on imaging non-XFEL sources and on themeasurement and inclusion of the coherence properties of the sources [1]. Wehave also begun exploring the development of novel highly coherent electronsources [2] which we believe promise to be an exciting emerging direction forvery high-resolution coherent imaging.

1. Whitehead, LW, et al.: Diffractive ImagingUsing Partially Coherent X Rays. Physical Review Letters 2009, 103(24):243902.

2. McCulloch, AJ, et al.: Arbitrarily shapedhigh-coherence electron bunches from cold atoms. Nature Physics 2011, 7(10):785-788.

Nov/08/2011
4:00 pm, 700 Clark Hall
J. Brad Marston, Brown University
Astrophysical and Geophysical Flows as Non-Equilibrium Condensed Matter
Astrophysical and geophysical fluids may be fruitfully viewed as forms of condensed matter driven far away from equilibrium. Such large-scale flows are not so nonlinear as to preclude their direct statistical simulation (DSS) by systemic expansions in cumulants. DSS offers a number of advantages over direct numerical simulation: (i) Low-order statistics are smoother in space and stiffer in time than the underlying instantaneous flows, hence statistically stationary or slowly varying fixed points can be described with fewer degrees of freedom and can also be accessed rapidly; (ii) Convergence with increasing resolution can be demonstrated; (iii) Finally and most importantly, DSS leads more directly to understanding, by integrating out fast modes, leaving only the slow modes that contain the most interesting information. The equations of motion for the equal-time cumulants form an infinite hierarchy. The simplest truncation is to set the third and higher order cumulants to zero, but more sophisticated closures are also of interest. DSS will be illustrated by application to a jet on a rotating sphere and also to a model of the solar tachocline driven by relaxation to an underlying flow with shear for which a joint instability arises from the combination of shearing forces and magnetic stress. Live demonstrations of DSS will be performed to show that equal-time statistics so obtained agree well with those accumulated by direct numerical simulation. I conclude by discussing possible extensions of the method both in terms of methods of non-equilibrium statistical mechanics and in the range of astrophysical and geophysical problems that are of interest.

Nov/01/2011
4:00 pm, 700 Clark Hall
Sidney Nagel, University of Chicago
Memories in Matter
Systems render information about their formation inaccessible to observers after they relax to equilibrium; a system that has not fully relaxed has the potential to retain memories of its creation. I will describe a class of out-of-equilibrium disordered systems that form memories in a remarkable fashion. The system “remembers” multiple values from a series of training inputs yet “forgets” nearly all of them at long times despite the inputs being continually repeated. However, with the addition of noise, the system retains all the memories indefinitely. I will argue that this is a generic process that may be seen in a variety of situations.

Oct/25/2011
4:00 p.m., 700 Clark Hall
Charles Kane, University of Pennsylvania
From Luttinger Liquid to Non-Abelian Quantum Hall States
We formulate a theory of strongly interacting two-dimensional topological phases by considering an anisotropic system consisting of coupled one-dimensional wires. We show that Abelian bosonization provides a simple framework for characterizing integer, fractional, hierarchical and non-Abelian quantum Hall states. This formulation provides a direct route between a solvable microscopic Hamiltonian formulated in terms of electronic degrees of freedom and the low energy description of quasiparticles and edge states in terms of conformal field theory. It also can be used to describe anisotropic lattice systems, and provides a starting point for models of fractional and non-Abelian Chern insulators. Implications for fractional topological insulators in two and three dimensions will be discussed.

Oct/18/2011
4:00 p.m., 700 Clark Hall
Veit Elser, Cornell University
Two Algorithms, One Talk, and No Free Lunch

Can you assemble the square tiles above into an 8×8 square so all the colored edges match and the border is all gray? I will describe a new algorithm for solving this problem and its more serious application to protein folding. Unrelated to this, I will describe another algorithm that seeks to reconstruct images of biomolecules from diffraction data that is almost pure noise.

Oct/04/2011
4:00 pm, 700 Clark Hall
Eric Cockayne, National Institute of Standards & Technology
To Be Announced

Sep/27/2011
4:00 pm, 700 Clark Hall
Paul Alivisatos, University of California, Berkeley
Toward Artificial Photosynthesis
There is a great need to be able to harvest the energy of the sun and to store the energy in a fuel. To be of practical use, that must be done at low cost and in a scalable manner, while also achieving reasonable efficiency. This talk will describe our recent work exploring multi-component nanocrystals, which are designed to absorb light, separate charges, and funnel the charges to catalysts to drive subsequent chemistry.

Sep/21/2011
4:00 p.m., 701 Clark Hall
John Campbell, University of Canterbury, New Zealand
Rutherford’s Path to the Nuclear Atom
2011 is the centennial of Rutherford’s nuclear atom, one of his three fundamental discoveries and the one for which he has most international fame. John will talk about Rutherford’s intriguing path to this discovery.

Sep/20/2011
4:00 pm, 700 Clark Hall
Fa Wang, Massachusetts Institute of Technology
Functional Renormalization Group Studies of Iron-Based Superconductors
Iron-based superconductors are a family of high T_c superconductors discovered a few years ago. They share several interesting features with the cuprates and other unconventional superconductors, for example the proximity of antiferromagnetism and superconductivity in the phase diagram and non-Fermi-liquid like properties in the normal state. Although their transition temperature (max. T_c ~ 55K) is not as impressive as the cuprates, iron-based superconductors hold promises for a better understanding of the mechanism of high T_c superconductivity. We studied electron correlation effects in these materials by the functional renormalization group methods, which is in principle an unbiased method in the weak correlation regime. We have also confirmed our results by Gutzwiller projected wavefunction studies which can treat intermediate and strong correlations. Our results predicted unconventional s-wave (s+-) pairing and gap anisotropy for the superconducting state, and furthermore several competing orders to superconductivity, including Fermi surface distortions. A strong evidence of the unconventional s-wave pairing state is the STM quasi-particle interference experiment, theoretically proposed by us and experimentally first realized by Hanaguri’s group. Experimental signatures of other theorectial predictions have also recently emerged. Comparing the results for iron-based materials and a model for (overdoped) cuprates we suggest that they share the same physics.

Sep/13/2011
4:00 pm, 700 Clark Hall
Peter Fischer, Lawrence Berkeley National Laboratory
Magnetic Soft X-ray Microscopy: A Path Towards Imaging Magnetism Down to Fundamental Length and Time Scales
One of thescientific and technological challenges in nanomagnetism research is to imagemagnetism down to fundamental magnetic length and time scales with elementalsensitivity in advanced multi-component materials. Magnetic soft X-raymicroscopy is a unique analytical technique combining X-ray magnetic circulardichroism (X-MCD) as element specific magnetic contrast mechanism with highspatial and temporal resolution. Fresnel zone plates used as X-ray opticalelements provide a spatial resolution down to currently 10nm thus approachingfundamental magnetic length scales such as magnetic exchange lengths. Imagescan be recorded in external magnetic fields giving access to studymagnetization reversal phenomena on the nanoscale and its stochastic characterwith elemental sensitivity. Utilizing the inherent time structure of currentsynchrotron sources, fast magnetization dynamics such as current induced walland vortex dynamics in ferromagnetic elements can be performed with astroboscopic pump-probe scheme with 70ps time resolution, limited by thelengths of the electron bunches. With a spatial resolutionapproaching the

Sep/06/2011
4:00 pm, 700 Clark Hall
Seongshik Oh, Rutgers University
Do We Really Have Topological Insulators Yet?
Overthe past few years, topological insulators (TIs) have emerged as a new platformfor coherent spin-polarized electronics and quantum computations. They are predictedto have an insulating bulk state and spin-momentum-locked metallic surfacestates. This spin-momentum-locking mechanism and their band structure topologyare predicted to prevent the surface metallic states from being localized dueto backscattering. However, in spite of many reports supporting the existenceof TI surface states, their bulk state has always turned out to be metallic,instead of insulating. In other words,current-generation TIs are not truly topological insulators, but rather topologicalmetals. These metallic bulk states notonly shunt the surface state conduction, thus making extractionof the surface contribution difficult, but also degrade the surface states byproviding scattering channels that are forbidden in true TIs. In this talk, I will discuss these challengesand our efforts to implement true topological insulators through thin filmengineering schemes.

Aug/30/2011
4:00 pm, 700 Clark Hall
Kenneth O’Hara, Pennsylvania State University
Using Ultracold Atoms to Model Nuclei and Nuclear Matter
It may seem surprising that a dilute, ultracold atomic gas could share anything in common with nuclei and nuclear matter in neutron stars since the details of their interactions and their absolute energy scales differ so dramatically. Yet when the deBroglie wavelength greatly exceeds the range of interactions and the scattering length is large, universal phenomena are expected. We have observed the Efimov effect, first predicted in the context of nuclear physics, wherein three particles form an infinite series of three-body bound states where successive states are related by a universal scale factor. In addition to providing experimental evidence for this remarkable 40-year-old prediction, this observation has set the stage for observing an analog of color superconductivity with cold atoms. I will also discuss measurements of the universal interaction energy for a strongly-interacting Fermi gas with zero-range interactions and a divergent scattering length that are relevant to the equation of state for extremely low-density neutron matter. By recently demonstrating a Fermi system with a large effective range in addition to a large scattering length, we hope to extend the pertinence of such measurements to neutron matter at densities of astrophysical interest.

Jul/07/2011

Michael Mwangi, Rockefeller University
Using Whole Genome Sequencing to Unravel the Mechanisms of Antibiotic Resistance to MRSA
The World Health Organization (WHO)has identified the emergence and spread of multidrug resistant pathogens as aglobal public health threat. Of these pathogens, arguably one of the mostimportant is multidrug resistant Staphylococcus aureus (MRSA). Not only is MRSAa major source of infection in immunocompromised patients in healthcaresettings, but it has begun to infect more and more people with no traditionalrisk factors in community settings. In MRSA, various types of resistance, likevancomycin resistance, appear to be highly polygenic traits, so much remains tobe understood about the emergence of resistance, especially in vivo. Recently,we used high-throughput DNA sequencing to identify many mutations conferringresistance. While many genes seem to be involved in resistance, we did notmerely end up with a laundry list of genetic determinants. Instead, we weresurprised to find a stunning degree of parsimony and universality.

May/18/2011
1:00 p.m., 416 PSB
Doron Bergman, California Institute of Technology
The Topological Insulator and a Parasitic Metal — Friends or Foes?
In the flurry of experiments looking for topological insulator materials, it has been recently discovered that some bulk metals very close to topological insulator electronic states support the same topological surface states that are the defining characteristic of the topological insulator. First observed in spin-polarized angle resolved photoemission spectroscopy (ARPES) in Sb [D. Hsieh et al., Science 323, 919 (2009)], the helical surface states in the metallic systems appear to be robust to at least mild disorder. We present here a theoretical investigation of the nature of these “helical metals”—bulk metals with helical surface states. We explore how the surface and bulk states can mix, in both clean and disordered systems. We also explore magnetoelectric coupling phenomena in these systems, which turn out to realize a higher dimensional analog of the intrinsic anomalous Hall effect.

May/11/2011
3:00 pm, 609 Clark Hall
Thanh-Phong Vo, Lyon Institute of Nanotechnology
Near-field coupling of slow light mode through a nano-aperture antenna
Kavli Institute at Cornell seminar

We demonstrate the efficiency of using the bowtie nano-aperture antenna (BNA) near-field probe as nano-collector and nano-polarizer to investigate the full optical properties of slow Bloch mode on defect-free honeycomb planar photonic crystal. BNA is opened at the rounded apex of a metal-coated optical fiber for near-field scanning optical microscopy (NSOM) set-up. The experimental coupling efficiency of near-field modes to BNA-tip is estimated two orders of magnitude meanwhile the spatial resolution of optical images reaches twofold better with respect to conventional metal-coating tip.

Host: Jiwoong Park

May/03/2011
4:00 p.m., 700 Clark Hall
Evgeny Tsymbal, University of Nebraska-Lincoln
Ferroelectric Tunnel Junctions: Controlling Electron and Spin Transport by Ferroelectric Polarization
AEP & LASSP Seminar

Tunneljunctions are useful electronic devices in which current-carrying electrons canquantum-mechanically be transmitted between two metal electrodes across a verythin insulating barrier layer. A particular example is a magnetic tunneljunction, where electrical resistance depends on magnetization orientation ofthe two ferromagnetic electrodes – the phenomenon known as tunnelingmagnetoresistance (TMR). So far, however, almost all the existing tunneljunctions were based on non-polar dielectrics. An exciting possibility toextend the functionality of tunnel junctions is to use a ferroelectricinsulator as a barrier to create a ferroelectric tunnel junction (FTJ). [1] Thekey property of FTJ is tunneling electroresistance (TER) that is the change inelectrical resistance of a FTJ with reversal of ferroelectric polarization.Functional properties of a FTJ can be further extended by ferromagneticelectrodes to create a multiferroic tunnel junction (MFTJ). In such a MFTJ the transport spin polarizationand TMR are affected by ferroelectric polarization of the barrier. [1] Thus, MFTJsrepresent four-state resistance devices that can be controlled both by electricand magnetic fields. This talk will address the physics of FTJs and MFTJs basedon recent modeling and experiments.

1. E. Y. Tsymbal and H. Kohlstedt, Science 313, 181 (2006).

Apr/26/2011
4:00 p.m., PSB 401
Minoru Yamashita, Kyoto University
Thermal-transport Studies of Quantum Spin Liquids
Quantum spins, coupling antiferromagnetically on a 2D triangular lattice, cannot simultaneously satisfy all interactions. This frustrated situation is expected to give rise to mysterious fluid-like states of spins without long-range order, so called quantum spin liquid (QSL). The ground state of QSL and its exotic phenomena, such as fractionalized excitation with an artificial gauge field, have been extensively discussed for decades, yet to be identified by lack of any real materials. This is why the recent discoveries of materials possessing an ideal 2D triangular lattice have spurred a great deal of interest. To understand the nature of QSL, knowledge of the low-lying excitation, particularly the presence/absence of an excitation gap, is of primary importance. We employ thermal transport measurements on newly discovered QSL candidates, κ-(BEDT-TTF)₂Cu₂(CN)₃ and EtMe₃Sb[Pd(dmit)₂]₂, and report that the two organic insulators possess different QSLs characterized by different elementary excitations. In κ-(BEDT-TTF)₂Cu₂(CN)₃ [1], heat transport is thermally activated in low temperatures, suggesting presence of a spin gap in this QSL. In stark contrast, in EtMe₃Sb[Pd(dmit)₂]₂ [2], a sizable temperature-linear term of thermal conductivity is clearly resolved in the zero-temperature limit, showing gapless excitation with long mean free path (~1,000 lattice distances), analogous to excitations near the Fermi surface in normal metals. These results are consistent with theoretical suggestions including 2D gapless spinons with a Fermi surface.

