5,868 research outputs found
Nonequilibrium mesoscopic conductance fluctuations
We investigate the amplitude of mesoscopic fluctuations of the differential
conductance of a metallic wire at arbitrary bias voltage V. For non-interacting
electrons, the variance increases with V. The asymptotic large-V
behavior is \sim V/V_c (where eV_c=D/L^2 is the Thouless energy),
in agreement with the earlier prediction by Larkin and Khmelnitskii. We find,
however, that this asymptotics has a very small numerical prefactor and sets in
at very large V/V_c only, which strongly complicates its experimental
observation. This high-voltage behavior is preceded by a crossover regime,
V/V_c \lesssim 30, where the conductance variance increases by a factor \sim 3
as compared to its value in the regime of universal conductance fluctuations
(i.e., at V->0). We further analyze the effect of dephasing due to the
electron-electron scattering on at high voltages. With the Coulomb
interaction taken into account, the amplitude of conductance fluctuations
becomes a non-monotonic function of V. Specifically, drops as 1/V
for voltages V >> gV_c, where g is the dimensionless conductance. In this
regime, the conductance fluctuations are dominated by quantum-coherent regions
of the wire adjacent to the reservoirs.Comment: 14 pages, 4 figures. Fig.2 and one more appendix added, accepted for
publication in PR
Vortex Viscosity in Magnetic Superconductors Due to Radiation of Spin Waves
In type-II superconductors that contain a lattice of magnetic moments,
vortices polarize the magnetic system inducing additional contributions to the
vortex mass, vortex viscosity, and vortex-vortex interaction. Extra magnetic
viscosity is caused by radiation of spin waves by a moving vortex. Like in the
case of Cherenkov radiation, this effect has a characteristic threshold
behavior and the resulting vortex viscosity may be comparable to the well-known
Bardeen-Stephen contribution. The threshold behavior leads to an anomaly in the
current-voltage characteristics, and a drop in dissipation for a current
interval that is determined by the magnetic excitation spectrum.Comment: 4 pages, 1 figur
Phases of a fermionic model with chiral condensates and Cooper pairs in 1+1 dimensions
We study the phase structure of a 4-fermi model with three bare coupling
constants, which potentially has three types of bound states. This model is a
generalization of the model discussed previously by A. Chodos et al. [Phys.
Rev. D 61, 045011 (2000)], which contained both chiral condensates and Cooper
pairs. For this generalization we find that there are two independent
renormalized coupling constants which determine the phase structure at finite
density and temperature. We find that the vacuum can be in one of three
distinct phases depending on the value of these two renormalized coupling
constants
c-Axis longitudinal magnetoresistance of the electron-doped superconductor Pr1.85Ce0.15CuO4
We report c-axis resistivity and longitudinal magnetoresistance measurements
of superconducting Pr1.85Ce0.15CuO4 single crystals. In the temperature range
13K<T<32K, a negative magnetoresistance is observed at fields just above Hc2.
Our studies suggest that this negative magnetoresistance is caused by
superconducting fluctuations. At lower temperatures (T<13K), a different
magnetoresistance behavior and a resistivity upturn are observed, whose origin
is still unknown.Comment: Accepted for publication in Phys. Rev.
Finite Temperature Phase Diagram of Quasi-Two-Dimensional Imbalanced Fermi Gases Beyond Mean-Field
We investigate the superfluid transition temperature of quasi-two-dimensional
imbalanced Fermi gases beyond the mean-field approximation, through the
second-order (or induced) interaction effects. For a balanced Fermi system the
transition temperature is suppressed by a factor . For imbalanced
Fermi systems, the polarization and transition temperature of the tricritical
point are significantly reduced as the two-body binding energy
increases.Comment: 6 pages, 3 figure
Ehrenfest time in the weak dynamical localization
The quantum kicked rotor (QKR) is known to exhibit dynamical localization in
the space of its angular momentum. The present paper is devoted to the
systematic first--principal (without a regularizer) diagrammatic calculations
of the weak--localization corrections for QKR. Our particular emphasis is on
the Ehrenfest time regime -- the phenomena characteristic for the
classical--to--quantum crossover of classically chaotic systems.Comment: 27 pages, 9 figure
Normal State Nernst Effect in Electron-doped Pr2-xCexCuO4: Superconducting Fluctuations and Two-band Transport
We report a systematic study of normal state Nernst effect in the
electron-doped cuprates PrCeCuO over a wide range of
doping (0.050.21) and temperature. At low temperatures, we
observed a notable vortex Nernst signal above T in the underdoped films,
but no such normal state vortex Nernst signal is found in the overdoped region.
