71 research outputs found
The dynamics of developing Cooper pairing at finite temperatures
We study the time evolution of a system of fermions with pairing interactions
at a finite temperature. The dynamics is triggered by an abrupt increase of the
BCS coupling constant. We show that if initially the fermions are in a normal
phase, the amplitude of the BCS order parameter averaged over the Boltzman
distribution of initial states exhibits damped oscillations with a relatively
short decay time. The latter is determined by the temperature, the
single-particle level spacing, and the ground state value of the BCS gap for
the new coupling. In contrast, the decay is essentially absent when the system
was in a superfluid phase before the coupling increase.Comment: 4 pages, figure fixe
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 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
Dissipationless BCS Dynamics with Large Branch Imbalance
In many situations a BCS-type superconductor will develop an imbalance
between the populations of the holelike and electronlike spectral branches.
This imbalance suppresses the gap. It has been noted by Gal'perin et al. [Sov.
Phys. JETP 54, 1126 (1981)] that at large imbalance, when the gap is
substantially suppressed, an instability develops. The analytic treatment of
the system beyond the instability point is complicated by the fact that the
Boltzmann approach breaks down. We study the short-time behavior following the
instability, in the collisionless regime, using methods developed by Yuzbashyan
et al. [J. Phys. A 38, 7831 (2005); Phys. Rev. B 72, 220503(R) (2005)].Comment: 12 pages, 3 figure
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
Normal and anomalous solitons in the theory of dynamical Cooper pairing
We obtain multi-soliton solutions of the time-dependent Bogoliubov-de Gennes
equations or, equivalently, Gorkov equations that describe the dynamics of a
fermionic condensate in the dissipationless regime. There are two kinds of
solitons - normal and anomalous. At large times, normal multi-solitons
asymptote to unstable stationary states of the BCS Hamiltonian with zero order
parameter (normal states), while the anomalous ones tend to eigenstates
characterized by a nonzero anomalous average. Under certain circumstances,
multi-soliton solutions break up into sums of single solitons. In the linear
analysis near the stationary states, solitons correspond to unstable modes.
Generally, they are nonlinear extensions of these modes, so that a stationary
state with unstable modes gives rise to a -soliton solution. We relate
parameters of the multi-solitons to those of the asymptotic stationary state,
which determines the conditions necessary for exciting solitons. We further
argue that the dynamics in many physical situations is multi-soliton.Comment: 41 pages, 14 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
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''
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 between the leads. The
superconducting order parameter 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 and of the
current between the leads. We have evaluated (1) the multi-valued
superconducting order parameter at given ; (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.
Thermodynamics of Nonstoichiometric Nickel Tellurides. I. Heat Capacity and Thermodynamic Functions of the Ύ Phase from 5 to 350°K
Heat capacities of the nickel tellurides were measured at compositions NiTe1.1 and NiTe2.0 (near limits of homogeneity of the ÎŽ phase) and at one intermediate composition, NiTe1.5, from 5 to 350°K. Heat capacity values and entropy and enthalpy increments are tabulated. No evidence of orderâdisorder transitions, or thermal anomalies, or of contributions to the thermal properties from the anisotropy or phonon scattering by the holes in the structure on approaching the composition NiTe2 was observed. Although simple additivity of the heat capacities of the constituent elements failed to represent that of the solution compositions adequately, a KoppâNeumann treatment in terms of the limiting compositions of the ÎŽ phase gives good agreement with the experimental heat capacity and entropy of NiTe1.5 and hence is useful in interpolating to other intermediate compositions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70090/2/JCPSA6-28-3-497-1.pd
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