3,265 research outputs found
Interaction corrections: temperature and parallel field dependencies of the Lorentz number in two-dimensional disordered metals
The electron-electron interaction corrections to the transport coefficients
are calculated for a two-dimensional disordered metal in a parallel magnetic
field via the quantum kinetic equation approach. For the thermal transport,
three regimes (diffusive, quasiballistic and truly ballistic) can be identified
as the temperature increases. For the diffusive and quasiballistic regimes, the
Lorentz number dependence on the temperature and on the magnetic field is
studied. The electron-electron interactions induce deviations from the
Wiedemann-Franz law, whose sign depend on the temperature: at low temperatures
the long-range part of the Coulomb interaction gives a positive correction,
while at higher temperature the inelastic collisions dominate the negative
correction. By applying a parallel field, the Lorentz number becomes a
non-monotonic function of field and temperature for all values of the
Fermi-liquid interaction parameter in the diffusive regime, while in the
quasiballistic case this is true only sufficiently far from the Stoner
instability.Comment: 11 pages, 5 figures. Appendix A revised, notes adde
Magnetoconductivity of low-dimensional disordered conductors at the onset of the superconducting transition
Magnetoconductivity of the disordered two- and three-dimensional
superconductors is addressed at the onset of superconducting transition. In
this regime transport is dominated by the fluctuation effects and we account
for the interaction corrections coming from the Cooper channel. In contrast to
many previous studies we consider strong magnetic fields and various
temperature regimes, which allow to resolve the existing discrepancies with the
experiments. Specifically, we find saturation of the fluctuations induced
magneto-conductivity for both two- and three-dimensional superconductors at
already moderate magnetic fields and discuss possible dimensional crossover at
the immediate vicinity of the critical temperature. The surprising observation
is that closer to the transition temperature weaker magnetic field provides the
saturation. It is remarkable also that interaction correction to
magnetoconductivity coming from the Cooper channel, and specifically the so
called Maki-Thompson contribution, remains to be important even away from the
critical region.Comment: 4 pages, 1 figur
Electron-electron interaction corrections to the thermal conductivity in disordered conductors
We evaluate the electron-electron interaction corrections to the electronic
thermal conductivity in a disordered conductor in the diffusive regime. We use
a diagrammatic many-body method analogous to that of Altshuler and Aronov for
the electrical conductivity. We derive results in one, two and three dimensions
for both the singlet and triplet channels, and in all cases find that the
Wiedemann-Franz law is violated.Comment: 8 pages, 2 figures Typos corrected in formulas (15) and (A.4) and
Table 1; discussion of previous work in introduction extended; reference
clarifying different definitions of parameter F adde
Universal Conductance Fluctuations in Mesoscopic Systems with Superconducting Leads: Beyond the Andreev Approximation
We report our investigation of the sample to sample fluctuation in transport
properties of phase coherent normal metal-superconductor hybrid systems.
Extensive numerical simulations were carried out for quasi-one dimensional and
two dimensional systems in both square lattice (Fermi electron) as well as
honeycomb lattice (Dirac electron). Our results show that when the Fermi energy
is within the superconducting energy gap , the Andreev conductance
fluctuation exhibits a universal value (UCF) which is approximately two times
larger than that in the normal systems. According to the random matrix theory,
the electron-hole degeneracy (ehD) in the Andreev reflections (AR) plays an
important role in classifying UCF. Our results confirm this. We found that in
the diffusive regime there are two UCF plateaus, one corresponds to the
complete electron-hole symmetry (with ehD) class and the other to conventional
electron-hole conversion (ehD broken). In addition, we have studied the Andreev
conductance distribution and found that for the fixed average conductance
the Andreev conductance distribution is a universal function that depends only
on the ehD. In the localized regime, our results show that ehD continues to
serve as an indicator for different universal classes. Finally, if normal
transport is present, i.e., Fermi energy is beyond energy gap , the AR
is suppressed drastically in the localized regime by the disorder and the ehD
becomes irrelevant. As a result, the conductance distribution is that same as
that of normal systems
Observation of mesoscopic conductance fluctuations in YBaCuO grain boundary Josephson Junctions
Magneto-fluctuations of the normal resistance R_N have been reproducibly
observed in high critical temp erature superconductor (HTS) grain boundary
junctions, at low temperatures. We attribute them to mesoscopic transport in
narrow channels across the grain boundary line. The Thouless energy appears to
be the relevant energy scale. Our findings have significant implications on
quasiparticle relaxation and coherent transport in HTS grain boundaries.Comment: Revised version, minor changes. 4 pages, 4 figure
Coulomb drag in quantum circuits
We study drag effect in a system of two electrically isolated quantum point
contacts (QPC), coupled by Coulomb interactions. Drag current exhibits maxima
as a function of QPC gate voltages when the latter are tuned to the transitions
between quantized conductance plateaus. In the linear regime this behavior is
due to enhanced electron-hole asymmetry near an opening of a new conductance
channel. In the non-linear regime the drag current is proportional to the shot
noise of the driving circuit, suggesting that the Coulomb drag experiments may
be a convenient way to measure the quantum shot noise. Remarkably, the
transition to the non-linear regime may occur at driving voltages substantially
smaller than the temperature.Comment: 6 pages, 2 figure
Current relaxation in nonlinear random media
We study the current relaxation of a wave packet in a nonlinear random sample
coupled to the continuum and show that the survival probability decays as . For intermediate times , the exponent
satisfies a scaling law where is
the nonlinearity strength and is the localization length of the
corresponding random system with . For and we find a universal decay with which is a signature of the
{\it nonlinearity-induced delocalization}. Experimental evidence should be
observable in coupled nonlinear optical waveguides.Comment: revised version, PRL in press, 4 pages, 4 figs (fig 3 with reduced
quality
Delocalization of weakly interacting bosons in a 1D quasiperiodic potential
We consider weakly interacting bosons in a 1D quasiperiodic potential
(Aubry-Azbel-Harper model) in the regime where all single-particle states are
localized. We show that the interparticle interaction may lead to the many-body
delocalization and we obtain the finite-temperature phase diagram.
Counterintuitively, in a wide range of parameters the delocalization requires
stronger cou- pling as the temperature increases. This means that the system of
bosons can undergo a transition from a fluid to insulator (glass) state under
heating
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