7,549 research outputs found
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
A constrained random-force model for weakly bending semiflexible polymers
The random-force (Larkin) model of a directed elastic string subject to
quenched random forces in the transverse directions has been a paradigm in the
statistical physics of disordered systems. In this brief note, we investigate a
modified version of the above model where the total transverse force along the
polymer contour and the related total torque, in each realization of disorder,
vanish. We discuss the merits of adding these constraints and show that they
leave the qualitative behavior in the strong stretching regime unchanged, but
they reduce the effects of the random force by significant numerical
prefactors. We also show that a transverse random force effectively makes the
filament softer to compression by inducing undulations. We calculate the
related linear compression coefficient in both the usual and the constrained
random force model.Comment: 4 pages, 1 figure, accepted for publication in PR
Thermal conductivity in a mixed state of a superconductor at low magnetic fields
We evaluate accurate low-field/low-temperature asymptotics of the thermal
conductivity perpendicular to magnetic field for one-band and two-band s-wave
superconductors using Keldysh-Usadel formalism. We show that heat transport in
this regime is limited by tunneling of quasiparticles between adjacent vortices
across a number of local points and therefore widely-used approximation of
averaging over circular unit cell is not valid. In the single-band case, we
obtain parameter-free analytical solution which provides theoretical lower
limit for heat transport in the mixed state. In the two-band case, we show that
heat transport is controlled by the ratio of gaps and diffusion constants in
different bands. Presence of a weaker second band strongly enhances the thermal
conductivity at low fieldsComment: 7 pages, 1 figure, discussion of the clean case and discussion of
experiment adde
Two-bands effect on the superconducting fluctuating diamagnetism in MgB₂
The field dependence of the magnetization above the transition temperature Tc
in MgB₂ is shown to evidence a diamagnetic contribution consistent with
superconducting fluctuations reflecting both the σ and π bands. In
particular, the upturn field Hup in the magnetization curve, related to the
incipient effect of the magnetic field in quenching the fluctuating pairs,
displays a double structure, in correspondence to two correlation lengths. The
experimental findings are satisfactorily described by the extension to the
diamagnetism of a recent theory for paraconductivity, in the framework of a
zero-dimensional model for the fluctuating superconducting droplets above Tc
Suppression or enhancement of the Fulde-Ferrell-Larkin-Ovchinnikov order in a one-dimensional optical lattice with particle correlated tunnelling
We study through controlled numerical simulation the ground state properties
of spin-polarized strongly interacting fermi gas in an anisotropic optical
lattice, which is described by an effective one-dimensional general Hubbard
model with particle correlated hopping rate. We show that the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type of state, while enhanced by a
negative correlated hopping rate, can be completely suppressed by positive
particle correlated hopping, yielding to an unusual magnetic phase even for
particles with on-site attractive interaction We also find several different
phase separation patterns for these atoms in an inhomogeneous harmonic trap,
depending on the correlated hopping rate
Electrodynamics of Fulde-Ferrell-Larkin-Ovchinnikov superconducting state
We develop the Ginzburg-Landau theory of the vortex lattice in clean
isotropic three-dimensional superconductors at large Maki parameter, when
inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov state is favored. We show that
diamagnetic superfluid currents mainly come from paramagnetic interaction of
electron spins with local magnetic field, and not from kinetic energy response
to the external field as usual. We find that the stable vortex lattice keeps
its triangular structure as in usual Abrikosov mixed state, while the internal
magnetic field acquires components perpendicular to applied magnetic field.
Experimental possibilities related to this prediction are discussed.Comment: 5 pages, 1 figur
Fluctuoscopy of Disordered Two-Dimensional Superconductors
We revise the long studied problem of fluctuation conductivity (FC) in
disordered two-dimensional superconductors placed in a perpendicular magnetic
field by finally deriving the complete solution in the temperature-magnetic
field phase diagram. The obtained expressions allow both to perform
straightforward (numerical) calculation of the FC surface
and to get asymptotic expressions in
all its qualitatively different domains. This surface becomes in particular
non-trivial at low temperatures, where it is trough-shaped with . In this region, close to the
quantum phase transition,
is non-monotonic, in
agreement with experimental findings. We reanalyzed and present comparisons to
several experimental measurements. Based on our results we derive a qualitative
picture of superconducting fluctuations close to and T=0
where fluctuation Cooper pairs rotate with cyclotron frequency
and Larmor radius , forming some kind of quantum liquid with long coherence
length and slow relaxation
().Comment: 26 pages, 13 figures, 3 tables, RevTex 4.
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
Conventional and charge six superfluids from melting hexagonal Fulde-Ferrell-Larkin-Ovchinnikov phases in two dimensions
We consider defect mediated melting of Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) and pair density wave (PDW) phases in two dimensions. Examining
mean-field ground states in which the spatial oscillations of the FFLO/PDW
superfluid order parameter exhibit hexagonal lattice symmetry, we find that
thermal melting leads to a variety of novel phases. We find that a spatially
homogeneous charge six superfluid can arise from melting a hexagonal
vortex-anitvortex lattice FFLO/PDW phase. The charge six superfluid has an
order parameter corresponding to a bound state of six fermions. We further find
that a hexagonal vortex-free FFLO/PDW phase can melt to yield a conventional
(charge two) homogeneous superfluid. A key role is played by topological
defects that combine fractional vortices of the superfluid order and fractional
dislocations of the lattice order.Comment: 8 pages, 3 figure
- …