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Publications

Softened elastic response and unzipping in chemical vapor deposition graphene membranes

Cornell Affiliated Author(s)
Author
C.S. Ruiz-Vargas
H.L. Zhuang
P.Y. Huang
A.M. Van Der Zande
S. Garg
P.L. McEuen
D.A. Muller
R.G. Hennig
J. Park
Abstract

We use atomic force microscopy to image grain boundaries and ripples in graphene membranes obtained by chemical vapor deposition. Nanoindentation measurements reveal that out-of-plane ripples effectively soften graphene's in-plane stiffness. Furthermore, grain boundaries significantly decrease the breaking strength of these membranes. Molecular dynamics simulations reveal that grain boundaries are especially weakening when subnanometer voids are present in the lattice. Finally, we demonstrate that two graphene membranes brought together form membranes with higher resistance to breaking.

Journal
Nano Letters
Date Published
Research Area
Group (Lab)
Paul McEuen Group

A257T linker region mutant of T7 helicase-primase protein is defective in DNA loading and rescued by T7 DNA polymerase

Cornell Affiliated Author(s)
Author
G. Patel
D.S. Johnson
B. Sun
M. Pandey
X. Yu
E.H. Egelman
M.D. Wang
S.S. Patel
Abstract

The helicase and primase activities of the hexameric ringshaped T7 gp4 protein reside in two separate domains connected by a linker region. This linker region is part of the subunit interface between monomers, and point mutations in this region have deleterious effects on the helicase functions. One such linker region mutant, A257T, is analogous to the A359T mutant of the homologous human mitochondrial DNA helicase Twinkle, which is linked to diseases such as progressive external opthalmoplegia.

Journal
Journal of Biological Chemistry
Date Published
Research Area
Group (Lab)
Michelle Wang Group

Quantum oscillations and the Fermi surface of high-temperature cuprate superconductors

Cornell Affiliated Author(s)
Author
Baptiste Vignolle
David Vignolles
David LeBoeuf
Stephane Lepault
Brad Ramshaw
Ruixing Liang
D.A. Bonn
W.N. Hardy
Nicolas Doiron-Leyraud
A. Carrington
N.E. Hussey
Louis Taillefer
Cyril Proust
Abstract

Over 20 years since the discovery of high temperature superconductivity in cuprates (Bednorz and Müller, 1986 [1]), the first convincing observation of quantum oscillations in underdoped YBa2Cu3O6.5 (Doiron-Leyraud et al., 2007 [2]) has deeply changed the theoretical landscape relevant to these materials. The Fermi surface is a basic concept of solid state physics, which underpins most physical properties (electrical, thermal, optical, etc.) of a metal. Even in the presence of interactions, this fundamental concept remains robust.

Journal
Comptes Rendus Physique
Date Published
Funding Source
EP/F038836/1
Group (Lab)
Brad Ramshaw Group

Strategies for processing diffraction data from randomly oriented particles

Cornell Affiliated Author(s)
Author
V. Elser
Abstract

The high intensity of free-electron X-ray light sources may enable structure determinations of viruses or even individual proteins without the encumbrance of first forming crystals. This note compares two schemes of non-crystalline diffraction data collection that have been proposed: serial single-shot data from individual particles, and averaged cross-correlation data from particle ensembles. The information content of these schemes is easily compared and we show that the single-shot approach, although experimentally more challenging, is always superior in this respect.

Journal
Ultramicroscopy
Date Published
Group (Lab)
Veit Elser Group

Evolution of the pseudogap in a polarized Fermi gas

Cornell Affiliated Author(s)
Author
E.J. Mueller
Abstract

We calculate the single-particle spectral density of a normal (nonsuperfluid) two-component gas of fermions in the BCS-BEC crossover within a T-matrix approximation. We review how noncondensed pairs lead to a spectral density reminiscent of the ordered state, and explore how a gaplike feature in the spectrum evolves as one changes the polarization of the gas. As the gas is polarized, we find that this pseudogap becomes more diffuse and moves away from the Fermi level, reflecting the fact that fewer pairs are present but that they still play an important role in the excitations.