This work was done in collaboration with N. Nakata, Y. Senshu, M. Nagata, Y. Kasahara, S. Fujimoto, T. Shibauchi, Y. Matsuda, T. Sasaki, N. Yoneyama, N. Kobayashi, H. M. Yamamoto and R. Kato.

[1] Minoru Yamashita et al., Nature Physics 5, 44-47 (2009).
[2] Minoru Yamashita et al., Science 328, 1246 (2010).

Apr/19/2011
4:00 p.m., PSB 401
Taylor Hughes, University of Illinois, Urbana-Champaign
Anomalies and Torsion in Condensed Matter Systems
In this talk I will review the common appearance of torsion in solids as well as some new developments. Torsion typically appears in condensed matter physics associated to topological defects known as dislocations. Now we are beginning to uncover new aspects of the coupling of torsion to materials. Recently, a dissipationless viscosity has been studied in the quantum Hall effect. I will connect this viscosity to a 2+1-d torsion Chern-Simons term and discuss possible thought experiments in which this could be measured. If time permits, I will briefly discuss a new topological defect in 3+1-d, a torsional skyrmion, which does not require a lattice deformation to exist in solids. If present, torsional skyrmions are likely to impact the propagation of electrons in materials with strong spin-orbit coupling such as topological insulators and spin-orbit coupled semiconductors.

Apr/12/2011
4:00 p.m., 700 Clark Hall
Wolfgang Ketterle, MIT
Toward Quantum Magnetism with Ultracold Atoms
Over the last 20 years,science with ultracold atoms has focused on motion: slowing down motion,population of a single motional state (Bose-Einstein condensation, atomlasers), superfluid motion of bosons and fermion pairs. In my talk, Iwill address the next challenge when motion is frozen out: Spinordering. A two-component boson or fermion mixture can form magneticphases such as ferromagnetic, antiferromagnetic ordering and a spinliquid. The challenge is to reach the low temperature and entropyrequired to observe these phenomena. I will describe our current effortsand progress towards this goal. This includes the study of fermions withstrong repulsive interactions where we obtained evidence for a phase transitionto itinerant ferromagnetism, and a new adiabatic gradient demagnetization coolingscheme which has enabled us to realize spin temperatures of less than 50picokelvin in optical lattices. These are the lowest temperatures evermeasured in any physical system.

Apr/05/2011
4:00 p.m., 700 Clark Hall
Itai Cohen, Cornell University
Using a Confocal Rheoscope to Investigate Soft Squishy Materials
Soft matter systems derive their bulk mechanical properties from their underlying microscale structure and it’s response to thermal fluctuations. In this talk I will discuss how we are using our newly developed Confocal Rheoscope to simultaneously measure changes in the mechanical behavior and structural organization of materials ranging from shear thinning and thickening colloidal suspensions to various strain stiffening biological tissues. In the colloidal systems, our studies have revealed the underlying entropic mechanisms, in-plane structural changes, and hydro-clusters that accompany transitions in the bulk material’s flow response. In the cartilage tissue, we have shown that the inhomogeneous tissue structure and composition lead to dramatic variations in the mechanical properties of the tissue. For example, we have discovered that that a 300 micron thick region located 100 microns below the articular surface is the primary location where shear energy is being dissipated in this tissue. Finally, I will comment on the implications of these studies to emerging technologies and medical procedures.

Mar/17/2011
4:00 p.m., 700 Clark Hall
Eric Brown, James Franck Institute, University of Chicago
Shear Thickening in Concentrated Suspensions
Particulate fluids such as colloids, suspensions, foams, emulsions, and granular materials can exhibit a variety of mechanical properties. One of the most dramatic of these properties is shear thickening in concentrated suspensions and colloids, in which the effective viscosity reversibly and discontinuously jumps by orders-of-magnitude as the shear rate is increased. This phenomenon has not been previously explained based on traditional hydrodynamic rheology models. Using rheometry and video microscopy measurements on suspensions, I will show this shear thickening is a response to expansion (dilatancy) of the particle packing under shear, a common feature of granular materials. The expansion strains the boundaries of the system, which respond with a confining stress, typically set by the stiffness of the walls or surface tension. The confining stress is transmitted through the packing along frictional contacts, causing the dramatically enhanced dissipation under shear which is observed as shear thickening. I will also discuss connections with the mechanics of other particulate fluids, and the consequences for models of concentrated particulate fluids.

Mar/15/2011
4:00 p.m., 700 Clark Hall
Lode Pollet, Institute of Theoretical Physics, ETH Zurich
Byteing the Supersolid
In recent years, dramatic improvements in algorithms allowed physicists to study models and materials with unprecedented accuracy and in hitherto inaccessible parameter regimes. I will discuss some of the most exciting ideas and results, and show how they slowly but surely push the frontier of what is doable and controllable. As a particular example, I will zoom in on path integral Monte Carlo simulations, which are the heart of our present understanding of the supersolid properties of Helium-4. This is a debate that took off with the observation of a non-classical moment of rotational inertia by Kim and Chan in 2004 when they loaded solid Helium-4 into a torsional oscillator and cooled it to temperature of 0.2 Kelvin. Those results are still topic of debate today. I will show that ideal crystals are insulating and purged from vacancies, but also that crystallographic defects such as grain boundaries and dislocations can become superfluid.

Mar/10/2011
4:00 p.m., PSB 401
Eleni Katifori, Rockefeller University
Design Principles in the Plant Kingdom: Loops, Optimality and the Architecture of Leaf Veins
Leaf venation is a pervasive example of a complex biological transport network that is necessary for the survival of land plants and thought to be highly optimized. Distribution networks optimized for efficiency have been shown to be loopless — yet, the architecture of the leaf vascular networks is dominated by loops. We consider possible reasons for the emergence of loops in biological transport networks and study optimizing functionals that can account for their ubiquity. We show that loops can emerge for a number of reasons, each with a unique signature in the network architecture. We sketch the development of a mathematical framework that is suitable to characterize planar redundant networks, dominated by (hierarchically nested) loops. Finally, we examine the metric topology and transport properties of the resulting optimized networks and compare them with exemplars of their real life counterparts, dicotyledonous leaves.

Mar/08/2011
4:00 p.m., 700 Clark Hall
Pierre Thibault, Technical University of Munich
X-ray Imaging in the Style of Bird Calls
Recent synchrotron sources and X-ray free-electron lasers produce X-ray beams of unprecedented quality. In combination with nearly ideal detectors, the highly coherent beams now available can be put to use for a variety of imaginative imaging techniques, where X-ray photons propagate freely after interacting with a specimen. These lensless imaging approaches transfer much of the image-formation task from hardware to software, bringing interesting problems in data analysis. In this talk I will give a broad overview of the techniques that can make full use of the exceptional properties of the new and foreseen technology, with a special emphasis on the data analysis challenges. In particular, I will describe a method called ptychography, whose detection scheme has deep connections with other fields, including quantum tomography and a common analysis method used to represent bird calls. I will present some of the most exciting future developments, including protein structure determination and time-resolved imaging at the nanometer scale. I will also describe how Cornell’s ERL project will be an invaluable tool for many of these novel techniques.

Mar/03/2011
4:00 p.m., 700 Clark Hall
Jing Xia, California Institute of Technology
Topological Phases and Their Competition with Symmetry-Breaking Orders
Topological order is a new kind of collective order beyond Landau’s symmetry-breaking classification. Interesting in its own right, certain topologically ordered materials including the “chiral p-wave” superconductors and “non-Abelian” fractional quantum Hall (FQH) states may be used to realize fault-tolerant “topological” quantum computers. We have used both optical and electrical techniques to identify topological phases and to study their competition with broken symmetry orders. As the first example, I will describe ultra-sensitive magneto-optic Kerr measurements on ruthenate superconductor Sr₂RuO₄ with a recently developed “loop-less” fiber-optic Sagnac interferometer, identifying Sr₂RuO₄ to be a “chiral p-wave” topological superconductor. As a second example, I will discuss in 2D electrons at filling factor 5/2 the intriguing competition between the “non-Abelian” topological FQH state, an electronic liquid crystal phase and a newly discovered “reentrant isotropic compressible” state. I will also provide evidence for a novel rotational-symmetry-breaking FQH state as a consequence of this competition. Finally, I will discuss other applications of Sagnac interferometer, including bio-medial imaging.

Mar/01/2011
4:00 p.m., 700 Clark Hall
Yusuke Nishida, Massachusetts Institute of Technology
Universal Physics with Ultracold Atoms: Efimov Effect, BCS-BEC Crossover, and Beyond
When particles interact by a short-range potential with large scattering length, the low-energy physics becomes universal, i.e., independent of the interaction potential. Ultracold atoms are ideal to study such universal quantum physics because the atom-atom interaction, dimensionality of space, and quantum statistics of particles can be controlled at will. Because of the universality, insights obtained by ultracold atoms are valuable across many subfields in physics. In this talk, I will take these advantages of ultracold atoms to deepen and broaden our perspectives on the universal quantum physics. In particular, I will introduce an idea of “mixed dimensions,” where different atomic species live in different spatial dimensions, and show that such new systems can be used to study a very rich variety of few-body and many-body physics and discuss the current experimental status and future prospects. This study significantly expands the scope of universal quantum physics and hopefully opens up a new research area in ultracold atoms.

Feb/22/2011
4:00 p.m., 700 Clark Hall
Lu Bai, Rockefeller University
Single-Cell Biophysical Study on Nucleosome-Depleted Region and Gene Regulation
Precise gene regulation is crucial for cell proliferation and differentiation, yet the genomic features that control transcription noise are poorly understood. In eukaryotic cells, nucleosome positioning on promoters plays an important role in transcription. Recent genome-wide nucleosome mapping revealed that >90% of yeast promoters, as well as many promoters in higher eukaryotes, contain nucleosome-depleted-regions (NDRs). I hypothesized that NDRs in promoters strongly affect transcription levels and noise, and my research has focused on the formation mechanism and functional consequence of NDR on a cell-cycle regulated yeast promoter, CLN2. I discovered two sets of DNA-binding factors in the CLN2pr NDR: activators and Nucleosome-depleting factors (NDFs), with the latter responsible for the NDR formation. Disruption of the NDFs, or spatial separation of the two sets of factors, brings nucleosome(s) onto the activator binding sites. By assaying transcription in single cells using time-lapse fluorescence microscopy, I found that the promoters with nucleosome-covered activator binding sites are bimodal: strong activation in some cell cycles and none in the others. In contrast, the same binding sites localized in NDR lead to reliable activation once-per-cycle. My results strongly support the role of the NDR in suppressing gene expression noise and provide insight for how to build a functional promoter.

Feb/15/2011
4:00 p.m., 700 Clark Hall
Murat Acar, California Institute of Technology
Network-Dosage Compensation in Gene Circuits
The number of copies of a gene network in a cell, or network dosage, has a direct effect on cellular phenotypes. Network dosage is altered in situations such as the switching of some organisms between haploid and diploid life forms, doubling of chromosomes during cell cycle, genome-wide duplication of genetic content, and global variation in gene expression. Coping with variations in network dosage is crucial for maintaining optimal function in gene networks. We explored how network structure facilitates network-level dosage compensation. By using the yeast galactose network as a model, we combinatorially deleted one of the two copies of its four regulatory genes and found that network activity was robust to the change in network dosage. A mathematical analysis revealed that a two-component genetic circuit with elements of opposite regulatory activity (activator and inhibitor) constitutes a minimal requirement for network-dosage invariance. Specific interaction topologies and a one-to-one interaction stoichiometry between the activating and inhibiting agents were additional essential elements facilitating dosage invariance. This mechanism of network-dosage invariance could represent a general design for gene network structure in cells.

Feb/08/2011
4:00 p.m., 700 Clark Hall
Steven Olmschenk, National Institute of Standards & Technology and the University of Maryland
Quantum Information with Atoms and Light
Rapid progress in coherent control of single trapped atomic ions and ensembles of ultracold atoms in optical lattices has enabled more precise atomic physics measurements, a probe of fundamental quantum physics, and progress towards quantum information processing. Here I present recent results on approaches to quantum information with each of these systems. Using ytterbium ions coupled to single photons, we demonstrate a quantum teleportation protocol between two distant matter qubits with a measured fidelity of 90(2)%. With atoms in an optical lattice, we perform randomized benchmarking of single qubit operations, measuring an average error per gate of 1.4(1) x 10^-4. A method to extend the coherence time by cancellation of up to 95(2)% of the differential light shift in the ground state of rubidium is also implemented. Finally, I discuss how these gate operations might be extended for more advanced applications.

Feb/01/2011
4:00 p.m., 700 Clark Hall
Lisa Manning, Princeton University
How Does Surface Tension Emerge From Structure in Biological Tissues?

Biological tissues often behave like elastic solidson short time scales and fluids on long time scales. Different tissue typesexhibit different characteristic macroscopic mechanical properties such as surfacetension and viscosity, and cell rearrangements in developing animal tissuesclosely resemble the behavior of immiscible liquids governed by their surfacetensions. But individual cells are not equivalent to molecules in afluid; cells resist shape changes and modulate adhesive contacts with neighborsin tightly packed, disordered structures. I will discuss a minimal model,based on feedback between mechanical energy and cellular structure, that successfullyexplains past experimental data and makes novel predictions about the shapes ofcells at the tissue surface, which we verify in zebrafish embryonic tissues. This model specifies how the collective property of surface tensionemerges from properties of individual cells such as cell-cell adhesion and”cortical tension.” I will discuss the implications of this model fortissue organization, and highlight open questions about the relationshipbetween local structure and cell rearrangements in these disordered, active materials.I will also briefly discuss related work which provides insight into the basic physics of flowin disordered non-biological materials.

Nov/30/2010
4:00 p.m., 700 Clark Hall
Jane Wang, Cornell University
Physics of Living Matter: Insect Flight and Related Problems
This talk will be on two themes. The first is about mechanistic understanding of living organisms that have a mind of their own. In particular, I will discuss how insects fly and turn. The second is about interactions among multiple objects: collective behavior of particles through hydrodynamic interactions, and how insects chase one another.

Nov/23/2010
4:00 p.m., 700 Clark Hall
Jeevak Parpia, Cornell University
A Pre-Thanksgiving Diet: Add Dirt to 3He, Squeeze, and Measure
I describe an ongoing experiment on ³He in uniaxially compressedaerogel. Two aspects will be discussed. First I’ll describe results from thenormal state where we link the mean free path inferred from the dissipation andmass coupled in a torsion pendulum to the impurity limited mean free path. Thismean free path is related to the scattering length in a dirty superconductor.As in the case of an s-wave superconductor where magnetic impuritiessuppress Tc, in ³He, impuritiesalso suppress Tc. Next in the superfluid state I will discuss the changes inthe phase diagram of superfluid ³He brought on by impurity scattering and the effects of introducing auniaxial compression. The naïve expectation that compression would introduce apreferred orientation of the l vector is not observed in ourexperiment. However, the metastable A phase is enhanced significantly.

