Electron localization in sound absorption oscillations in the quantum Hall effect regime

The absorption coefficient for surface acoustic waves in a piezoelectric insulator in contact with a GaAs/AlGaAs heterostructure (with two-dimensional electron mobility $\mu= 1.3\times 10^5 cm^2/V\cdot s)$ at T=4.2K) via a small gap has been investigated experimentally as a function of the frequency of the wave, the width of the vacuum gap, the magnetic field, and the temperature. The magnetic field and frequency dependencies of the high-frequency conductivity (in the region 30-210 MHz) are calculated and analyzed. The experimental results can be explained if it assumed that there exists a fluctuation potential in which current carrier localization occurs. The absorption of the surface acoustic waves in an interaction with two-dimensional electrons localized in the energy "tails" of Landau levels is discussed.Comment: RevTeX 6 pages+6 EPS pic

Molecular random walks and invariance group of the Bogolyubov equation

Statistics of molecular random walks in a fluid is considered with the help of the Bogolyubov equation for generating functional of distribution functions. An invariance group of solutions to this equation as functions of the fluid density is discovered. It results in many exact relations between probability distribution of the path of a test particle and its irreducible correlations with the fluid. As the consequence, significant restrictions do arise on possible shapes of the path distribution. In particular, the hypothetical Gaussian form of its long-range asymptotic proves to be forbidden (even in the Boltzmann-Grad limit). Instead, a diffusive asymptotic is allowed which possesses power-law long tail (cut off by ballistic flight length).Comment: 23 pages, no figures, LaTeX AMSART, author's translation from Russian of the paper accepted to the TMPh (``Theoretical and mathematical physics''

Nonequilibrium Cooper pairing in the nonadiabatic regime

We obtain a complete solution for the mean-field dynamics of the BCS paired state with a large, but finite number of Cooper pairs in the non-adiabatic regime. We show that the problem reduces to a classical integrable Hamiltonian system and derive a complete set of its integrals of motion. The condensate exhibits irregular multi-frequency oscillations ergodically exploring the part of the phase-space allowed by the conservation laws. In the thermodynamic limit however the system can asymptotically reach a steady state.Comment: 4 pages, no figure

Vortex mediated microwave absorption in superclean layered superconductors

In the superclean case the spectrum of vortex core excitations in the presence of disorder is not random but consists of two series of equally-spaced levels. The I-V characteristics of such superconductors displays many interesting phenomena. A series of resonances is predicted at frequencies commensurate with the spacing of the vortex excitations. These resonances reveal an even-odd anomaly. In the presence of one weak impurity the excitation levels can approach each other and almost cross. Absorption at very low frequencies is identified with the resonances arising in this case. The results of such microscopic theory coincide up to the order of magnitude with both the theory employing kinetic equation and the experiment. The non-linear effects associated with Zener transitions in such crossings are studied. These phenomena can be used as a probe of vortex core excitations.Comment: 11 pages, 2 Postscript figure

Bi-stability in voltage-biased NISIN structures

As a generic example of a voltage-driven superconducting structure we study a short superconductor connected to normal leads by means of low transparency tunnel junctions, with a voltage bias $V$ between the leads. The superconducting order parameter $\Delta$ is to be determined self-consistently. We study the stationary states of the system as well as the dynamics after a perturbation. We find a region in parameter space where there are two stable stationary states at a given voltage. These bi-stable states are distinguished by distinct values of the superconducting order parameter $\Delta$ and of the current between the leads. We have evaluated (1) the multi-valued superconducting order parameter $\Delta$ at given $V$; (2) the current between the leads at a given V; and (3) the critical voltage at which superconductivity in the island ceases. With regards to dynamics, we find numerical evidence that the stationary states are stable and that no complicated non-stationary regime can be induced by changing the voltage. This result is somewhat unexpected and by no means trivial, given the fact that the system is driven out of equilibrium. The response to a change in the voltage is always gradual, even in the regime where changing the interaction strength induces rapid anharmonic oscillations of the order parameter.Comment: 15 pages, 11 figures, submitted to Phys. Rev.

Static and high-frequency hole transport in p-Si/SiGe heterostructures in the extreme quantum limit

Complex high-frequency (HF), sigma(AC) = sigma(1) - i sigma(2), and static, sigma(DC), conductivities, as well as current-voltage characteristics, have been measured in p-Si/SiGe heterostructures with a low hole density (p = 8.2 x 10(10) cm(-2)) at temperatures T = 0.3-4.2 K in the ultraquantum limit, when the filling factor is v > |sigma(2)|, the negative sign of sigma(2), the threshold behavior of the current-voltage characteristic, and the dependence I proportional to exp(-A/V-0.3) in the subthreshold region-indicate the formation of a pinned Wigner crystal (glass) in the ultraquantum limit (T = 0.3-0.8 K, B > 14 T)