Journal
Physical Review A - Atomic, Molecular, and Optical Physics
Date Published
Funding Source
0758104

Structure and Scm3-mediated assembly of budding yeast centromeric nucleosomes

Cornell Affiliated Author(s)
Author
M.L. Dechassa
K. Wyns
M. Li
M.A. Hall
M.D. Wang
K. Luger
Abstract

Much controversy exists regarding the structural organization of the yeast centromeric nucleosome and the role of the nonhistone protein, Scm3, in its assembly and architecture. Here we show that the substitution of H3 with its centromeric variant Cse4 results in octameric nucleosomes that organize DNA in a left-handed superhelix. We demonstrate by single-molecule approaches, micrococcal nuclease digestion and small-angle X-ray scattering that Cse4-nucleosomes exhibit an open conformation with weakly bound terminal DNA segments.

Journal
Nature Communications
Date Published
Research Area
Group (Lab)
Michelle Wang Group

Interplay of rotational, relaxational, and shear dynamics in solid 4He

Cornell Affiliated Author(s)
Author
E.J. Pratt
B. Hunt
V. Gadagkar
M. Yamashita
M.J. Graf
A.V. Balatsky
J.C. Davis
Abstract

Using a high-sensitivity torsional oscillator (TO) technique, we mapped the rotational and relaxational dynamics of solid helium-4 (4He) throughout the parameter range of the proposed supersolidity. We found evidence that the same microscopic excitations controlling the torsional oscillator motions are generated independently by thermal and mechanical stimulation. Moreover, a measure for the relaxation times of these excitations diverges smoothly without any indication for a critical temperature or critical velocity of a supersolid transition.

Journal
Science
Date Published
Group (Lab)
J.C. Seamus Davis Group

Fast X-ray microdiffraction techniques for studying irreversible transformations in materials

Cornell Affiliated Author(s)
Author
S.T. Kelly
J.C. Trenkle
L.J. Koerner
S.C. Barron
N. Walker
P.O. Pouliquen
M.W. Tate
Sol Gruner
E.M. Dufresne
T.P. Weihs
T.C. Hufnagel
Abstract

A pair of techniques have been developed for performing time-resolved X-ray microdiffraction on irreversible phase transformations. In one technique capillary optics are used to focus a high-flux broad-spectrum X-ray beam to a 60 m spot size and a fast pixel array detector is used to achieve temporal resolution of μ55 s.

Journal
Journal of Synchrotron Radiation
Date Published
Group (Lab)
Eric Dufresne Group
Sol M. Gruner Group

Nucleation at the DNA supercoiling transition

Cornell Affiliated Author(s)
Author
B.C. Daniels
J.P. Sethna
Abstract

Twisting DNA under a constant applied force reveals a thermally activated transition into a state with a supercoiled structure known as a plectoneme. Using transition-state theory, we predict the rate of this plectoneme nucleation to be of order 104 Hz. We reconcile this with experiments that have measured hopping rates of order 10 Hz by noting that the viscous drag on the bead used to manipulate the DNA limits the measured rate.

Journal
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics
Date Published
Funding Source
0705167
1005479
Research Area
Group (Lab)
James Sethna Group

Paddling mode of forward flight in insects

Cornell Affiliated Author(s)
Author
L. Ristroph
A.J. Bergou
J. Guckenheimer
Z.J. Wang
Itai Cohen
Abstract

By analyzing high-speed video of the fruit fly, we discover a swimminglike mode of forward flight characterized by paddling wing motions. We develop a new aerodynamic analysis procedure to show that these insects generate drag-based thrust by slicing their wings forward at low angle of attack and pushing backwards at a higher angle. Reduced-order models and simulations reveal that the law for flight speed is determined by these wing motions but is insensitive to material properties of the fluid.

Journal
Physical Review Letters
Date Published
Research Area
Group (Lab)
Itai Cohen Group
Z. Jane Wang Group