Experiments carried out by Rob Bennett and NikZhelev, Eric Smith, Andrew Fefferman.

Nov/19/2010
1:15 p.m., 700 Clark Hall
Tony F. Heinz, Columbia University
Seeing Electrons in Two-Dimensions: Optical Spectroscopy of Graphene
Optical spectroscopy provides an excellent complement totransport measurements as a means of understanding the distinctive propertiesof electrons in the two-dimensional system of graphene. Within the simplest picture, one has a(zero-gap) semiconductor with direct transitions between the well-known conicalbands. This picture gives rise toa predicted absorption that is frequency independent and has a strength for amonolayer of πα = 2.3%, where α is the fine-structureconstant. We will demonstrate thatthis relation is indeed satisfied in an appropriate spectral range in the nearinfrared, but that at higher photon energies electron-hole interactionssignificantly modify this result through the formation of a saddle-point exciton. In addition to providing a windowinto the observation of such carrier-carrier interactions, optical spectroscopypermits a detailed analysis of how the linear bands of graphene, correspondingto massless Dirac Fermions, are modified to yield massive electrons throughinterlayer interactions in bilayer graphene sheets. The observation of a tunable band gap in bilayer graphenewill also be discussed, as will be the evolution of the behavior towards thatof bulk 3-D graphite.

Nov/18/2010
3:00 p.m., 701 Clark Hall
Max Metlitski, Harvard University
The Ising-Nematic Transition in Two-Dimensional Metals
Strongly Correlated Informal Thursday Seminar Series

In recent years a lot of interest has been raised by quantum phase transitions involving a smooth disappearance of a Fermi-surface. As one approaches such critical points, the Landau-quasiparticle weight and velocity tend to zero, nevertheless, the Fermi surface remains sharply defined. One proposed example of such a transition is the development of Ising-nematic order in a metal. This order, associated with electronic correlations, which spontaneously break the square lattice symmetry to that of a rectangular lattice, has been observed in the enigmatic normal state of the cuprate superconductors by a number of recent experiments.

Motivated by these findings, I will present the scaling theory of the Ising-nematic transition in a two-dimensional metal. The critical point is described by an infinite set of 2+1 dimensional local field theories, labeled by points on the Fermi surface. Scaling forms for the response functions are proposed, and supported by computations up to three loops. Our results extend also to the theory of a Fermi surface coupled to a U(1) gauge field, which describes a number of spin and charge-liquid states.

Nov/11/2010
1:30 p.m., 701 Clark Hall
Edan Lerner, Weizmann Institute of Science
Elasto-Plasticity of Athermal Amorphous Solids
When amorphous solids aresubjected to external strains, they tend to reach a stationary non-equilibriumsteady flow state after an initial transient of mostly elastic response. Atvanishingly low temperatures and strain rates, the steady flow state consistsof largely correlated plastic events in the form of avalanches, characterizedby subextensive statistics. In my talk I will give a thorough description ofthe steady state statistics, studied by the means of computer simulations ofmodel glass formers. I will then discuss the atomistic theory of athermalelastic constants, and derive an equation of motion describing the mechanicalresponse at the onset of mechanical instabilities. I will show how thisequation determines the scaling exponents measured in the steady flow state.Finally, I will explain how the results presented extend to finite strain ratesby introducing a length scale which emerges naturally from the observed scalinglaws. I will exemplify the role played by the emerging length scale in thedecorrelation of plastic flow events.

Nov/09/2010
4:00 p.m., 700 Clark Hall
Clare Yu, University of California, Irvine
Glasses, Stress, and Attenuation
Awide variety of amorphous materials exhibit similar behavior in their thermalproperties. Examples include universal features in the specific heat, thermalconductivity, and ultrasonic attenuation. Recent experiments from the Parpiagroup find that high stress silicon nitride thin film resonators exhibit aremarkably high Q factor, exceeding that of amorphous SiO₂ by 2 to 3orders of magnitude over a broad range of temperatures, and even exceeding thatof single crystal silicon at room temperature. We present a model of why thestress reduces the attenuation. The basic assumption is that high stressincreases the potential barriers of the excitations of defects that produce theloss, thus reducing the effective density of lossy fluctuators. We discuss implications for thespecific heat and thermal conductivity.

Nov/02/2010
4:00 p.m., 700 Clark Hall
Peter Johnson, Brookhaven National Laboratory
The Pseudogap Phase of the Cuprate Superconductors; Are We Getting Closer?
TheCuprate Superconductors continue to present some of the biggest challenges forcondensed matter physics. In the under-dopedor pseudo-gap phase of these materials, a significant portion of the Fermisurface is still gapped at temperatures above the transition temperature T_c.Further instead of a closed Fermi surface, photoemission studies indicate that thelow-energy electronic excitations appear to form unconnected Fermi arcsseparated by the gapped regions. Here we report high-resolution photoemissionstudies of this underdoped regime. New methods of analysis point to the possibility that the Fermi arcs mayin fact be one side of Fermi pockets, consistent with the underlying nature ofthe spin liquids in these materials. By examining a range of reduced doping levels down into the non-superconductingregime, below 5% doping level, it appears that the areas of the hole pocketsscale with the doping level. Aparticle-hole asymmetry observed in the nodal region is clear evidence that electronpairing does not originate from the Fermi arcs in the normal state. However incontrast the particle-hole symmetry observed in the anti-nodal region isinterpreted as evidence for singlet pairs forming along the copper-oxygen bonddirections at temperatures above the superconducting transition temperature T_c.

Oct/26/2010
4:00 p.m., 700 Clark Hall
Masudul Haque, Max Planck Institute for the Physics of Complex Systems
Interaction Induced Hierarchy of Edge-Locking Effects
In 1D lattices with open boundaries, I will present a ‘fractal’ structure high up in the energy spectrum, and associated out-of-equilibrium consequences.

The non-equilibrium consequences are a hierarchy of ‘edge-locking’ effects.

I will show versions of the phenomenon for three classic condensed-matter models: (1) the Bose-Hubbard model; (2) the spinless fermion model with nearest-neighbor repulsion; (3) the XXZ spin chain.

Oct/19/2010
4:00 p.m., 700 Clark Hall
Ivan Bozovic, Brookhaven National Laboratory
Unscrambling the Physics of High-Temperature Superconductivity by Atomic-Layer Engineering

Using a unique molecular beamepitaxy system we synthesize digitally — one atomic layer at a time — thinfilms, multilayers and superlattices of cuprates and other complex oxides. Theconstituent layers can be just one-unit-cell thick, and the interfacesatomically perfect. Various heterostructures are designed to enable novelexperiments that probe the basic physics of high-temperature superconductivity(HTS).

In this talk, I will review ourrecent experiments on such films and superlattices, some of which involvedadvanced research tools such as synchrotron-based Resonant soft X-rayscattering and COBRA surface crystallography, Angle-resolved time-of-flight ionscattering, Ultrafast electron diffraction, Low-energy muon spin resonance, THzspectroscopy, etc. The results answer some key questions in HTS physics — aboutthe dimensionality, relevant interactions, the roles of (in)homogeneity andfluctuations — as follows.

(i) In a single CuO₂ plane withoutdoping, quantum spin liquid forms.

(ii) A single CuO₂ plane doped with holescan sustain HTS, even with enhanced T_c.

(iii) Some HTS cuprate samples are quite homogeneous(have a very sharp and uniform SC gap, etc.)

(iv) HTS and anti-ferromagnetic phases separate on Ångstromscale and are not degenerate in energy.

(v) Pseudo-gap state mixes with the SC state over alarge length scale (“Giant Proximity Effect”).

(vi) In-planecharge excitations are strongly coupled to out-of-planelattice vibrations.

(vii) Strong phase fluctuations drive the SCtransition, but do not persist more than 10-15 K above T_c.

[1] A. Suter et al., submitted for publication.

[2] I.Bozovic et al., PRL 89, 107001(2002); A. Gozar et al., Nature 455, 782 (2008); S. Smadici et al.,PRL (2009) 102, 107004 (2009), G. Logvenov et al.,Science 326, 699 (2009), V. Butko et al.,Adv. Mater. 21, 1 (2009), H.Zhou et al., PNAS 107, 8103(2010).

[3]P. Abbamonte et al., Science 297, 581 (2002); H.Shim et al., PRL 101, 247004 (2008),A.Gozar et al., unpublished.

[4]I. Bozovicet al., Nature 422, 873 (2003).

[5] I. Bozovic et al., PRL 93,157002 (2004); E. Morenzoni et al., submitted.

[6] N. Gedik et al., Science 316, 425 (2007); Z. Radovic et al., PRB 77, 092508(2008); J. Demsar et al., submitted.

[7] I. Sochnikov et al., Nature Nanotech.5, 516 (2010); L. Bilbro et al.,submitted; A. Bollinger et al., unpublished.

Oct/05/2010
4:00 p.m., 700 Clark Hall
Sharon Glotzer, University of Michigan
From Aristotle to Onsager and Beyond: Packing and Assembling Tetrahedra
The packing of shapes has interested humankind for millennia. We investigate the packing of hard, regular tetrahedra using computer simulation and show that entropy alone can order them into unique structures both simple and unexpectedly complex, including a dodecagonal quasicrystal. These structures can be compressed to packing densities much greater than that of spheres, and as high as 85.63%, the current world record. We Investigate the stability and robustness of the quasicrystal phase, and explore adding interactions to nanoscale tetrahedra, which promotes their assembly into sheets, wires, helices, and other complex structures.

Sep/28/2010
4:00 p.m., 700 Clark Hall
Louis Taillefer, University of Sherbrooke
Fermi Surface Reconstruction and Quantum Criticality in Cuprate Superconductors
I will show how measurements of theresistivity, Hall, Seebeck and Nernst coefficients in various hole-dopedcuprates shed light on the nature of the pseudogap phase [1-6]. Inside thisphase, they elucidate the mechanism that causes the Fermi surface toreconstruct. At the quantum critical point where the phase ends, the resistivityis found to be linear in temperature as T→ 0. A correlation between linearresistivity and T_c pointsto an intimate link between scattering and pairing [7].

I then compare the properties ofcuprates to those of quasi-1D organic superconductors [8], wherespin-density-wave order and fluctuations are at the heart of Fermi surfacereconstruction, quantum criticality and pairing. The strong similaritiessuggest that spin-density-wave order and fluctuations also play a fundamentalrole in the cuprates.

[1] N. Doiron-Leyraud et al., Nature 447, 565 (2007).

[2] D. LeBoeuf et al., Nature 450, 533 (2007).

[3] R. Daou et al., Nature Physics 5, 31 (2009).

[4] O. Cyr-Choinière et al., Nature 458, 743 (2009).

[5] R. Daou et al., Nature 463, 519 (2010).

[6] J. Chang et al., PRL 104, 057005 (2010).

[7] L. Taillefer, arXiv:1003.2972.

[8] N. Doiron-Leyraud et al., PRB 80, 214531 (2009).

Sep/21/2010
4:00 p.m., 700 Clark Hall
Jerry Gollub, Haverford College and University of Pennsylvania
Remarkable Phenomena in Low Reynolds Number Flow
In this talk I willreport on surprising physics that happens in the domain of low Reynolds numberflows, where speeds and spatial scales are small, or fluids are viscous. (a) The flow of suspensions of soft colloidal particles near the jammingconcentration reveals non-Newtonian flow and a phase transition with criticalbehavior. We also demonstrate a novel method by which the elasticproperties of individual micron-sized particles can be measured. (b) Theflows induced around swimming algal cells doing what amounts to a “breaststroke” have been measured for the first time on a scale of 3 microns,which allows us to determine the time-dependent power the cells dissipate asthey swim. We show that the oscillatory nature of their swimming causes themto waste a large part of the power they expend in locomotion.

Sep/14/2010
4:00 p.m., 700 Clark Hall
Peter Abbamonte, University of Illinois, Urbana–Champaign
The Effective Fine Structure Constant of Graphene
Electrons in graphenebehave like Dirac Fermions, permitting phenomena familiar from high energyphysics to be studied in a solid state setting. A key question is whetherthese Fermions behave as if they are noninteracting, or if they are criticallyinfluenced by Coulomb correlations. In this talk I will describeinelastic x-ray scattering experiments on single crystals of graphite which,when coupled with advanced reconstruction algorithms, allowed us to dynamicallyimage screening processes in a (freestanding) graphene sheet. We foundthat the polarizability of graphene is larger than theoretically predicted, bya factor of 3.5, because of excitonic effects in the pi band. Ourmeasurements suggest that the strength of correlations depends on the scale onwhich the system is probed, and is measured by an effective fine structureconstant, alpha(k,omega), which tends toward the value 1/7 in the limit of zerofrequency and small k. I will discuss the implications of this result forvarious phenomena, particularly the response of graphene to charged impurities.

Sep/07/2010
4:00 p.m., 700 Clark Hall
Helmut Schiessel, Leiden University
Chromatin: A Multi-Scale Jigsaw Puzzle in Biophysics

Chromatin, the DNA-proteincomplex that fills the nuclei of our cells, is a giganticthree-dimensional jigsaw puzzle. Propeller-twisted basepair plates stackon top of each other to form the DNA double-helix. The helix wraps aroundprotein cylinders resulting in a string of wedge-shaped DNA-spools,so-called nucleosomes. The nucleosomes pack into chromatin fibers thatfold into higher-order structures up to the 46 chromosomal territoriesthat fill the entire nucleus.

Recent experimental progresshas opened the door for developing theoretical models for chromatin, oneof the most studied and yet one of the least understood structures inmolecular biology — right at the heart of the cell. In this talk Idiscuss some of our theoretical attempts to “put the pieces together”:twisting DNA into plectonemes, unwrapping DNA from protein cylinders andstacking nucleosomes into helical fibers.

Aug/31/2010
4:00 p.m., 700 Clark Hall
David Huse, Princeton University
Many-body Anderson Localization, Heat Baths, and the Breakdown of Equilibrium Quantum Statistical Mechanics
An isolatedstrongly-interacting system of many spins, qubits, or particles with staticdisorder may be many-body localized at high temperatures. In that case it failsto relax to thermal equilibrium, even in the long-time limit. There is aquantum phase transition at nonzero temperature between the “ergodic”phase, where the system does thermalize and serves as its own heat bath, andthe localized (or glass) phase, where equilibrium quantum statistical mechanicsno longer captures the long-time averages and the thermal conductivityvanishes.

May/21/2010

Kostya Trachenko, Queen Mary University of London
Understanding Liquids and Glass Transition on the Basis of Elastic Interactions
Existing textbook expressions for the energyand heat capacity of gases and solids are widely taught in physicscourses.However, no such expression exists for a liquid. The reason for this wassummarized by Landau as “liquids have no small parameter”, anddiscussed in detail in Landau&Lifshitz Statistical Physicstextbook. Basedon the old idea of J Frenkel, I formulate the problem in the language ofphonons, and calculate liquid energy and heat capacity for bothclassical andquantum cases. The resulting equation relates liquid heat capacity toits viscositywith no fitting parameters, and is compared with the experimental dataofmercury.