The superconducting fluctuations in the underdoped region are most likely
incoherent phase fluctuations as found in hole-doped cuprates. At high
temperatures, a large normal state Nernst signal is found at dopings from
slightly underdoped to highly overdoped. Combined with normal state
thermoelectric power, Hall effect and magnetoresistance measurements, the large
Nernst effect is compatible with two-band model. For the highly overdoped
films, the large Nernst effect is anomalous and not explainable with a simple
hole-like Fermi surface seen in photoemission experiments.Comment: 9 pages, 8 figures, accepted in PR
Fulde-Ferrell-Larkin-Ovchinnikov Superconducting State in CeCoIn5
We report specific heat measurements of the heavy fermion superconductor
CeCoIn5 in the vicinity of the superconducting critical field H_{c2}, with
magnetic field in the [110], [100], and [001] directions, and at temperatures
down to 50 mK. The superconducting phase transition changes from second to
first order for field above 10 T for H || [110] and H || [100]. In the same
range of magnetic field we observe a second specific heat anomaly within the
superconducting state. We interpret this anomaly as a signature of a
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) inhomogeneous superconducting state. We
obtain similar results for H || [001], with FFLO state occupying a smaller part
of the phase diagram.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Letter
The Renormalization Group and the Superconducting Susceptibility of a Fermi Liquid
A free Fermi gas has, famously, a superconducting susceptibility that
diverges logarithmically at zero temperature. In this paper we ask whether this
is still true for a Fermi liquid and find that the answer is that it does {\it
not}. From the perspective of the renormalization group for interacting
fermions, the question arises because a repulsive interaction in the Cooper
channel is a marginally irrelevant operator at the Fermi liquid fixed point and
thus is also expected to infect various physical quantities with logarithms.
Somewhat surprisingly, at least from the renormalization group viewpoint, the
result for the superconducting susceptibility is that two logarithms are not
better than one. In the course of this investigation we derive a
Callan-Symanzik equation for the repulsive Fermi liquid using the
momentum-shell renormalization group, and use it to compute the long-wavelength
behavior of the superconducting correlation function in the emergent low-energy
theory. We expect this technique to be of broader interest.Comment: 9 pages, 2 figure
Dynamics of vortex penetration, jumpwise instabilities and nonlinear surface resistance of type-II superconductors in strong rf fields
We consider nonlinear dynamics of a single vortex in a superconductor in a
strong rf magnetic field . Using the London theory, we
calculate the dissipated power , and the transient time scales
of vortex motion for the linear Bardeen-Stephen viscous drag force, which
results in unphysically high vortex velocities during vortex penetration
through the oscillating surface barrier. It is shown that penetration of a
single vortex through the ac surface barrier always involves penetration of an
antivortex and the subsequent annihilation of the vortex antivortex pairs.
Using the nonlinear Larkin-Ovchinnikov (LO) viscous drag force at higher vortex
velocities results in a jump-wise vortex penetration through the surface
barrier and a significant increase of the dissipated power. We calculate the
effect of dissipation on nonlinear vortex viscosity and the rf vortex
dynamics and show that it can also result in the LO-type behavior,
instabilities, and thermal localization of penetrating vortex channels. We
propose a thermal feedback model of , which not only results in the LO
dependence of for a steady-state motion, but also takes into account
retardation of temperature field around rapidly accelerating vortex, and a
long-range interaction with the surface. We also address the effect of pinning
on the nonlinear rf vortex dynamics and the effect of trapped magnetic flux on
the surface resistance calculated as a function or rf frequency and
field. It is shown that trapped flux can result in a temperature-independent
residual resistance at low , and a hysteretic low-field dependence of
, which can {\it decrease} as is increased, reaching a minimum
at much smaller than the thermodynamic critical field .Comment: 18 figure
- …