Publications
Fast X-ray microdiffraction techniques for studying irreversible transformations in materials
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.
Nucleation at the DNA supercoiling transition
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.
Paddling mode of forward flight in insects
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.
Plasmon resonance in individual nanogap electrodes studied using graphene nanoconstrictions as photodetectors
We achieve direct electrical readout of the wavelength and polarization dependence of the plasmon resonance in individual gold nanogap antennas by positioning a graphene nanoconstriction within the gap as a localized photodetector. The polarization sensitivities can be as large as 99%, while the plasmon-induced photocurrent enhancement is 2-100. The plasmon peak frequency, polarization sensitivity, and photocurrent enhancement all vary between devices, indicating the degree to which the plasmon resonance is sensitive to nanometer-scale irregularities. © 2011 American Chemical Society.
Minimal model of plasma membrane heterogeneity requires coupling cortical actin to criticality
We present a minimal model of plasma membrane heterogeneity that combines criticality with connectivity to cortical cytoskeleton. The development of this model was motivated by recent observations of micron-sized critical fluctuations in plasma membrane vesicles that are detached from their cortical cytoskeleton. We incorporate criticality using a conserved order parameter Ising model coupled to a simple actin cytoskeleton interacting through point-like pinning sites. Using this minimal model, we recapitulate several experimental observations of plasma membrane raft heterogeneity.
Universality beyond power laws and the average avalanche shape
The study of critical phenomena and universal power laws has been one of the central advances in statistical mechanicsduring the second half of the past century, explaining traditional thermodynamic critical points 1 , avalanche behaviour near depinning transitions 2,3 and a wide variety of other phenomena 4 . Scaling, universality and the renormalization group claim to predict all behaviour at long length and timescales asymptotically close to critical points.
Erratum: Stress-based vapor sensing using resonant microbridges (Applied Physics Letters (2010) 96 (163503))
Optical-lattice hamiltonians for relativistic quantum electrodynamics
We show how interpenetrating optical lattices containing Bose-Fermi mixtures can be constructed to emulate the thermodynamics of quantum electrodynamics (QED). We present models of neutral atoms on lattices in 1+1, 2+1, and 3+1 dimensions whose low-energy effective action reduces to that of photons coupled to Dirac fermions of the corresponding dimensionality. We give special attention to (2+1)-dimensional quantum electrodynamics (QED3) and discuss how two of its most interesting features, chiral symmetry breaking and Chern-Simons physics, could be observed experimentally.
Structural origin of apparent Fermi surface pockets in angle-resolved photoemission of Bi2Sr2-xLaxCuO 6+δ
We observe apparent hole pockets in the Fermi surfaces of single-layer Bi-based cuprate superconductors from angle-resolved photoemission. From detailed low-energy electron diffraction measurements and an analysis of the angle-resolved photoemission polarization dependence, we show that these pockets are not intrinsic but arise from multiple overlapping superstructure replicas of the main and shadow bands.
Local versus global equilibration near the bosonic Mott-insulator- superfluid transition
We study the time scales for adiabaticity of trapped cold bosons subject to a time-varying lattice potential using a dynamic Gutzwiller mean-field theory. We explain apparently contradictory experimental observations by demonstrating a clear separation of time scales for local dynamics (∼ms) and global mass redistribution (∼1s). We provide a simple explanation for the short and fast time scales, finding that while density or energy transport is dominated by low energy phonons, particle-hole excitations set the adiabaticity time for fast ramps.