I subsequently address the old and verycontroversial problem of glass transition. The first part of theproblem, the “thermodynamic”problem, is to explain the jump of heat capacity at the glass transitiontemperatureTg without asserting the existence of a distinct solid glass phase. Thisproblem is also common to other disordered systems, including spinglasses. Ipropose that if Tg is defined as the temperature at which the liquidstopsrelaxing at the experimental time scale, the jump of heat capacity at Tgfollows as a necessary consequence due to the change of system’selastic,vibrational and thermal properties. In this picture, I discusstime-dependenteffects of glass transition, including the widely observed logarithmicincreaseof Tg with the quench rate.

The second part of the glass transition problem,the “dynamic problem”, is to explain the number of anomalous relaxationlawsabove Tg, including the Vogel-Fulcher-Tammann law, slowstretched-exponential relaxationand dynamic crossovers. I propose a solution that is based on elasticwaves ina liquid. Central to this discussion is the range of propagation ofhigh-frequency elastic waves in a liquid, which I call “liquidelasticitylength d”. d measures the range over which local relaxation events in aliquid elastically interact with each other via the elastic waves theyinduce.The non-trivial point is that d increases with liquid relaxation timetau (orviscosity), contrary to the usual decrease of d with viscosity forcommonlydiscussed hydrodynamic waves. Hence, d is small at high temperature butincreases on lowering the temperature. This sets the cooperativity ofmolecularrelaxation in a liquid and explains a number of dynamic relaxation laws.

May/04/2010
4:30 p.m., 700 Clark Hall
Alexei Kitaev, California Institute of Technology
Mathematical Classification of Gapped Free-Fermion Hamiltonians

I describe a general classification scheme, which is applicabletoIQHE systems, topological insulators, and topological superconductors.The symmetryand spatial dimension determines a general universality class, whichcorrespondsto one of the 2 types of complex and 8 types of real Clifford algebras.Thephases within a given class are further characterized by a topologicalinvariant, an element of some Abelian group that can be 0, Z, orZ₂.The interface between two infinite phases with different topologicalnumbersmust carry some gapless mode.

Apr/27/2010
4:30 p.m., 700 Clark Hall
Elbio Dagotto, Oak Ridge National Laboratory
Studying Models for Strongly Correlated Electronic Systems Using Computational Techniques

The field of Strongly Correlated Electrons (SCE) includesimportant families of materials such as those exhibiting high temperaturesuperconductivity and colossal magnetoresistance (CMR). A model Hamiltonianapproach is needed to address, at least qualitatively, the physics of thesecompounds. However, solving models for SCE is difficult becausenon-perturbative techniques are needed. Our group studies models for SCE usinga variety of computational techniques and in this presentation a summary of ourrecent efforts will be provided. As a typical example of a case where competingstates lead to interesting effects, first the CMR effect will be shown toappear in Monte Carlo simulations of double exchange models coupled to latticedistortions. Manganites can also present multiferroic characteristics and theyare often part ofinteresting oxide hetero-structures. Computational results for both cases willalso be presented. After this overview of Mn oxides, some recent Monte Carlo,mean field, and exact diagonalization results for the high-Tc superconductorswill be discussed. In particular, the importance of the difficult intermediateHubbard coupling regime to rationalize some of the properties of the pnictideswill be addressed.

Work supported by NSF and DOE. More information about ourgroup can be found in http://sces.phys.utk.edu/

Apr/20/2010
4:30 p.m., 700 Clark Hall
Jon Machta, University of Massachusetts
Physics and Phase Transitions in Parallel Computational Complexity

In this talk I will discuss applications ofcomputational complexity theory to statistical physics and vice versa. Althoughwork at the interface between physics and computational complexity has centeredlargely on NP-hard problems, there are also many interesting questions to beaddressed within complexity class P of tractable problems. These questions arebrought into relief by considering problems from the point of parallelcomputation. The talk will begin with a review of parallel computationalcomplexity: the PRAM and Boolean circuit family models of computation, theclasses P and NC and the notion of P-completeness. I will then discussseveral statistical physics models from this perspective. The last partof the talk will be about phase transitions in the Circuit Value Problem—acanonical P-complete problem.

Apr/13/2010
4:30 p.m., 700 Clark Hall
Richard Scalettar, University of California, Davis
The Hubbard Model: From Condensed Matter to Atomic Physics

The Hubbard Hamiltonian is extensively studiedand applied to important problems in condensed matter physics. It purports toexplain the qualitative origin of magnetism and insulating behavior intransition metal oxides, and even superconductivity and density inhomogeneitiesin high temperature superconductors. Quantum Monte Carlo (QMC)simulations provide an exact treatment of such interacting electron models on latticesof up to 1000 sites or so. In this seminar I will first present anoverview of our QMC results on the spin, density, and pairing correlations inthe two dimensional Hubbard Hamiltonian. I will then turn to a discussionof the use of this model in the understanding of ultracold, optically trappedatoms. Do these systems really provide more faithful experimentalrealizations of the Hubbard Hamiltonian?

Apr/06/2010
4:30 p.m., 700 Clark Hall
Susanne Yelin, University of Connecticut
Nonlinear Optical Phenomena in Strongly Interacting Media
Polar moleculeshave commanded a lot of interest lately, now existing at ultra-coldtemperatures and being hailed as the next best option for quantum informationand simulation. I will, in particular, discuss their use for single-photonnonlinear optics, thus using their interaction properties. The sameinteraction, dipole-dipole interaction, also lies at the heart of superradiantphenomena. Applications of superradiance include diverse systems as polarmolecules, Rydberg atoms, and solid state materials.

Mar/30/2010
4:30 p.m., 700 Clark Hall
Boris Svistunov, University of Massachusetts
Supersolidity of Helium-4

A supersolid is a solid that can conduct itsown atoms without friction.

The supersolidity of He-4 is one of the biggestpuzzles in the modern low-temperature physics, and a subject of intensiveexperimental and theoretical studies during the last 6 years, followingKim and Chan’s discovery that solid He-4 decouples from a torsionaloscillator. First- principles numeric simulations exclude supersolidity of aperfect He-4 crystal, while suggesting a number of scenarios ofdisorder-induced supersolidity. In particular, simulations revealsuperfluidity in the cores of dislocations, shedding considerable light on theeffect of giant isochoric compressibility recently observed by Hallock and Ray inthe experiment on DC supertransport in solid He-4. Despite impressiveexperimental and theoretical progress of recent years, the field still remainshighly controversial.

Mar/09/2010
4:30 p.m., 700 Clark Hall
Robert Willett, Lucent Technology
Alternating e/4 and e/2 Period Interference Oscillations as Evidence for Filling Factor 5/2 Non-Abelian Quasiparticles

It is atheoretical conjecture that 5/2 fractional quantum Hall state charge e/4excitations may obey exotic non-Abelian statistics. In edge stateinterference these purported non-Abelian quasiparticles should display periode/4 Aharonov-Bohm oscillations if the interfering quasiparticle encircles aneven number of localized e/4 charges, but suppression of oscillations if an oddnumber is encircled. To test this hypothesis, here we perform swept areainterference measurements at 5/2. We observe an alternating pattern ofe/4 and e/2 period oscillations in resistance. This aperiodic alternation isconsistent with proposed non-Abelian properties: the e/4 oscillations occur forencircling an even number of localized quasiparticles, e/2 oscillations areexpressed when encircling an odd number. Aperiodic alternation corresponds tothe expected area sweep sampling the localized quasiparticles. Importantly, adding localized quasiparticles to the encircled area bychanging magnetic field induces interchange of the e/4 and e/2 oscillationperiods, specifically consistent with non-Abelian e/4 quasiparticles.

Mar/02/2010
4:30 p.m., 700 Clark Hall
Stephen B. Cronin, University of Southern California
Unique One- and Two-Dimensional Phenomena Observed in Carbon Nanotubes and Graphene

Our ability to fabricate nearly defect-free, suspendedcarbon nanotubes (CNTs) has enabled us to observe several phenomena never seenbefore in CNTs, including breakdown of the Born-Oppenheimer approximation [1], mode selective electron-phonon coupling [2], leading to negativedifferential resistance (NDR) and non-equilibrium phonon populations, and aMott insulator transition [3]. In this work, Ramanspectroscopy is used to measure individual, suspended CNTs under applied gateand bias potentials. Ramanspectroscopy of periodic ripple formation in suspended graphene will also bereported. As will be shown,preparing clean, defect-free devices is an essential prerequisite for studyingthe rich low-dimensional physics of CNTs and graphene.

1. Bushmaker, A.W., Deshpande. V.V., Hsieh, S.,Bockrath, M.W., and Cronin, S.B., “Direct Observation of Born-OppenheimerApproximation Breakdown in Carbon Nanotubes,” Nano Letters, 9, 607 (2009).

2. Bushmaker, A.W., Deshpande. V.V., Bockrath, M.W.,and Cronin, S.B., “Direct Observation of Mode Selective Electron-PhononCoupling in Suspended Carbon Nanotubes,” NanoLetters, 7, 3618 (2007).

3. Bushmaker,A.W., Deshpande. V.V., Hsieh, S., Bockrath, M.W., and Cronin, S.B., “LargeModulations in the Intensity of Raman-Scattered Light from Pristine Carbon Nanotubes,”Phys. Rev. Letts., 103, 067401 (2009).

Feb/23/2010
4:30 p.m., 700 Clark Hall
Mark Saffman, University of Wisconsin
Neutral Atom Quantum Gates via Rydberg Excitations

Neutral atoms in their electronicground states interact only weakly. Excitation of the atoms to high lyingRydberg levels results in a dipolar interaction that is 12 orders of magnitudelarger. I will show how we have harnessed this extremely strong interaction toimplement a quantum CNOT gate between two atoms. In addition to quantum gates the long range Rydberg interactionis well suited for generating many particle entanglement, and deterministicquantum interfaces between matter and photonic qubits. I will discuss someexamples of these quantum information applications.

Feb/16/2010
4:30 p.m., 700 Clark Hall
Andrew Mackenzie, University of St. Andrews
Thermodynamic Studies of Phase Formation in the Vicinity of Quantum Criticality in Sr3Ru2O7

The layered perovskite metal Sr₃Ru₂O₇has generated interest because of the discovery of nematic-like electricaltransport properties at low temperatures. The unusual properties are seen in the vicinity of a metamagneticquantum critical point. Theyappear to be the result of the formation of a new phase, which can be observedonly in the highest purity single crystals, with mean free paths of severalthousand angstroms. Recently, mygroup has concentrated on understanding this phase and determining itsboundaries using thermodynamic probes. In this talk I will review the physics that we believe underlies ourobservations, and then report on the recent progress, showing how measurementsof the specific heat and magneto-caloric effect enable the determination of acomplete ‘entropy landscape’ of phase formation in the vicinity of a quantumcritical point. I will alsodiscuss the discovery of de Haas-van Alphen oscillations within the putativeelectronic nematic phase.

Feb/09/2010
4:30 p.m., 700 Clark Hall
Robert Bennett, Cornell University
Studies of Superfluid 3He, Confined to a Single 0.6 µm Thick Slab, Using DC SQUID NMR

As a resultof suppression of the order parameter at a boundary the phase diagram ofsuperfluid ³He in regular confined geometries, of length scale onthe order of the coherence length ξ,is predicted to differ significantly from that of the bulk liquid, and suchgeometries may also give rise to previously unobserved order parameters.Motivated by the prediction^[1]of an inhomogeneous ‘stripe’ phase occurring in thin slabs, we have studied a d=0.6 µm thick slab of superfluid ³Hecontained inside a nanofabricated silicon and glass cell. This cryogenicmicrofluidic chamber provides a well-defined environment for the superfluid inwhich the sample geometry and boundary conditions may be well characterized.The cell walls are 3 mm thick to allow pressure tuning of the effectiveconfinement, d/ξ. We use NMR to ‘fingerprint’ the superfluid order parameter. Toenable us to obtain high-quality signals from such a small sample we havedeveloped a spectrometer,^[1]incorporating a two-stage dc SQUID amplifier, with unprecedented sensitivity.In this talk I will describe the cell fabrication and characterization, the NMRspectrometer and present recent experimental results.

1. A.B. Vorontsov and J. Sauls, Phys. Rev. Lett., 98,045301 (2007).

2. L.V. Levitin etal., Appl. Phys. Lett., 91,262507 (2007)

Feb/02/2010
4:30 p.m., 700 Clark Hall
Matthew Fisher, California Institute of Technology
Spin Bose-Metals in Weak Mott Insulators
The existence of two-dimensional (2d) Mott insulators which exhibit no symmetry breaking yet
gapless excitations is suggested by both experiment and theory. Such putative “critical spin liquids”
will possess power law spin correlations which can oscillate at various wavevectors. In a sub-class
dubbed “Spin Bose-Metals” the singularities reside along surfaces in momentum space, analogous
to a Fermi surface but without long-lived quasiparticle excitations. I will recap the evidence for
such a 2d spin liquid in a class of organic materials, and describe recent theoretical progress in
accessing such states via controlled numerical and analytical studies on quasi-1d model systems.

Jan/26/2010
4:30 p.m., 700 Clark Hall
Antonio Badolato, University of Rochester
Cavity Quantum Electrodynamics with Single Quantum Dots

Single atomscoupled to single radiation field modes of a cavity represent the elementalstructures in Cavity Quantum Electrodynamics (CQED). Landmark experiments onatoms in cavities have revealed fundamental aspects of coherence in quantumsystems and made CQED a central paradigm for the study of open quantum systems.The lack of suitable artificial atoms and cavity technology hindered for a longtime the implementation of CQED in solid state. However, constant progresses incrystal growth and nanofabrication have been changing the scenariosignificantly.

In this talk Iwill show that semiconductor systems can access CQED effects, offering anunprecedented level of control and engineering of light-matter coherentinteraction. I will focus on three aspects: (i) The effective atomic behaviorof electrons spatially confined in InAs/GaAs semiconductor self assembledquantum dots (QDs). (ii) The spatial positioning and spectral tuning betweensingle QDs and photonic crystals microcavity (PCM) modes. (iii) Thedemonstration of a deterministic coupling between single QDs and single PCMmodes [1]. I will illustrate the quantum nature of the reached strong-couplingregime [2] and how the fine structure of the QD is revealed in this regime [3].

