arXiv subjects
Publications and source records attributed to A. V. Shytov.
Electric charge density in a metallic nanoparticle fluctuates due to inhomogeneities of various kinds. While in the bulk these fluctuations are compensated via Thomas--Fermi screening, residual surface charges give rise to a permanent electric dipole moment. We show that this dipole moment increases linearly with particle size, and fluctuates strongly from particle to particle
A new regime of coherent quantum dynamics of a qubit is realized at low driving frequencies in the strong driving limit. Coherent transitions between qubit states occur via the Landau-Zener process when the system is swept through an energy-level avoided crossing. The quantum interference mediated by repeated transitions gives rise to an oscillatory dependence of the qubit population on the driving field amplitude and flux detuning. These interference fringes, which at high frequencies consist of individual multiphoton resonances, persist even for driving frequencies smaller than the decoherence rate, where individual resonances are no longer distinguishable. A theoretical model that incorporates dephasing agrees well with the observations.
Interesting electrical polarity in nanoclusters usually requires the polarizability to exceed the value R^3 of the classical sphere of radius R. We clarify how this occurs naturally in single electron quantum systems, and relate it to the giant polarizability of Na_14F_13, and to spontaneous dipole formation on niobium clusters. Many-body effects generally reduce the polarizability through screening. The usual RPA treatment retrieves the classical answer, but it significantly overestimates screening in few-electron systems. The system of two electrons on the surface of a sphere is solved numerically, to account for the Coulomb repulsion. At high densities, numerical results agree with RPA model with properly subtracted self-interaction effects. At low densities, the system performs quantum oscillations around the classical ground state. We calculate the lowest anharmonic correction to the polarizability, which also agrees well with numerical evaluation of the polarizability.
An arbitrarily small concentration of impurities can affect the spin Hall conductivity in a two-dimensional semiconductor system. We develop a Boltzmann-like equation that can be used for impurity scattering with arbitrary angular dependence, and for arbitrary angular dependence of the spin-orbit field b(k) around the Fermi surface. For a model applicable to a 2D hole system in GaAs, if the impurity scattering is not isotropic, we find that the spin Hall conductivity depends on the derivative of b with respect to the energy and on deviations from a parabolic band structure, as well as on the angular dependence of the scattering. In principle, the resulting spin Hall conductivity can be larger or smaller than the ``intrinsic value'', and can have opposite sign. In the limit of small angle scattering, in a model appropriate for small hole concentrations, where the band is parabolic and b ~ k^3, the spin Hall conductivity has opposite sign from the intrinsic value, and has larger magnitude. Our analysis assumes that the spin-orbit splitting $b$ and the transport scattering rate tau^{-1} are both small compared to the Fermi energy, but the method is valid for for arbitrary value of b*tau.
We derive the transport equations for two-dimensional electron systems with spin-orbit interaction and short-range spin-independent disorder. In the limit of slow spatial variations of the electron distribution we obtain coupled diffusion equations for the electron density and spin. Using these equations we calculate electric-field induced spin accumulation in a finite-size sample for arbitrary ratio between spin-orbit energy splitting and elastic scattering rate. We demonstrate that the spin-Hall conductivity vanishes in an infinite system independent of this ratio.
We analyse the behavior of the density of states in a singlet s-wave superconductor with weak magnetic impurities in the clean limit. By using the method of optimal fluctuation and treating the order parameter self-consistently we show that the density of states is finite everywhere in the superconducting gap, and that it varies as $\ln N(E)\propto -|E-Δ_0|^{(7-d)/4}$ near the mean field gap edge $Δ_0$ in a d-dimensional superconductor. In contrast to most studied cases the optimal fluctuation is strongly anisotropic.
One may probe intrinsic coherence of a qubit by periodically sweeping its control parameter. The qubit is then excited by Landau-Zener (LZ) mechanism. The interference between multiple LZ transition leads to the unusual oscillatory dependence of the energy absorption rate on sweeping amplitude and period. This interference pattern allows to determine the decoherence time of the qubit. We introduce a simple phenomenological model describing this "interferometer", and find the form of the interference pattern.
Electron energy distribution in a mesoscopic diffusive wire in the presence of an external ac voltage is obtained. At low temperatures the distribution has a form of a multi-step staircase, with the step width equal to the field energy quantum. Analytic results for the field frequency high and low compared to Thouless energy are obtained, while the intermediate frequency regime is analyzed numerically. Manifestations in the tunneling spectroscopy and noise measurements are discussed.
We study the S-matrix for the transitions at an avoided crossing of several energy levels, which is a multilevel generalization of the Landau-Zener problem. We demonstrate that, by extending the Schroedinger evolution to complex time, one can obtain an exact answer for some of the transition amplitudes. Similar to the Landau-Zener case, our result covers both the adiabatic regime (slow evolution.) and the diabatic regime (fast evolution). The form of the exact transition amplitude coincides with that obtained in a sequential pairwise level crossing approximation, in accord with the conjecture of Brundobler and Elser.
