Crossover from quantum to Boltzmann transport in graphene

We compare a fully quantum mechanical numerical calculation of the conductivity of graphene to the semiclassical Boltzmann theory. Considering a disorder potential that is smooth on the scale of the lattice spacing, we find quantitative agreement between the two approaches away from the Dirac point. At the Dirac point the two theories are incompatible at weak disorder, although they may be compatible for strong disorder. Our numerical calculations provide a quantitative description of the full crossover between the quantum and semiclassical graphene transport regimes.Comment: 4 pages, 4 figures; published versio

Weyl node with random vector potential

We study Weyl semimetals in the presence of generic disorder, consisting of a random vector potential as well as a random scalar potential. We derive renormalization group flow equations to second order in the disorder strength. These flow equations predict a disorder-induced phase transition between a pseudo-ballistic weak-disorder phase and a diffusive strong-disorder phase for sufficiently strong random scalar potential or for a pure three-component random vector potential. We verify these predictions using a numerical study of the density of states near the Weyl point and of quantum transport properties at the Weyl point. In contrast, for a pure single-component random vector potential the diffusive strong-disorder phase is absent.Comment: published version with minor change

Continuous recognition memory for spoken words in noise

Previous research has shown that talker variability affects recognition memory for spoken words (Palmeri et al., 1993). This study examines whether additive noise is similarly retained in memory for spoken words. In a continuous recognition memory task, participants listened to a list of spoken words mixed with noise consisting of a pure tone or of high-pass filtered white noise. The noise and speech were in non-overlapping frequency bands. In Experiment 1, listeners indicated whether each spoken word in the list was OLD (heard before in the list) or NEW. Results showed that listeners were as accurate and as fast at recognizing a word as old if it was repeated with the same or different noise. In Experiment 2, listeners also indicated whether words judged as OLD were repeated with the same or with a different type of noise. Results showed that listeners benefitted from hearing words presented with the same versus different noise. These data suggest that spoken words and temporally-overlapping but spectrally non-overlapping noise are retained or reconstructed together for explicit, but not for implicit recognition memory. This indicates that the extent to which noise variability is retained seems to depend on the depth of processin

Fluctuating "order parameter" for a quantum chaotic system with partially broken time-reversal symmetry

The functional defined as the squared modulus of the spatial average of the wave function squared, plays the role of an ``order parameter'' for the transition between Hamiltonian ensembles with orthogonal and unitary symmetry. Upon breaking time-reversal symmetry, the order parameter crosses over from one to zero. We compute its distribution in the crossover regime and find that it has large fluctuations around the ensemble average. These fluctuations imply long-range spatial correlations in the eigenfunction and non-Gaussian perturbations of eigenvalues, in precise agreement with results by Fal'ko and Efetov and by Taniguchi, Hashimoto, Simons, and Altshuler. As a third implication of the order-parameter fluctuations we find correlations in the response of an eigenvalue to independent perturbations of the system.Comment: 4 pages, REVTeX-3.0, 1 figure. Reference added to Y. V. Fyodorov and A. D. Mirlin, Phys. Rev. B 51, 13403 (1995

Signatures of Klein tunneling in disordered graphene p-n-p junctions

We present a method for obtaining quantum transport properties in graphene that uniquely combines three crucial features: microscopic treatment of charge disorder, fully quantum mechanical analysis of transport, and the ability to model experimentally relevant system sizes. As a pertinent application we study the disorder dependence of Klein tunneling dominated transport in p-n-p junctions. Both the resistance and the Fano factor show broad resonance peaks due to the presence of quasi bound states. This feature is washed out by the disorder when the mean free path becomes of the order of the distance between the two p-n interfaces.Comment: 4 pages, 4 figure

Finite size effects and localization properties of disordered quantum wires with chiral symmetry

Finite size effects in the localization properties of disordered quantum wires are analyzed through conductance calculations. Disorder is induced by introducing vacancies at random positions in the wire and thus preserving the chiral symmetry. For quasi one-dimensional geometries and low concentration of vacancies, an exponential decay of the mean conductance with the wire length is obtained even at the center of the energy band. For wide wires, finite size effects cause the conductance to decay following a non-pure exponential law. We propose an analytical formula for the mean conductance that reproduces accurately the numerical data for both geometries. However, when the concentration of vacancies increases above a critical value, a transition towards the suppression of the conductance occurs. This is a signature of the presence of ultra-localized states trapped in finite regions of the sample.Comment: 5 figures, revtex

Ehrenfest-time dependence of weak localization in open quantum dots

Semiclassical theory predicts that the weak localization correction to the conductance of a ballistic chaotic cavity is suppressed if the Ehrenfest time exceeds the dwell time in the cavity [I. L. Aleiner and A. I. Larkin, Phys. Rev. B {\bf 54}, 14424 (1996)]. We report numerical simulations of weak localization in the open quantum kicked rotator that confirm this prediction. Our results disagree with the `effective random matrix theory' of transport through ballistic chaotic cavities.Comment: 4 pages, 2 figure

Electromotive force and internal resistance of an electron pump

We present a scattering theory of the electromotive force and internal resistance of an electron pump. The characterization of the device performance in terms of only two parameters requires the assumption of incoherent multiple scattering within the circuit and complete thermalization among electrons moving in a given direction. The electromotive force is shown to be of the order of the driving frequency in natural units. In an open setup, the electromotive force adds to the voltage difference between reservoirs to drive the current, both facing a contact resistance which is absent in the case of a closed circuit of uniform width