[1] A. Badolato,K. Hennessy et al., Science 308, 1158 (2005).
[2] K. Hennessy,A. Badolato et al., Nature 445, 896 (2007).
[3] M. Winger, A.Badolato et al., Phys. Rev. Lett. 101, 226808 (2008); M. Winger, A.Badolato, et al., Phys. Rev.Lett. 103, 207403 (2009)

Nov/17/2009
4:30 p.m. 700 Clark Hall
John Tranquada, Brookhaven National Laboratory
Striped Superconductors

High temperature superconductivity occurs in closeproximity to antiferromagnetism in both the long-studied copper oxides and thenewer iron-based superconductors. Magnetic excitations are commonly believed toplay an important role in the electron pairing necessary for thesuperconducting state, but there are divergent ideas on how they are involved. I will discuss our studies of oneparticular system, La_2-xBa_xCuO₄, whichexhibits coexistence of locally antiferromagnetic order and an unusual superconductingstate. Focusing on the x=1/8composition, neutron and x-ray diffraction studies show that charge and spinstripe order develop below 53~K and 40~K, respectively [1,2]. The usual bulk superconductivity isstrongly suppressed, with a transition temperature T_c ~ 5 K;nevertheless, susceptibility and resistivity measurements provide evidence forthe onset of two-dimensional superconducting correlations at 40 K, togetherwith the spin stripe order [2,3]. To explain the electronic decoupling of the CuO₂ layers, aform of striped superconductivity, also called a pair density wave, has beenproposed [4,5]. I will discuss thepotential relevance of these results to other cuprates.

[1] M. Fujita et al.,Phys. Rev. B 70, 104517 (2004).

[2] J. M. Tranquada etal., Phys. Rev. B 78, 174529(2008).

[3] Q. Li et al.,Phys. Rev. Lett. 99, 067001 (2007).

[4] E. Berg et al.,Phys. Rev. Lett. 99, 127003 (2007).

[5] E. Berg et al.,arXiv:0901.4826.

Nov/10/2009
4:30 p.m. 700 Clark Hall
Raffi Budakian, University of Illinois at Urbana-Champaign
Observation of Fractional Fluxoid States in Mesoscopic Rings of Sr2RuO4 by Ultrasensitive Cantilever Magnetometry
In the past decade, there has emerged strong evidence to support spin-triplet superconductivity in the layered-perovskite Sr₂RuO₄, whose ground state is thought to be analogous to the A-phase of ³He. It is believed that the spin and orbital degrees of freedom of the superconducting order parameter can give rise to states with remarkable properties, such as chiral domains and half-quantum vortices (HQV) that may obey non-Abelian statistics. With regards to the latter, recent theoretical work suggests that the HQV state could be made energetically favorable in me-soscopic samples [1]. In this talk, I will present a new method for ultrasensitive cantilever mag-netometry that allows us to probe the magnetic response of mesoscopic samples of Sr₂RuO₄. Using this technique, we have detected the entry of individual vortices into micron-size rings of Sr₂RuO₄. Our most intriguing observation is the appearance of fractional fluxoid states that have half the magnetic moment of the full (integer) fluxoid. We find that the stability region of the fractional fluxoid state grows linearly with the magnitude of the in-plane magnetic field applied to the crystal. While the physical origin of the fractional state is yet unknown, I will present a recent theoretical proposal that predicts spontaneous spin polarization in the HQV state [2] which could explain the observed field dependence.

[1] S. B. Chung, H. Bluhm, and E. A. Kim, Phys. Rev. Lett. 99, 197002 (2007).
[2] V. Vakaryuk, and A. J. Leggett, Phys. Rev. Lett. 103, 057003 (2009).

Nov/03/2009
4:30 p.m. 700 Clark Hall
Craig Fennie, Cornell University
Controlling Ferroelectric and Magnetic Order in Complex Materials

Due to their highly tunable groundstates, structurally and chemically complex oxides are a promising class ofmaterials in which to realize new emergent phenomena that could not onlychallenge our current understanding of condensed matter but also provide realsolutions for technological advances in for example the energy sciences andelectronics. In this talk I will discuss two examples of our recent work on thetheoretical, first-principles design of and subsequent experimental discoveryof complex oxide materials rarely found in nature – ferromagnetic-ferroelectric oxides in which a spontaneousmagnetism not only coexists with but also is strongly coupled to a spontaneouselectric polarization. In one case control over the interplay of spins andoptical phonons leads to a competition between different ordered statesproducing a colossal magnetoelectric effect. In a second case, a ferroelectricdistortion can be manipulated to induce weak-ferromagnetism facilitating theelectric-field control of a switchable magnetization.

Oct/27/2009
4:30 p.m. 700 Clark Hall
Nathan Gemelke, James Franck Institute, University of Chicago
In situ Microscopy of an Atomic Mott Insulator

High-resolution microscopy ofultracold atomic gases confined in optical lattice potentials now permitshighly detailed studies of inhomogeneous mixtures of superfluid and Mottinsulating phases, and the quantum phase transition which lies between. Working with two-dimensional samples,line-of-sight integration can be avoided, and the surface density unambiguouslydetermined everywhere in the gas in a single snapshot image. This has for thefirst time allowed direct observation of the so-called ‘wedding-cake’ structureof the inhomogeneous gas. Byanalyzing images, local compressibility can be determined, allowing for anefficient extraction of the low-temperature phase diagram for comparison withthe Bose-Hubbard model; careful study of onset of the insulating phase yields adirect measure of the opening of the excitation gap. By studying many images, fluctuations can be studied in allphases, and can be compared to that expected from the fluctuation-dissipationtheorem. Combining in-situ density measurements withnear-field matter-wave interference permits a “two-quadrature” measurement, comparing thedegree of local phase coherence with local density fluctuations. Extension of these studiespromise access to quantum criticality and dynamics in a relatively clean and well-understoodenvironment.

Oct/20/2009
4:30 p.m. 700 Clark Hall
Wei Ku, Brookhaven National Laboratory
Simplifying Local Excitations in Correlated Charge-Transfer Insulators

Recenttheoretical developments to simplifying understanding of strongly bound localFrenkel excitons in “charge-transfer insulators” will bepresented. First, the descriptionof charge-transfer insulators will be unified with that of the Mott insulatorsvia the use of symmetry-respecting first-principles Wannier functions. Second, formal theoretical framework ofTD-DFT with LDA+U will be developed and applied to a case study illustratingits capability and deficiency. Third, systematic framework of strong coupling approach will be presentedto account for the multiplets beyond the typical first-principlesapproximations. Finally, atwo-particle hopping kernel is introduced to describe propagation of the localexcitations. Case studies includeanisotropy of local excitations in NiO [1], “missing” neutronspectral weight of the cuprates [2], and space-time propagation of excitons inLiF [3].

[1] B. Larsonet al., Phys. Rev. Lett 99, 026401(2007).

[2] I.Zaliznyak et al., to appear in Nature Physics (2009).

[3] P. Abbamonte et al., Proc. Natl. Acad. Sci. 105, 12159 (2008).

Oct/07/2009
4:30 p.m. 700 Clark Hall
Meredith Betterton, University of Colorado
Theory of Microtubule Depolymerization by the Kinesin-8 Kip3p
Joint Biophysics-LASSP Colloquium

Oct/01/2009
4:30 p.m. 700 Clark Hall
Austen Lamacraft, University of Virginia
Low Energy Dynamics of Spinor Condensates
Arguably the biggest conceptual novelty encountered in the study of Bose and Fermi condensates of ultracold atomic gases is the high spin of the particles involved. Indeed, prior to the “ultracold revolution” the only Bose superfluid that could be studied in the laboratory was He4, which has zero spin. With the advent of optical trapping of Bose condensates of alkali atoms, which allows for a fully rotationally invariant setting, the experimental study of spin ordering within a hyperfine multiplet came within reach.

In such systems, novel kinds of magnetic ordering are expected to be more common. For example, some ground states predicted by mean-field theory have zero magnetic moment but non-vanishing nematicity (quadrupole moment). In this talk I will describe the possible magnetic orders in spinor Bose condensates and derive the low energy dynamics on the order parameter manifold, making connections to recent experiments and the analogous problem in conventional magnetism.

Sep/22/2009
4:30 p.m. 700 Clark Hall

No Seminar

Sep/15/2009
4:30 p.m. 700 Clark Hall
Subir Sachdev, Harvard University
Where is the Quantum Critical Point in the Cuprate Superconductors?
I will review recent experiments on the cuprate superconductors in the context of a proposed phase diagram as a function of applied magnetic field, temperature, and hole/electron-doping. The theory of spin-fluctuation mediated pairing at large doping must be modified to apply in the underdoped regime. The modified theory includes the pocket structure of the Fermi surface and effects associated with the proximity of the Mott insulator. The theory predicts the competition between spin density wave order and superconductivity, and I discuss how its quantum criticality is connected with the global cuprate phase diagram.

Sep/08/2009
4:30 p.m. 700 Clark Hall
Garnet Chan, Cornell University
Thinking about wavefunctions

Sep/01/2009
4:30 p.m., 700 Clark Hall
Mark Bowick, Syracuse University
Defects, Drops and Structured Vesicles
I will discuss the ordering of various states of matter on curved two-dimensional manifolds and the effect of spatial curvature on the basic structure and distribution of elementary topological defects in such systems. Spatial curvature can drive the delocalization of disclination (curvature) defects, the unbinding of dislocation defects and the fractionalization of interstitial and vacancy defects. I will also discuss the use of block copolymers to make vesicles with internal smectic and nematic order.

Jul/16/2009
12:30 p.m., 609 Clark Hall
Mo Hamidian and Aryeh Warmflash
LASSP Theory Pizza Talks
LASSP Theory Pizza Talks

Mo Hamidian
“Probing the Nature of Heavy Fermions and Hidden Order: Scanning, Tunneling Spectroscopic Studies of URu₂Si₂”

Aryeh Warmflash
“How To Make a Frog: Early Patterning and Developmental Pathways in Xenopus”

Jul/09/2009
12:30 p.m., 609 Clark Hall
Elliot Kapit and Kaden Hazzard
LASSP Theory Pizza Talks
Elliot Kapit, Cornell University
“Superconductivity in a Strongly Interacting, 2-d Hubbard-like Model”

Kaden Hazzard, Cornell University
“Realizing Fractional Quantum Hall States in Cold Atoms”

Jun/26/2009
2:30 p.m., 701 Clark Hall
Professor Vladimir E. Fortov, Russian Academy of Sciences
Intense Shock Waves and Extreme States of Plasmas
The physical properties of strongly coupled plasma at high energy densities are analyzed in a broad region of the phase diagram. The theoretical and experimental methods of hot dense plasma investigations are also discussed. Major attention is paid to shock wave methods. Intense shock, rarefaction and radiative waves in gaseous, solid and porous samples, electrical explosion and bulk electron and ion heating were used for generation of extremely high temperatures and high pressures. Highly time-resolved diagnostics allow us to measure the thermodynamical, radiative and mechanical properties of strongly coupled plasmas in a broad region of the phase diagram, starting from compressed condensed solid states up to the low density gas region, including high temperature evaporation curves with near-critical states of metals, and metal-insulator transition regions.

Thermodynamical parameters of metal critical points are analyzed and compared with the theoretical predictions. Shock-wave-induced non-equilibrium phenomena at fast melting and adiabatic condensation are analyzed in the framework of the interspinodal decomposition model. Theoretical interpretations of opacity measurements demonstrate strong deformation of discrete spectra in coupled plasmas. Pressure ionization phenomena in hydrogen, iodine, silica, sulfur, fullerenes, and some metals are analyzed on the basis of multiple shock compression experiments. The effect of “dielectrization” for some metal plasma (Li, Na, AlH3) are discussed on the basis of a multiple shock compression experiment.

Experiments with strongly non-ideal dusty plasmas are performed under microgravity conditions onboard International Space Station and under laboratory conditions. The physical properties of plasma “liquids” and plasma “crystals” are analyzed. Computer simulations of parameters of strongly coupled matter generated by intense shock wave sources are presented.

Jun/24/2009
12:30 p.m., 609 Clark Hall
Vinay Ambegaokar and John Shumway
LASSP Theory Pizza Talks
Vinay Ambegaokar, Cornell University
“Reliably estimating errors in Monte Carlo simulations of the Ehrenfest model”

John Shumway, Arizona State University
“Path integral Monte Carlo calculations on trapped cold atomic gases”

Apr/28/2009
4:30 p.m., 700 Clark Hall
Hae-Young Kee, University of Toronto
Microscopic Route to the Effective Interaction for the Nematic Phase in Ruthenates
Anisotropic metallic phase in the presence of magnetic fields bounded by two consecutive metamagnetic transitions was reported in ultra-clean bilayer Ruthenates. It was proposed that an effective momentum-dependent interaction leads to an anisotropic metal dubbed the electronic nematic phase which suppresses the Lifshitz transition and accompanies the metamagnetic transitions. A microscopic origin of such an effective interaction incorporating all relevant orbitals will be presented. We identify Fermi surface topology consistent with recent angle resolved photoemission spectroscopy data, and show that the dominant interaction leading to the nematic order is the intra-orbit Hubbard interaction. Experimental consequences and other competing orders will be discussed.

Apr/21/2009
4:30 p.m., 700 Clark Hall
Takashi Imai, McMaster University
Why Does Undoped FeSe Become a High-Tc Superconductor under Pressure? — 77Se NMR Study
Electrons doped into iron-arsenides destabilize the SDW (Spin Density Wave) ground state, and induce high T_c superconductivity in the FeAs layers with T_c as high as 55 K. Interestingly, electron-doping can be accomplished by alloying Fe atoms with Co atoms, which donate their extra electrons. This is in remarkable contrast with the high fidelity required by the CuO₂ layers of high T_c cuprate superconductors; in cuprates, one can’t replace Cu or O atoms with any other elements without destroying superconductivity. It turns out that even As-atoms are not essential for high T_c superconductivity in iron-based systems; the stoichiometric FeSe is not SDW ordered, but is a superconductor with modest Tc ~ 9 K [1]. Furthermore, application of high pressure raises T_c to as high as 37 K [2]. What are the differences and similarities with FeAs systems? Why does undoped FeSe become a high T_c superconductor under pressure? We will describe our microscopic investigation into the electronic properties of FeSe using ⁷⁷Se NMR spectroscopy under high pressure up to 2.2GPa [3].

[1] For an early review about the electronic properties of FeSe, see “The Iron Age of Superconductivity” by M. Johannes, Physics 1, 28 (2008).
[2] S. Medvedev, T. M. McQueen, R. J. Cava et al., cond-mat/0903.2143.
[3] T. Imai, K. Ahilan, F.L. Ning, T.M. McQueen, and R. J. Cava, Phys. Rev. Lett. (in press); cond-mat/0902.3832.

