4,290 research outputs found

    Quantum dot dephasing by edge states

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    We calculate the dephasing rate of an electron state in a pinched quantum dot, due to Coulomb interactions between the electron in the dot and electrons in a nearby voltage biased ballistic nanostructure. The dephasing is caused by nonequilibrium time fluctuations of the electron density in the nanostructure, which create random electric fields in the dot. As a result, the electron level in the dot fluctuates in time, and the coherent part of the resonant transmission through the dot is suppressed

    Thomas-Fermi-Dirac-von Weizsacker hydrodynamics in laterally modulated electronic systems

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    We have studied the collective plasma excitations of a two-dimensional electron gas with an arbitrary lateral charge-density modulation. The dynamics is formulated using a previously developed hydrodynamic theory based on the Thomas-Fermi-Dirac-von Weizsacker approximation. In this approach, both the equilibrium and dynamical properties of the periodically modulated electron gas are treated in a consistent fashion. We pay particular attention to the evolution of the collective excitations as the system undergoes the transition from the ideal two-dimensional limit to the highly-localized one-dimensional limit. We also calculate the power absorption in the long-wavelength limit to illustrate the effect of the modulation on the modes probed by far-infrared (FIR) transmission spectroscopy.Comment: 27 page Revtex file, 15 Postscript figure

    Kohn Anomalies in Superconductors

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    I present the detailed behavior of phonon dispersion curves near momenta which span the electronic Fermi sea in a superconductor. I demonstrate that an anomaly, similar to the metallic Kohn anomaly, exists in a superconductor's dispersion curves when the frequency of the phonon spanning the Fermi sea exceeds twice the superconducting energy gap. This anomaly occurs at approximately the same momentum but is {\it stronger} than the normal-state Kohn anomaly. It also survives at finite temperature, unlike the metallic anomaly. Determination of Fermi surface diameters from the location of these anomalies, therefore, may be more successful in the superconducting phase than in the normal state. However, the superconductor's anomaly fades rapidly with increased phonon frequency and becomes unobservable when the phonon frequency greatly exceeds the gap. This constraint makes these anomalies useful only in high-temperature superconductors such as La1.85Sr.15CuO4\rm La_{1.85}Sr_{.15}CuO_4.Comment: 18 pages (revtex) + 11 figures (upon request), NSF-ITP-93-7

    Spin orbit effects in a GaAs quantum dot in a parallel magnetic field

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    We analyze the effects of spin-orbit coupling on fluctuations of the conductance of a quantum dot fabricated in a GaAs heterostructure. We argue that spin-orbit effects may become important in the presence of a large parallel magnetic field B_{||}, even if they are negligble for B_{||}=0. This should be manifest in the level repulsion of a closed dot, and in reduced conductance fluctuations in dots with a small number of open channels in each lead, for large B_{||}. Our picture is consistent with the experimental observations of Folk et al.Comment: 5 page

    DiVinE-CUDA - A Tool for GPU Accelerated LTL Model Checking

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    In this paper we present a tool that performs CUDA accelerated LTL Model Checking. The tool exploits parallel algorithm MAP adjusted to the NVIDIA CUDA architecture in order to efficiently detect the presence of accepting cycles in a directed graph. Accepting cycle detection is the core algorithmic procedure in automata-based LTL Model Checking. We demonstrate that the tool outperforms non-accelerated version of the algorithm and we discuss where the limits of the tool are and what we intend to do in the future to avoid them

    Dephasing of Electrons by Two-Level Defects in Quantum Dots

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    The electron dephasing time τϕ\tau_{\phi} in a diffusive quantum dot is calculated by considering the interaction between the electron and dynamical defects, modelled as two-level system. Using the standard tunneling model of glasses, we obtain a linear temperature dependence of 1/τϕ1/\tau_{\phi}, consistent with the experimental observation. However, we find that, in order to obtain dephasing times on the order of nanoseconds, the number of two-level defects needs to be substantially larger than the typical concentration in glasses. We also find a finite system-size dependence of τϕ\tau_{\phi}, which can be used to probe the effectiveness of surface-aggregated defects.Comment: two-column 9 page

    Dynamic exchange-correlation potentials for the electron gas in dimensionality D=3 and D=2

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    Recent progress in the formulation of a fully dynamical local approximation to time-dependent Density Functional Theory appeals to the longitudinal and transverse components of the exchange and correlation kernel in the linear current-density response of the homogeneous fluid at long wavelength. Both components are evaluated for the electron gas in dimensionality D=3 and D=2 by an approximate decoupling in the equation of motion for the current density, which accounts for processes of excitation of two electron-hole pairs. Each pair is treated in the random phase approximation, but the role of exchange and correlation is also examined; in addition, final-state exchange processes are included phenomenologically so as to satisfy the exactly known high-frequency behaviours of the kernel. The transverse and longitudinal spectra involve the same decay channels and are similar in shape. A two-plasmon threshold in the spectrum for two-pair excitations in D=3 leads to a sharp minimum in the real part of the exchange and correlation kernel at twice the plasma frequency. In D=2 the same mechanism leads to a broad spectral peak and to a broad minimum in the real part of the kernel, as a consequence of the dispersion law of the plasmon vanishing at long wavelength. The numerical results have been fitted to simple analytic functions.Comment: 13 pages, 11 figures included. Accepted for publication in Phys. Rev.

    Coulomb drag in compressible quantum Hall states

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    We consider the Coulomb drag between two layers of two-dimensional electronic gases subject to a strong magnetic field. We first focus on the case in which the electronic density is such that the Landau level filling fraction ν\nu in each layer is at, or close to, ν=1/2\nu=1/2. Discussing the coupling between the layers in purely electronic terms, we show that the unique dependence of the longitudinal conductivity on wave-vector, observed in surface acoustic waves experiments, leads to a very slow decay of density fluctuations. Consequently, it has a crucial effect on the Coulomb drag, as manifested in the transresistivity ρD\rho_D. We find that the transresistivity is very large compared to its typical values at zero magnetic field, and that its temperature dependence is unique -- ρDT4/3\rho_D \propto T^{4/3}. For filling factors at or close to 1/41/4 and 3/43/4 the transresistivity has the same TT-dependence, and is larger than at ν=1/2\nu = 1/2. We calculate ρD\rho_D for the ν=3/2\nu=3/2 case and propose that it might shed light on the spin polarization of electrons at ν=3/2\nu=3/2. We compare our results to recent calculations of ρD\rho_D at ν=1/2\nu=1/2 where a composite fermion approach was used and a T4/3T^{4/3}-dependence was obtained. We conclude that what appears in the composite fermion language to be drag induced by Chern-Simons interaction is, physically, electronic Coulomb drag.Comment: 11 pages, REVTeX with two Postscript figure
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