Publications
Detection of DNA and poly-l-lysine using CVD graphene-channel FET biosensors
A graphene channel field-effect biosensor is demonstrated for detecting the binding of double-stranded DNA and poly-l-lysine. Sensors consist of chemical vapor deposition graphene transferred using a clean, etchant-free transfer method. The presence of DNA and poly-l-lysine are detected by the conductance change of the graphene transistor. A readily measured shift in the Dirac voltage (the voltage at which the graphene's resistance peaks) is observed after the graphene channel is exposed to solutions containing DNA or poly-l-lysine.
Avoided valence transition in a plutonium superconductor
The d and f electrons in correlated metals are often neither fully localized around their host nuclei nor fully itinerant. This localized/itinerant duality underlies the correlated electronic states of the high-Tc cuprate superconductors and the heavy-fermion intermetallics and is nowhere more apparent than in the 5f valence electrons of plutonium. Here, we report the full set of symmetry-resolved elastic moduli of PuCoGa5 - the highest Tc superconductor of the heavy fermions (Tc = 18.5 K) - and find that the bulk modulus softens anomalously over a wide range in temperature above Tc.
Higgs criticality in a two-dimensional metal
We analyze a candidate theory for the strange metal near optimal hole doping in the cuprate superconductors. The theory contains a quantum phase transition between metals with large and small Fermi surfaces of spinless fermions carrying the electromagnetic charge of the electron, but the transition does not directly involve any broken global symmetries. The two metals have emergent SU(2) and U(1) gauge fields respectively, and the transition is driven by the condensation of a real Higgs field, carrying a finite lattice momentum and an adjoint SU(2) gauge charge.
A high frame rate hybrid X-ray image sensor
This paper describes a solid-state image sensor for high-speed X-ray imaging. The sensor is made up of a light sensitive detector layer bump-bonded to a readout integrated circuit (ROIC). The detector layer is high resistivity n-type silicon and is fully depleted in operation. The p-implanted islands are used to define pixel regions with 100-μm × 100-μm area. The detector layer contains 852 × 209 pixels indium bump-bonded to four identical CMOS ROICs. Each ROIC contains 213 × 209 pixels and is fabricated using a 0.25-μm CMOS process.
Transverse collisional instabilities of a Bose-Einstein condensate in a driven one-dimensional lattice
Motivated by recent experiments, we analyze the stability of a three-dimensional Bose-Einstein condensate loaded in a periodically driven one-dimensional optical lattice. Such periodically driven systems do not have a thermodynamic ground state but may have a long-lived steady state which is an eigenstate of a "Floquet Hamiltonian." We explore collisional instabilities of the Floquet ground state which transfer energy into the transverse modes.
Evanescent-field optical readout of graphene mechanical motion at room temperature
Graphene mechanical resonators have recently attracted considerable attention for use in precision force- and mass-sensing applications. To date, readout of their oscillatory motion typically requires cryogenic conditions to achieve high sensitivity, restricting their range of applications. Here we report the demonstration of an evanescent optical readout of graphene motion, using a scheme which does not require cryogenic conditions and exhibits enhanced sensitivity and bandwidth at room temperature.
Spicing up continuum solvation models with SaLSA: The spherically averaged liquid susceptibility ansatz
Continuum solvation models enable electronic structure calculations of systems in liquid environments, but because of the large number of empirical parameters, they are limited to the class of systems in their fit set (typically organic molecules). Here, we derive a solvation model with no empirical parameters for the dielectric response by taking the linear response limit of a classical density functional for molecular liquids.
Imaging Dirac-mass disorder from magnetic dopant atoms in the ferromagnetic topological insulator Crx(Bi0.1Sb0.9)2-xTe3
To achieve and use the most exotic electronic phenomena predicted for the surface states of 3D topological insulators (TIs), it is necessary to open a "Dirac-mass gap" in their spectrum by breaking timereversal symmetry. Use of magnetic dopant atoms to generate a ferromagnetic state is the most widely applied approach. However, it is unknown how the spatial arrangements of the magnetic dopant atoms influence the Dirac-mass gap at the atomic scale or, conversely, whether the ferromagnetic interactions between dopant atoms are influenced by the topological surface states.
A microfabricated fixed path length silicon sample holder improves background subtraction for cryoSAXS
The application of small-angle X-ray scattering (SAXS) for high-throughput characterization of biological macromolecules in solution is limited by radiation damage. By cryocooling samples, radiation damage and required sample volumes can be reduced by orders of magnitude. However, the challenges of reproducibly creating the identically sized vitrified samples necessary for conventional background subtraction limit the widespread adoption of this method. Fixed path length silicon sample holders for cryoSAXS have been microfabricated to address these challenges.
Identifying the 'fingerprint' of antiferromagnetic spin fluctuations in iron pnictide superconductors
Cooper pairing in the iron-based high-T c superconductors is often conjectured to involve bosonic fluctuations. Among the candidates are antiferromagnetic spin fluctuations and d-orbital fluctuations amplified by phonons. Any such electron-boson interaction should alter the electron's 'self-energy', and then become detectable through consequent modifications in the energy dependence of the electron's momentum and lifetime.