Apr/14/2009
4:30 p.m., 700 Clark Hall
Jenny Hoffman, Harvard University
Scanning Tunneling Spectroscopy and Vortex Imaging in the Iron-Pnictide High-Tc Superconductors
Last year, 22 years after the discovery of high-T_c superconductivity in the cuprates, superconductivity was discovered up to 55K in a second family of materials: the iron-pnictides. This new discovery has generated tremendous excitement for several reasons. First, there is hope that the iron-pnictides will finally provide the foil necessary to understand the enormous yet puzzling body of research on the cuprates. Second, initial reports of low anisotropy and strong vortex pinning in these new materials have spurred optimism that the iron-pnictides may finally lead to the widespread technological applications which have been elusive for cuprates. In this talk, I will summarize the current state of iron-pnictides research, before presenting our own work: the first scanning tunneling spectroscopic imaging study of a single crystal iron-pnictide superconductor in high magnetic fields. We study optimally doped BaFe_1.8Co_0.2As₂ with T_c=25.3K, finding a ∼6 meV superconducting gap with nanoscale inhomogeneity, which leads to an average reduced gap of 2Δ/kBT_c∼5.7. We further observe a static disordered vortex lattice at 9 T, and demonstrate that vortices are pinned in the bulk of this material, a promising observation for practical application.

Apr/09/2009
1:15 p.m., 609 Clark Hall
M. Zahid Hasan, Princeton University
Dirac Fermions, Topological Phases and Experimental Observation of Quantum Hall-Like Effects Without Magnetic Field
Strongly Correlated Informal Thursday Seminar Series

Most states of condensed-matter are categorized by spontaneously broken symmetries (Landau paradigm). The remarkable discovery of quantum Hall effects (1980s) revealed that there exists an organizational principle of matter based not on the broken symmetry but only on the topological distinctions in the presence of time-reversal symmetry breaking. In the past few years, theoretical developments suggest that new classes of topological states of matter might exist that are purely topological in nature in the sense that they do not break time-reversal symmetry hence can be realized without any applied magnetic field: “Quantum Hall-like effects without magnetic field.” In this presentation, I report a series of experimental results [1-3] documenting and demonstrating the existence of such a topologically ordered time-reversal-invariant state of matter and discuss the exotic electromagnetic (Wilczek’s QCD theta vacuum [4]) properties this novel phase of quantum matter might exhibit and outline their potential use.

Apr/07/2009
4:30 p.m., 700 Clark Hall
Douglas Durian, University of Pennsylvania
Granular Unsteadiness
The flow of granular materials is of widespread practical and fundamental interest. One challenge to understanding and controlling behavior is that the response is nonlinear, with a forcing threshold below which the medium is static. Furthermore, just above threshold the response may be intermittent even though the forcing is steady. Two familiar examples are avalanches on a heap and clogging in a silo. Another example is dynamical heterogeneities for systems brought close to jamming, where intermediate-time motion is correlated in the form of intermitted string-like swirls. Here this will be
illustrated with experiments on air-driven beads, where jamming is approached by lowering the effective temperature, as well as by experiments on rapid heap flow, where jamming is approached as a function of depth from the free surface. Use of novel statistical quantities and optical spectroscopies reveal a growing dynamical length scale on approach to jamming.

Mar/31/2009
4:30 p.m., 700 Clark Hall
Joel Moore, University of California, Berkeley and Lawrence Berkeley National Laboratory
Topological Insulators and Magnetoelectric Coupling in Solids
The recently discovered “topological insulators” are three-dimensional nonmagnetic materials in which spin-orbit coupling generates an insulating phase characterized by topologically protected surface states. These surface states, analogous to the edge states of the quantum Hall effect in one lower dimension, have recently been observed in BiSb alloys and Bi2Se3. We review this rapidly developing field and explain how investigation of the origin of this phase leads to a Berry-phase picture of magnetoelectric polarizability in three-dimensional solids, closely analogous to the Berry-phase theory of polarization in insulators. In closing we discuss how topological insulators, which are essentially independent-electron materials, can nevertheless be used to create new correlated phases.

Mar/26/2009
1:15 p.m., 701 Clark Hall
Mustansir Barma, Director, Tata Institute of Fundamental Research
Clustering Induced by Fluctuating Forces
We study a system of passive particles sliding under gravity on a randomly fluctuating surface. We find an interesting coarsening behavior leading to a steady state with a large degree of clustering. Two-point correlation functions exhibit scaling, but unlike normal phase-separating systems, here the scaling function is singular, a manifestation of giant fluctuations in the system. The results also pertain to passive scalars advected by a noisy Burgers flow.

Mar/12/2009
1:15 p.m., 701 Clark Hall
Vladimir Hinkov, Max Planck Institute for Solid State Research, Stuttgart
Electronic Liquid Crystals and Magnetic-field Enhanced Spin Correlations in Underdoped YBCO
Strong electron correlations and phase competition lead to the rich physics observed in underdoped cuprates. Here I will report the spontaneous onset of a one-dimensional incommensurate (IC) modulation of the low-energy (et al., Science 319, 597). Our finding provides evidence for the occurrence of an electronic liquid-crystal state breaking C4-symmetry. When reducing the temperature further to values below 30 K static spin correlations are induced, with the same IC geometry as the spin excitations. Their intensity is enhanced by a factor of two upon application of a magnetic field of 15 T (Haug et al., arXiv:0902.3335). We attribute the recently reported quantum oscillations in underdoped YBCO (Doiron-Leyraud et al., Nature 447, 565) to a Fermi surface reconstruction induced by the IC spin modulations we observe. Due to the hitherto missing experimental evidence for such IC modulations, this mechanism was not pursued in the past, although it offers a straight-forward explanation for Fermi-surface reconstruction.

Mar/10/2009
4:30 p.m., 700 Clark Hall
Anatoli Polkovnikov, Boston University
Quantum Nearly Adiabatic Dynamics in Closed Systems
In this talk I will first discuss the connection between quantum and thermodynamic adiabatic theorems and will suggest there are three generic regimes of response of closed many-body systems to slow external perturbations. I will argue that in all three regimes thermodynamic adiabatic theorem follows from the quantum adiabatic theorem. I will illustrate these regimes using several examples including sweeps through a quantum critical point. Based on this connection I will introduce microscopic expressions for the heat and diagonal entropy and will show that their behavior is consistent with the first and the second laws of thermodynamics.

Time permitting I will also briefly talk about expansion of quantum dynamics in interacting systems in quantum fluctuations near the classical limit. This expansion is based on extending the ideas of Wigner-Weyl quantization or equivalently phase space methods.

Mar/03/2009
4:30 p.m., 700 Clark Hall
Paul Fendley, University of Virginia
Topological Quantum Computation with Non-Abelian Anyons
I discuss how systems with non-abelian anyons can be used to build a topological quantum computer. Operations are performed by braiding the anyons; because the outcome of braiding is a purely topological property, such quantum computers should be robust against local errors. I will give several examples of how such anyons arise in fractional quantum Hall systems and quantum “net” models.

Feb/26/2009
1:15 p.m., 701 Clark Hall
Frank Kruger, University of Illinois, Urbana-Champaign
Fermion Minus Signs, Quantum Criticality, and the Fractal Nodal Surface
Special LASSP Seminar SCITS Series, ‘Strongly Correlated Informal Theory Seminar’

The complete lack of theoretical understanding of the quantum critical states found in the heavy fermion metals and the normal states of the high-Tc superconductors is routed in deep fundamental problem of condensed matter physics: the infamous minus signs associated with Fermi-Dirac statistics render the path integral non-probabilistic and do not allow to establish a connection with critical phenomena in classical systems. Using Ceperley’s constrained path-integral formalism we demonstrate that the workings of scale invariance and Fermi-Dirac statistics can be reconciled. The latter is self-consistently translated into a geometrical constraint structure. We prove that this “nodal hypersurface” encodes the scales of the Fermi liquid and we conjecture that it turns fractal when the system becomes quantum critical. To illustrate this we calculate nodal surfaces and electron momentum distributions of Feynman backflow wave functions and indeed find that with increasing backflow strength the quasiparticle mass gradually increases, to diverge when the nodal structure becomes fractal. Such a collapse of a Fermi liquid at a critical point has been observed in the heavy-fermion intermetallics in a spectacular fashion.

Feb/24/2009
4:30 p.m., 700 Clark Hall
Nicholas Bigelow, University of Rochester
Making and Manipulating Dipolar Quantum Gases

Feb/19/2009
1:15 p.m., 701 Clark Hall
Fiona Burnell, Princeton University
A Geometrical Perspective on Topological Lattice Models
Solvable models have contributed greatly to our understanding of topological phases of matter. We show how one such general construction—string nets—is related to the so-called chain-mail invariant studied in the context of knot theory. This link invariant has a natural interpretation as the partition function of a Hamiltonian acting on a lattice; excitations with anionic statistics can be constructed by adding labeled strings to this picture. The result in 2+1 dimensions is a geometric interpretation of string-net models; we discuss possible extensions of the theory into 3+1 dimensions.

Feb/19/2009
3:00 p.m., 701 Clark Hall
Tobias Micklitz, Argonne National Laboratory
Conductance of Fully and Partially Equilibrated Quantum Wires
We investigate the effect of equilibration on the conductanceof a quantum wire, connected to non-interacting leads. Concentrating onthe regime of weak electron-electron interaction and low temperature Twe find a correction to the quantized value of conductance. Thecorrection is temperature-dependent, scales for short wires with theirlength, and saturates for long wires to a value quadratic in T andindependent of the details of the interaction mechanism responsible forequilibration. The crossover regime occurs on a length-scale set bythree-particle scattering processes which involve states at the bottomof the band and, therefore, is ‘exponentially long’.

Feb/17/2009
4:30 p.m., 700 Clark Hall
Michael S. Fuhrer, University of Maryland
Massless and Massive Electrons in Aotmically Thin Carbon
Graphene, a single atom-thick plane of graphite, has recently been isolated and studied experimentally. In this two-dimensional hexagonal lattice of carbon atoms, the electrons obey the Dirac equation for massless particles, complete with a two-component spinor degree of freedom that mimics the spin of a relativistic particle. Graphene thus represents a unique opportunity to study ultra-relativistic Dirac Fermions in the laboratory. In addition, the extraordinary materials parameters of graphene are attractive for a range of applications from high-speed electronic devices to flexible, transparent conducting coatings. I will first discuss the electronic structure of graphene, and its implications for electronic properties. I will then describe experiments to elucidate the electron scattering mechanisms in graphene in the diffusive transport regime, which illustrate some unique consequences of graphene’s massless electronic dispersion relation. Finally, I will discuss experiments on mesoscopic graphene samples which provide a direct probe of the massless Fermion particle-in-a-box states in graphene, as well as the massive Fermion particle-in-a-box states in its bilayer counterpart.

Feb/10/2009
4:30 p.m., 700 Clark Hall
Wendy Zhang, James Franck Institute, University of Chicago
Memory as Vibration in a Disconnecting Air Bubble
Focusing a finite amount of energy dynamically into a vanishingly small amount of material requires that the initial condition be perfectly symmetric. In reality, imperfections are always present and cut-off the approach towards the focusing singularity. The disconnection of an underwater bubble provides a simple example of this competition between asymmetry and focusing. We use a combination of theory, simulation and experiments to show that the dynamics near disconnection contradicts the prevailing view that the disconnection dynamics converges towards a universal, cylindrically-symmetric singularity. Instead an initial asymmetry in the shape of the bubble neck excites vibrations that persist until disconnection. We argue that such memory-encoding vibrations may arise whenever initial asymmetries perturb the approach towards a singularity whose dynamics has an integrable form.

Feb/09/2009
2:30 p.m., 243 Clark Hall
David Schwab, UCLA
Disorder Effects on Quantum Phase Transitions
We give a heuristic argument for disorder rounding of a first-order quantum phase transition into a continuous phase transition. From both weak and strong disorder analysis of the N-color quantum Ashkin-Teller model in one spatial dimension, we find that for N≥3, the first-order transition is rounded to a continuous transition and the physical picture is the same as the random transverse field Ising model for a limited parameter regime. The results are strikingly different from the corresponding classical problem in two dimensions where the fate of the renormalization group flows is a fixed point corresponding to N-decoupled pure Ising models. We then study the competition between interactions and disorder in two dimensions. Within a simple model with short-ranged repulsive interactions, we show that—even in the limit of strong interaction—the Mott gap and its associated broken symmetry are completely washed out by disorder for dimensions D≤2, leading to a glassy state.

Feb/03/2009
4:30 p.m., 700 Clark Hall
Peng Chen, Cornell University
Single-Molecule Imaging of Nanoscale Catalysis and Electrocatalysis

Jan/29/2009
1:15 p.m., 609 Clark Hall
Jung-Jung Su, University of Texas
Excitonic Condensation in Graphene and in Other Systems
Strongly Correlated Informal Thursday Seminar

Among the many examples of Bose condensation considered in physics, exciton condensation has maintained special interest because of controversy about condensate properties. In the first part of my talk, I will discuss the excitonic condensate in graphene. Because of the gapless nature and the nearly identical conduction and valence bands, graphene-based bilayers are attractive candidates for high-temperature electron-hole pair condensation. The Kosterlitz-Thouless temperatures we estimated for these two-dimensional counterflow superfluids approach room temperature make it possible for device applications. In the second part of my talk, I will discuss the circuit conditions required to induce a steady-state counterflow superfluid. In addition, I will discuss interpretations of tunnel, drag and counterflow experiments in quantum Hall exciton condensates.

Jan/27/2009
4:30 p.m., 700 Clark Hall
Jennifer Schwarz, Syracuse University
Structure in Active Filament Networks
Consider a system of filaments in which filaments polymerize at one end, depolymerize at the other end, crosslink, branch off prexisting filaments and get severed. The skeleton of the cell is such a system. Since the cytoskeleton is a dynamic entity, it not only maintains the shape of the cell but can change its shape to initiate cell locomotion, for example. We analytically and numerically study the steady-state morphology of the cytoskeleton in the absence of cross-linking and discover that the system optimizes for changes in morphology with the variation of just one parameter. We also investigate the role of the 70° branching angle typically found in polymerizing networks of branched actin filaments at the leading edge of crawling cell. Using numerical simulations, we test the conjecture that an optimal branching angle is found by looking at the competition between polymerization along the direction of motion and branching with a component perpendicular to the direction of motion to provide collective structural support.

Jan/23/2009
1:15 p.m., 609 Clark Hall
Daniel Agterberg, University of Wisconsin, Milwaukee
FFLO and Pair Density Wave Superconductivity
Special LASSP Seminar

With the groundbreaking work of Fulde, Ferrell, Larkin and Ovchinnikov (FFLO), it was realized that superconducting order can also break translational invariance, leading to a phase in which the Cooper pairs develop a coherent periodic spatially oscillating structure. Such pair density wave (PDW) superconductivity has become relevant in a diverse range of systems, including cuprates, organic superconductors, heavy-fermion superconductors, cold atoms, and high-density quark matter. Here I focus on two aspects of such PDW superconductors. The first is the generic realization of such phases of non-centrosymmetric superconductors. The second is the vortex –like topological defects and their consequences in these superconductors.