Tunnelling density of states (DoS) in Luttinger liquid has a dip at zero energy, commonly known as the zero-bias anomaly (ZBA). In the presence of a magnetic field, in addition to the zero-bias anomaly, the DoS develops two peaks separated from the origin by the Zeeman energy. We find the shape of these peaks at arbitrary strength of the electron-electron interaction. The developed theory is applicable to various kinds of quantum wires, including carbon nanotubes.
We analyse the behavior of the density of states in a singlet s-wave superconductor with weak magnetic impurities in the clean limit by using the method of optimal fluctuation. We show that the density of states varies as $\ln N(E)\propto -|E-Δ_0|^{(7-d)/4}$ near the mean field gap edge $Δ_0$ in a d-dimensional superconductor. The optimal fluctuation in d>1 is strongly anisotropic. We compare the density of states with that obtained in other recent approaches.
Electrons moving in a conductor can transfer momentum to the lattice via collisions with impurities and boundaries, giving rise to a fluctuating mechanical stress tensor. The root-mean-squared momentum transfer per scattering event in a disordered metal (of dimension L greater than the mean free path l and screening length xi) is found to be reduced below the Fermi momentum by a factor of order l/L for shear fluctuations and (xi/L)^2 for pressure fluctuations. The excitation of an elastic bending mode by the shear fluctuations is estimated to fall within current experimental sensitivity for a nanomechanical oscillator.
The electrodynamical response of the edge of a compressible Quantum Hall system affects tunneling into the edge. Using the composite Fermi liquid theory, we derive an effective action for the edge modes interacting with tunneling charge. This action generalizes the chiral Luttinger liquid theory of the Quantum Hall edge to compressible systems in which transport is characterized by a finite Hall angle. In addition to the standard terms, the action contains a dissipative term. The tunneling exponent is calculated as a function of the filling fraction for several models, including short-range and long-range Coulomb interaction. We find that tunneling exponents are robust and to a large extent insensitive to the particular model. We discuss recent tunneling measurements in the overgrown cleaved edge systems, and demonstrate that the profile of charge density near the edge is very sensitive to the parameters of the system. In general, the density is non-monotonic, and can deviate from the bulk value by up to 30%. Implications for the correspondence to the chiral Luttinger edge theories are discussed.
In a recent letter (PRL 84, 3930 (2000)) Nakhmedov et al. claimed that the Van Hove singularity at $ε=0$ in the density of states (DoS) of the two-dimensional crystal with half-filled tight-binding band survives the addition of substitutional impurities. This derivation suffers from several inconsistencies. We resolve them and show that the DoS at the band center is finite, as one might naively expect.
The Landau-Zener transition in a two level system can be suppressed or enhanced by coupling to an environment, depending on the temperature and the environment spectral function. We consider the marginal spectral function, when the dissipation effects are important for arbitrarily slow motion. Landau-Zener transition rate demonstrates a non-trivial dependence of the on the ``bias'', i. e. on the rate of the two energy levels relative motion. The Landau-Zener transition is fully suppressed for the values of the bias below a threshold bias set by the coupling strength. Above the threshold, the transition rate for zero temperature is found using the instanton method. At finite temperature, the Landau-Zener transition rate has a non-monotonic dependence on the coupling strength, being suppressed at the strong coupling.
We discuss localization of quasiparticles in an extended NS structure in the situation when the reflection from the NS interface is mostly of Andreev kind. The localization of quasiparticle states arises due to trajectory retracing caused by Andreev reflection. This effect is semiclassical in the sense that in the classical limit the states become fully localized, while quantum effects destroy localization behaviour. We derive the criteria for this localization and show that it takes place only for sufficiently smooth disorder with large correlation length, whereas short-range disorder destroys the effect. Localization of quasiparticle states gives rise to resonance peaks in the local tunneling density of states.
We present a composite fermion theory of tunneling into the edge of a compressible quantum Hall system. The tunneling conductance is non-ohmic, due to slow relaxation of electromagnetic and Chern-Simons field disturbances caused by the tunneling electron. Universal results are obtained in the limit of a large number of channels involved in the relaxation. The tunneling exponent is found to be a continuous function of the filling factor nu, with a a slope that is discontinuous at nu=1/2 in the limit of vanishing bulk resistivity rho_xx. When nu corresponds to a principal fractional quantized Hall state, our results agree with the chiral Luttinger liquid theories of Wen, and Kane, Fisher and Polchinski.
We present an effective action approach for the problem of Coulomb blocking of tunneling. The method is applied to the ``strong coupling'' problem arising near zero bias, where perturbation theory diverges. By a semiclassical argument, we obtain electrodynamics in imaginary time, and express the anomaly through exact conductivity of the system $σ(ω, q)$ and exact interaction. The calculation is tested by comparing with the known perturbation theory result for the diffusive anomaly. Also, we use the method to study the anomaly enhancement due to external magnetic field, and the effect of screening by electrodes.