R.P Kaur, D.F. Agterberg, and M. Sigrist, Phys. Rev. Lett. 94, 137002 (2005).
D.F. Agterberg and H. Tsunetsugu, Nature Physics 4, 639 (2008).

Dec/02/2008
4:30 p.m., 700 Clark Hall
Kathleen J. Stebe, University of Pennsylvania
Oriented Assembly of Anisotropic Particles by Capillary Interactions
Particles situated at fluid interfaces occur in nature, with the particles ranging from pollen to insects which walk on water. Particles at interfaces are exploited in classical applications like Pickering emulsions, in which particles stabilize emulsions, and froth flotation, in which ore particle adsorption to fluid interfaces is used to separate and recover metal ores. Particles at interfaces also occur in emerging applications in which nanomaterials are organized at interfaces.

The assembly of particles into ordered structures via capillary interactions is studied. Early work in this field focused primarily on spherical particles that distort fluid interfaces and create excess area. The particles assembled by capillary interactions which occur because the excess area created by the particles decreases as the particles approach each other. Here, particles with shape anisotropy are studied. Such particles create undulations with excess area that can be locally elevated at certain locations around the particle. The local elevation of excess area makes these sites locations for preferred assembly. Hence, particles orient and aggregate in preferred orientations. Such self assembly is often termed directed assembly. Three key issues in directed assembly are means of controlling the object orientation, alignment, and the sites for preferred assembly, including means of promoting registry of features on particles. Each of these issues is addressed in detail in for the example of a right circular cylinder using analysis, experiment and numerics. A series of other shapes are then studied to illustrate the generality of the concepts developed.

Nov/25/2008
4:30 p.m., 700 Clark Hall
Eva Y. Andrei, Rutgers University
Graphene Seen Through Transport and Tunneling Measurements
Graphene, a one-atom thick layer of crystalline carbon possesses extraordinary electronic properties which make it a prime candidate for novel nano-electronic devices, at the same time raising the prospect to observe phenomena hitherto unseen in bench top experiments. These unusual properties are due to quasiparticle excitations that mimic massless Dirac fermions. I will present scanning tunneling microscopy and transport experiments that provide access to these quasiparticles giving insight into their motion, the impact of magnetic field, effects of interactions with each other and with the host lattice.

Nov/24/2008
2:30 p.m., 700 Clark Hall
Sriram Ramaswamy, Indian Institute of Science, Bangalore
Active Fluids, Films, and Filaments
Special LASSP Seminar

I shall present a summary of our research on the collective properties of active matter. The talk will begin with our earlier results in this area, in settings ranging from living cells to granular matter. I shall then discuss our more recent work on the instabilities of the free surface of an active thin fluid film, and the dynamics of a single semi-flexible filament suspended in an active medium.

Nov/18/2008
4:30 p.m., 700 Clark Hall
Britton L.T. Plourde, Syracuse University
Tailored Superconducting Channels for Controlling Vortex Dynamics
The dynamics of vortex flow in confined geometries can be explored with nanostructured weak-pinning channels of superconducting a-NbGe surrounded by strong-pinning NbN channel edges. The lack of pinning allows the vortices to move through the channels with the dominant interaction determined by the shape of the channel walls. We have fabricated such weak-pinning channels with asymmetric sawtooth edges for controlling the motion of vortices. This design results in substantial asymmetries in the vortex dynamics in the channels, thus forming a ratchet for producing net vortex motion in response to an oscillatory drive. Using these weak-pinning channels, we are able to explore the influence of vortex interactions on the ratchet response by fabricating strips with different channel spacings and measuring these over a range of vortex densities. We have also investigated vortices flowing in a single circular ratchet channel arranged in a Corbino disk geometry, where the asymmetric response is even more pronounced compared with our measurements of straight vortex ratchet channels.

Nov/13/2008
3:00 p.m., 206 Hollister Hall
Stefano Zapperi, INFM-CNR
Size Effects in the Failure of Disordered Media
Special LASSP / Fractal Seminar

Understanding the specimen size dependence of materials strength represents a fundamental scientific problem with important practical implications. I will discuss the role played disorder and pre-existing notches in determining size effects. Numerical simulations of disordered lattice models for fracture reveal a crossover between a disorder-induced statistical regime and a stress-concentration controlled regime ruled by continuum fracture mechanics. The numerical results are interpreted by a scaling law involving the presence of a statistical fracture process zone which can be quantified by averaging over several disordered configurations of the model. The theoretical fracture strength scaling law exhibits an excellent agreement with experimental results obtained from notch papered paper samples.

Nov/11/2008
4:30 p.m., 700 Clark Hall
Michael Lawler, SUNY Binghamton and Cornell University
Emergent Paramagnetic Phases in the Hyper-Kagome Quantum Antiferromagnet Na4Ir3O8
After many years of searching for a realization of a quantum spinliquid, an exotic quantum paramagnet, there is much excitement in thefield driven by the recent successful fabrication of a number of newfrustrated spin 1/2 antiferromagnets. Of these new systems,Na₄Ir₃O₈ stands out for its novel genuinely three-dimensional”hyper-kagome” lattice, its high level of frustration, and its simplestoichiometric structure. Like other candidate systems for hosting aquantum spin liquid phase, it shows no sign of magnetic order down tothe lowest temperatures studied. However, early measurements have shownthat it has a roughly T² heat capacity(C) and a large finite susceptibility (Χ) at low temperatures. It isdifficult to explain these observations with typical theories of spinliquids as they often suggest the same temperature dependence in C/Tand Χ. I will discuss a resolution to this problem achieved throughexploring a variety of possible candidate spin liquid theories. Inparticular, a zero parameter fit to the heat capacity within one ofthese theories, lends strong weight to a quantum spin liquid scenario.

Nov/04/2008
4:30 p.m., 700 Clark Hall
Mona Berciu, University of British Columbia
Spectral Weight Transfer for Holstein Polarons
Finding simple yet accurate analytical approximations for the Green’s function of various systems, especially at intermediary coupling strengths, has been a major challenge in recent years. In this talk, I will present such an approximation, called the Momentum Average (MA) approximation, relevant for polaronic systems. For the Holstein polaron, MA spectral weight has been shown to satisfy exactly 6 sum rules and to remain highly accurate for all higher order sum rules, for all coupling strengths. This accuracy has also been confirmed by comparison against results of numerical simulations. Moreover, MA can be systematically improved, at an increased (but still trivial) computational cost. Our MA results allow us to elucidate the evolution of the spectral weight of a Holstein polaron as the electron-phonon coupling is turned on. I will also briefly discuss recent progress in generalizing this approximation to more complicated models.

Oct/28/2008
4:30 p.m., 700 Clark Hall
Cindy Regal, JILA, University of Colorado
Cooling and Detecting Nanomechanical Motion with a Microwave Cavity
With the advent of micro and nanoscale mechanical resonators, researchers are rapidly progressing toward a tangible harmonic oscillator whose motion requires a quantum description. Challenges include freezing out the thermomechanical motion to leave only zero-point quantum fluctuations and, equally importantly, realizing a Heisenberg-limited displacement detector. We have created a microwave detector of mechanical motion that can be in principle quantum limited and is also capable of efficiently coupling to the motion of small mass, nanoscale objects, which have the most accessible zero-point motion. Specifically we have measured the displacement of a nanomechanical beam using a superconducting transmission-line microwave cavity. We realize excellent mechanical force sensitivity (∼aN√pHz), detect thermal motion at 10’s of milliKelvin temperatures, and achieve a displacement imprecision of 30 times the standard quantum limit.¹ In our most recent measurements we have observed damping and cooling effects on the mechanical oscillator due to the microwave radiation field in the resolved-sideband limit; these results complement the recent observation of such cooling effects in the optical domain. Our current experiments achieve an occupation of n = 140, and I discuss the prospects for employing this dynamical backaction technique to cool a mechanical mode entirely to its quantum ground state with microwaves.²

¹C. A. Regal, J. D. Teufel, and K. W. Lehnert, “Measuring nanomechanical motion with a microwave cavity interferometer,” Nature Physics, 4, 555 (2008).

²J. D. Teufel, J. W. Harlow, C. A. Regal, and K. W. Lehnert, “Dynamical backaction of microwave fields on a nanomechanical oscillator,” Phys. Rev. Lett., in press (2008).

Oct/21/2008
4:30 p.m., 700 Clark Hall
Gianfranco Durin, Istituto Nazionale di Ricerca Metrologica
Crackling Noise and Complexity in Natural Systems
Is there any relation between a devastating earthquake, a snow avalanche, some water drops in a sponge, or a ferromagnet under a magnetic field? Despite huge differences, all these natural systems respond to external perturbations by a burst of events on a broad range of sizes. This random response, known as “crackling noise”, is common to many complex natural systems. In this talk, after a general excursion on the main features of these ‘crackling’ systems, we aim to present the most recent results on a particular crackling noise produced in soft magnetic materials, known as Barkhausen noise. Many of the theoretical and experimental results obtained in ferromagnets can in fact be applied with success to the study of other complex systems. This helps us to understand complexity within a more general theoretical framework, and focus the role of criticality and universality to understand the noise properties.

Oct/07/2008
4:30 p.m., 700 Clark Hall
Antti-Pekka Jauho, Technical University of Denmark
Title to be Announced

Oct/03/2008
1:00 p.m., 609 Clark Hall
Thomas Vojta, Missouri University of Science and Technology
Quantum Phase Transitions and Disorder: From Harris Criterion to Infinite Randomness and Smearing
Phase transitions are fascinating phenomena in nature with consequences ranging from the large scale structure of the universe to exotic quantum phases at low temperatures. Many realistic systems contain impurities, defects and other forms of quenched disorder. This talk explores the consequences of such randomness on the properties of phase transitions. At zero-temperature quantum phase transitions, randomness can have particularly peculiar and strong effects: In many random quantum Ising magnets the critical point is of infinite-randomness type, i.e., counter-intuitively, the disorder strength increases without limit under coarse graining. This gives rise to activated exponential scaling rather than the usual power-law scaling. In addition, rare disorder fluctuations lead to strong thermodynamic singularities, called the quantum-Griffiths singularities, in the vicinity of the actual transition. In metallic systems the effects of these rare fluctuations can be even stronger, leading to a destruction of the phase transition by smearing. We suggest a classification of these rare region effects based on the effective dimensionality of the defects, and we illustrate it using examples from classical, quantum, and nonequilibrium phase transitions.

Sep/30/2008
4:30 p.m., 700 Clark Hall
Gerald D. Mahan, Pennsylvania State University
Vibrations of Nanotubes and Nanowires
We describe the phonons in single wall carbon nanotubes, and in several semiconductor nanowires. The talk discusses theory (my group) and experiment (Peter Eklund’s group). We discovered surface optical phonons in nanowires of polar semiconductors. We also predicted all of the vibrational modes. We discuss those that can be measured by either Raman or IR. In SWCNT we discuss the flexure modes, which were absent from the earliest theories. This error was due to a failure of Born-von Karman theory! Our calculations were the first to get these modes in SWCNTs. We provided the first theory of IR absorption in graphite and in SWCNTs.

Sep/26/2008
10:00 a.m., 112 Rockefeller Hall
David J. Singh, Oak Ridge National Laboratory
Superconductivity in Layered Iron Compounds
For a long time, copper has been a magical element in superconductivity in the sense that layered cuprates show high temperature superconductivity above 160K and no other chemistry approached this. Therefore, the discovery this year of high temperature superconductivity in layered Fe compounds with critical temperatures exceeding 50K came as a surprise. In this talk I overview these materials and discuss their properties. Although the field is still very young, certain remarkable features are emerging – very low carrier density, proximity to magnetism, itinerant electron physics, unexpected tolerance to disorder and very high critical fields. Many of these properties are perplexingly different from those of cuprate superconductors. Work supported by the Department of Energy, Division of Materials Sciences and Engineering.

Sep/23/2008
4:30 p.m., 700 Clark Hall
Seth Fraden, Brandeis University
PhaseChip 2.0: Manipulating Phase Diagrams with Microfluidics
Structure implies function is the mantra of the structural biologists. X-ray diffraction of protein crystals reveals protein structure, which is needed to advance fundamental understanding of protein function and for drug development. Currently the physical process of crystallization is the bottleneck in protein structure determination.

We describe a microfluidic device denoted PhaseChip 1.0, specifically designed to optimize and decouple the mechanisms of nucleation and growth. PhaseChip 1.0 is fabricated with multiple microfluidic components in order to precisely meter, mix, and store nanoliter volumes of sample, solvent, and other reagents. Hundreds of nanoliter drops of different protein solutions can be stored and individually processed using a total of 10 μg protein. While sophisticated microfluidic technology is beguiling, simplicity is a virtue as demonstrated in our second generation design, PhaseChip 2.0.

Movies illustrating the Phase Chip in action are available online.
http://www.elsie.brandeis.edu/

Sep/16/2008
4:30 p.m., 700 Clark Hall
John Shumway, Arizona State University and Cornell University
A Path Integral Approach to Computational Nanoscience
Path integrals are an intuitive formulation of quantum mechanics that complement the canonical formulation of operators and wavefunctions. While path integrals are widely used in high energy quantum field theory and in many-body condensed matter theory, they are rarely used in applications of quantum mechanics to materials, quantum chemistry, and nanoscale electronics. In this talk I will describe how we use path integrals as a practical computational tool, building on Ceperley’s groundbreaking work on superfluid helium in the 1980′s and 90′s. I will present examples in three contemporary problems: (1) the antiferromagnetic coupling of two spins in double quantum dots and optical lattices, (2) the quantum-confined Stark effect in self-assembled quantum dots, and (3) quantized conductance in nano-wires and quantum point contacts. These three examples illustrate three concepts in quantum path integrals, namely instantons, imaginary-time correlation functions, and the emergence of collective behavior.

Sep/09/2008
4:30 p.m., 700 Clark Hall
Cynthia Reichhardt, Los Alamos National Laboratory
Local Probes at the Nanoscale: Avalanches, Melting, and Jamming Transitions
The equilibrium and nonequilibrium dynamics of a nanoscale medium can be readily accessed by driving a single particle through the system. This type of local probe couples directly to the size and energy scales of heterogeneities in the material, permitting the observation and control of rare events which may trigger intermittent or avalanche-like processes. I will describe how magnetic force microscopy can be used to manipulate individual magnetic vortices in a superconductor, and show through simulations that this local probe can be used to study vortex melting, entanglement, and avalanches. I demonstrate that local probes can also be applied to soft and biological matter, including studies of the jamming transition in granular media.

Sep/02/2008
4:30 p.m., 700 Clark Hall
Anthony Dinsmore, University of Massachusetts, Amherst
Nucleation and Sublimation of Crystals: Insights from Experiments with Colloids
The process by which crystals melt or nucleate is an interesting topic that, despite being very common, has proved difficult to study experimentally because of the high speed and small size of atoms. Using colloidal spheres as model ‘atoms,’ we experimentally monitor the kinetics of the melting and freezing of crystallites in the presence of a short-ranged (depletion) attraction, whose strength is adjusted by means of the temperature. We follow the motions of hundreds of individual particles with high resolution. In both experiments and simulations of sublimation, we find a two-stage process: at first, large crystallites sublimate by escape of particles from the perimeter. The rate of crystallite shrinkage is then greatly enhanced as the size falls below a cross-over value that ranges between 20 and 50 in different regions of the phase diagram. Simultaneous with the enhanced sublimation rate, the crystallites transform to a dense amorphous structure, which then rapidly vaporizes. The two-step kinetics are also seen in freezing at sphere area fractions near 0.3, but not at substantially higher or lower area fractions. The two-step kinetics are attributed to a thermodynamically meta- or unstable amorphous phase. The results should be relevant in diverse systems including colloids, proteins, and atoms such as Argon.

Apr/29/2008
4:30 p.m., 700 Clark Hall
Paul Francois, Rockefeller University
Deriving Structure from Computational Evolution
I propose a general algorithm to design genetic networks by evolution in silico. This procedure first aims at designing motifs performing a given function in a single cell such as switches and oscillators, and the network topologies found are very similar to well-known genetic modules, such as circadian clocks. This algorithm also allows us to evaluate on computational models how predictive evolution can be. Using as an example in silico evolution of biochemical adaptation, I will show how problems such as dependency on the fitness function or on the mutation rates can be bypassed in some cases.

In an “evo-devo” context, a simpler model is finally used to explore evolutionary pathways leading to segmentation networks. In superior animals, segmentation is the patterning process that leads to the formation of body metameric units such as segments in insects or vertebrae in vertebrates. When the evolution is guided by a generic fitness function that just counts the number of segment boundaries, we invariably observe a very constrained evolutionary path. Surprisingly, the system spontaneously evolves towards a genetic network that implements the phenomenological “clock and wavefront” proposed by Cooke and Zeeman in 1976. These computations illustrate how complex traits can evolve by the incremental addition of new functions on top of preexisting traits.

Apr/22/2008
4:30 p.m., 700 Clark Hall
Eric Akkermans, Technion and Yale University
Photon Localization and Dicke Superradiance: A Crossover to Small World Networks
We study photon localization in a gas of cold atoms, using a Dicke Hamiltonian that accounts for photon mediated atomic dipolar interactions. The photon escape rates are obtained from a new class of random matrices. A scaling behavior is observed for photons escape rates as a function of disorder and system size. Photon localization is described using statistical properties of random networks which display a “small world” cross-over. Those results are compared to the Anderson photon localization transition.

Apr/15/2008
4:30 p.m., 700 Clark Hall
Steven Chu, Lawrence Berkeley National Laboratory, University of California. Berkeley
Coherent Control of Ultra-Cold Matter
Bethe Lecturer

Apr/08/2008
4:30 p.m., 700 Clark Hall
Egor Babaev, University of Massachusetts, Amherst
Counterflow Condensates: From Projected Metallic Hydrogen to Multicomponent Bose-Einstein and Excitonic Condensates

Apr/01/2008
4:30 p.m., 700 Clark Hall
Weining Man, Princeton University
Geometry and Symmetry in Photonics and Material Science
Building block geometry and overall structural symmetry play an important role in the design of new materials. In particular this talk investigates ellipsoid packings and photonic quasicrystals, problems in which the geometry of the building blocks and the structural symmetry determine the physical properties. Photonic quasicrystals are constructed from dielectric material arranged in a quasiperiodic pattern whose rotational symmetry is forbidden for periodic crystals. Because quasicrystals have higher point group symmetry than ordinary crystals, they can have more uniform bandgaps. Since calculating the band structure of 3D photonic quasicrystals is fundamentally challenging, and to date beyond the range of computation in a reasonable time, we decided to answer the question experimentally. We constructed the world’s first and largest (in terms of the number of units) 3D icosahedral Photonic quasicrystal (compose of polymer) using stereolithography. With our novel method to make polar plots of its microwave transmission vs. frequency and incident angle, we obtained the first-ever visualization of the Brillouin zone of a quasicrystal. Before our experimental work it was not at all clear that Brillouin zones existed or had physical meaning in quasicrystals. We proved that the nearly spherical Brillouin zones of 3D icosahedral quasicrystals make them one of the most promising candidates for complete photonic bandgaps found to date. For ellipsoidal granular material packing, we found in both experiments and simulations that ellipsoids can pack randomly more densely than spheres because of their extra degree of freedom associated with their rotational axes. Discovering the fact that the packing fraction has a cusp-like minimum for spheres and increases rapidly with aspect ratio differ from unity, is important for both theoretical modeling and practical applications.

Mar/25/2008
4:30 p.m., 700 Clark Hall
Philipp Werner, Columbia University
Diagrammatic Monte Carlo Methods for Fermions
Quantum impurity models play an important role as representations of molecular conductors and in the “dynamical mean field” method, currently one of the most promising tools for studying the physics of fermionic lattice models. Despite their zero-dimensional nature, the numerical simulation of impurity models remains a challenging task. Progress has been achieved with the recent development of diagrammatic Monte Carlo techniques [1,2,3]. This simulation approach relies on a diagrammatic expansion of the partition function and the stochastic sampling of collections of diagrams. I will explain the key ideas behind this first diagrammatic method for fermionic systems, and illustrate its power and flexibility with dynamical mean field results for multi-orbital models [4]. I will also briefly discuss the on-going effort to adapt the diagrammatic Monte Carlo technique to real-time dynamics in non-equilibrium systems.

[1] A. N. Rubtsov, V. V. Savkin, and A. I. Lichtenstein, Phys. Rev. B 72, 035122 (2005).
[2] P. Werner, A. Comanac, L. De’ Medici, M. Troyer, and A. J. Millis, Phys. Rev. Lett. 97, 076405 (2006).
[3] P. Werner and A. J. Millis, Phys. Rev. B 74, 155107 (2006).
[4] P. Werner and A. J. Millis, Phys. Rev. Lett. 99, 126405 (2007).

Mar/04/2008
4:30 p.m., 700 Clark Hall
Mukund Vengalattore, University of California, Berkeley
Equilibrium Phases of a Dipolar Magnetic Superfluid
Spinor Bose gases—multicomponent quantum fluids with a spin degree of freedom—have emerged as a promising model system for understanding complex orders in many-body systems and the quantum magnetism of ultracold atomic gases. I will describe a high resolution imaging technique to probe the magnetic properties of such spinor fluids. Employing this technique, we have realized a precision magnetic microscope with a field sensitivity that surpasses the performance of modern SQUID magnetometers. I will also describe the use of this technique to explore the quench dynamics of degenerate spinor gases across a quantum phase transition.

More recently, we have observed the effects of magnetic dipolar interactions in these ultracold spinor gases. The competing influences of the ‘local’ ferromagnetic interaction and the ‘long-range’ dipolar interaction result in a host of dynamical effects in these superfluids including the spontaneous formation of spatially modulated spin textures and the nucleation of topological defects. I will present measurements of the finite-temperature phase diagram of these dipolar superfluids and conclude with some proposals for future research avenues.

Feb/26/2008
4:30 p.m., 700 Clark Hall
Ahmet Yildiz, University of California, San Francisco
Single Molecule Studies of Motor Protein Movement with Nanometer Precision
LASSP and School of Applied and Engineering Physics Seminar

Motor proteins move along the linear tracks of cytoskeleton by hydrolyzing ATP to transport vital cargoes within a cell. The central question in the motor field was whether these two-headed motors move by ‘walking’, ‘sliding’, or a ‘biased diffusion’. To resolve this controversy, I have developed a novel fluorescence imaging technique (FIONA) that locates the center of the diffraction-limited image of a single molecule with 1 nm precision. In FIONA assay, I observed that fluorescently labeled heads of a motor alternately take a step and pass each other, similar to human walking. The motor must keep its identical heads out of phase, such that the rear head moves forward as the front head stays bound. I have tested whether the communication is mediated by intramolecular tension generated by a linker region that interconnects the two heads. When this tension is reduced by inserting artificial peptides into the linker, I observed that the motor slows down due to impaired coupling between ATP hydrolysis and forward stepping. However, speed recovers to nearly normal levels under assisting loads provided by optical trap. The results showed that mechanical tension generated through an ATP-driven conformational change provides the bias for directional movement and facilitates communication between the two motor heads.

Feb/21/2008
4:30 p.m., 700 Clark Hall
Markus Mueller, Harvard University
Nernst Effect and Magnetohydrodynamics Near Quantum Criticality in Superconductors, Graphene and Black Holes
We present a general hydrodynamic theory of transport in the vicinity of quantum critical points in two spatial dimensions that are described by “Lorentz-invariant” low energy theories. This applies in particular to the superfluid-insulator transition or to lightly doped graphene, where we allow for a weak impurity scattering rate, a magnetic field, and a deviation in the density from the critical value. We show that the frequency-dependent thermoelectric response functions, including the Nernst coefficient, are fully determined by a single transport coefficient (a universal electrical conductivity), the impurity scattering rate, and a few thermodynamic state variables. Our results predict a magnetic field and temperature dependence of the Nernst signal which resembles measurements in the cuprates, including the overall magnitude. Our theory predicts a collective, relativistic cyclotron resonance which should be observable in ultrapure samples and, in particular, in graphene—even at room temperature.

We will also discuss exact results for the zero frequency transport coefficients of a supersymmetric conformal field theory (CFT), which is solvable by the AdS/CFT correspondence via a mapping to a black hole problem in quantum gravity. These exact results are found to be in full agreement with the general predictions of our hydrodynamic analysis in the appropriate limiting regime, and provide an interesting example where the transport coefficients can be computed exactly.

Feb/19/2008
4:30 p.m., 700 Clark Hall
William Ryu, Princeton University
A Biophysicist Looks at the Thermal Response and Motor Behavior of E. coli and C. elegans
E. coli has a natural behavioral variable—the direction of rotation of its flagellar rotary motor. Monitoring this one-dimensional behavioral response in reaction to chemical perturbation has been instrumental in the understanding of how E. coli performs chemotaxis at the genetic, physiological, and computational level. We are applying this experimental strategy to the study of bacterial thermotaxis —a sensory mode that is less well understood. To investigate bacterial thermosensation we subject single cells to well defined thermal stimuli such as impulses of heat produced by an IR laser and analyze their response. Higher organisms may have more complicated behavioral responses because their motions have more degrees of freedom. Here we provide a comprehensive analysis of motor behavior of such an organism—the nematode C. elegans. Using tracking video-microscopy we capture a worm’s image and extract the skeleton of the shape as a head-to-tail ordered collection of tangent angles sampled along the curve. Applying principal components analysis we show that the space of shapes is remarkably low dimensional, with four dimensions accounting for > 95% of the shape variance. We also show that these dimensions align with behaviorally relevant states. As an application of this analysis we study the thermal response of worms stimulated by laser heating. Our quantitative description of C. elegans movement should prove useful in a wide variety of contexts, from the linking of motor output with neural circuitry to the genetic basis of adaptive behavior.

Feb/12/2008
4:30 p.m., 700 Clark Hall
Hui Deng, California Institute of Technology
The Matter-Light Quantum Interfaces for Scalable Quantum Networks
With the promise to control the physical world in ever smaller scales, quantum systems are coming into the center stage of modern technology. A forerunner in this quest is the science of quantum information. Few-qubits and simple quantum computing operations have been implemented in diverse matter systems. Quantum communication with guaranteed perfect security has been demonstrated for up to 150 km with optical systems. However, scalability remains a grand challenge.

Combining matter and light quantum systems, quantum networks promise an avenue toward scalable quantum technology. A quantum network consists of matter nodes to store and process quantum information, optical channels to transport quantum information, and more importantly, matter-light quantum interfaces between the nodes and the channels that enable efficient distribution of quantum information across the network.

Using atomic ensembles as matter nodes, we establish efficient quantum interfaces between atoms and photons based on collective matter-light interactions, and demonstrate the essential capabilities of quantum networks—generation, storage and distribution of entanglement between remote quantum nodes.

Feb/07/2008
4:30 p.m., 700 Clark Hall
Eun-Ah Kim, Stanford University
The Theory of the Nodal Nematic Quantum Chirality
In the last several years, experimental evidence has accumulated in a variety of highly correlated electronic systems of new quantum phases which (for purely electronic reasons) spontaneously break the rotational (point group) symmetry of the underlying crystal. Such “nematic” phases have been seen in quantum Hall systems, in the metamagnetic SrRuO, and more recently in magnetic neutron scattering studies of the high temperature superconductor, YBCO.

The quantum dynamics of the nematic order parameter, especially near a putative quantum critical point, naturally couples strongly to low energy fermionic excitations (quasiparticles). Such coupling can affect the nature and even the fate of quasiparticles making a study of nematic quantum criticality a highly nontrivial problem. In this talk, I will discuss our recent results on one case in which this problem can be attacked in a tractable manner: the nodal nematic quantum criticality. Deep inside a d-wave superconducting state in a two-dimensional tetragonal crystal, the phase space for the low energy fermions reduce from the entire Fermi surface to four nodal points. The limited phase space for scattering allows for a controlled analysis of the nematic quantum criticality for nodal fermions.

We study the character of this quantum phase transition. We investigated the problem by solving a model system with N flavors of quasiparticles in the large N limit. We find the transition to be continuous in this limit and the critical nematic fluctuations to have drastic effects on the nature of nodal quasiparticles. To leading order in 1/N, quantum fluctuations enhance the dispersion anisotropy of the nodal excitations, and cause strongly angle dependent scattering leading to a nontrivial structure in the single particle spectral function. I will discuss possible implications of our results for cuprate physics from the spectral function of the nematic mode and the single fermion spectral function.

Feb/05/2008
4:30 p.m., 700 Clark Hall
Shinsei Ryu, University of California, Santa Barbara
3D Topological Insulators and 2D Anderson Delocalization
Recently, a new type of insulators, called Z2 topological insulator, has been discovered. They can be thought of as a close cousin of the integer quantum Hall insulators, but different from the IQHE in many essential ways: Z2 topological insulators exist in systems that respect time reversal symmetry, can be either in 2D or 3D, and are characterized by a Z2 topological number, unlike the integral Hall conductance in the IQHE. Several candidate materials possessing the non-trivial Z2 topological features, such as HgTe quantum wells, and Bismuth-Antimony alloys, have been proposed and tested experimentally. In this talk, we will discuss quantum transport of 2D surface massless Dirac fermion states supported by 3D Z2 topological insulators terminated by a 2D boundary. Although these Dirac fermion states, from the symmetry point of view, belong to the 2D spin-orbit (symplectic) symmetry class of Anderson localization, they inherit the Z2 topological character of the bulk, and exhibit different response to impurities from conventional disorderedconductors with spin-orbit interactions. We also briefly discuss that this surface physics of 3D Z2 topological insulators can be simulated by graphene.