4,382 research outputs found
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
Upper critical field from normal state fluctuations in BiSrCuO
The in-plane magnetoresistance of an epitaxial BiSrCuO
thin film was systematically investigated as a function of doping, above .
The orbital magnetoconductance is used to extract the crossover field line
in the fluctuation regime. This field is found in good agreement
with the upper critical field obtained from resistivity data below , and
exhibits a similar upward curvature, thus pointing toward the existence of a
critical correlation length. The consequences regarding the nature of the
resistive transition are discussed
Long-range thermoelectric effects in mesoscopic superconductor-normal metal structures
We consider a mesoscopic four-terminal superconductor/normal metal (S/N)
structure in the presence of a temperature gradient along the N wire. A
thermoemf arises in this system even in the absence of the thermoelectric
quasiparticle current if the phase difference between the superconductors is
not zero. We show that the thermoemf is not small in the case of a negligible
Josephson coupling between two superconductors. It is also shown that the
thermoelectric voltage has two maxima: one at a low temperature and another at
a temperature close to the critical temperature. The obtained temperature
dependence of the thermoemf describes qualitatively experimental data.Comment: 9 pages, 6 figure
Spin Screening and Antiscreening in a Ferromagnet/Superconductor Heterojunction
We present a theoretical study of spin screening effects in a
ferromagnet/superconductor (F/S) heterojunction. It is shown that the magnetic
moment of the ferromagnet is screened or antiscreened, depending on the
polarization of the electrons at the Fermi level. If the polarization is
determined by the electrons of the majority (minority) spin band then the
magnetic moment of the ferromagnet is screened (antiscreened) by the electrons
in the superconductor. We propose experiments that may confirm our theory: for
ferromagnetic alloys with certain concentration of Fe or Ni ions there will be
screening or antiscreening respectively. Different configurations for the
density of states are also discussed.Comment: 5 pages; 4 figures. to be published in Phys. Rev,
Long-time behavior of the momentum distribution during the sudden expansion of a spin-imbalanced Fermi gas in one dimension
We study the sudden expansion of spin-imbalanced ultracold lattice fermions
with attractive interactions in one dimension after turning off the
longitudinal confining potential. We show that the momentum distribution
functions of majority and minority fermions approach stationary values quickly
due to a quantum distillation mechanism that results in a spatial separation of
pairs and majority fermions. As a consequence, Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) correlations are lost during the expansion. Furthermore, we argue that
the shape of the stationary momentum distribution functions can be understood
by relating them to the integrals of motion in this integrable quantum system.
We discuss our results in the context of proposals to observe FFLO
correlations, related to recent experiments by Liao et al., Nature 467, 567
(2010).Comment: 8 pages including supplementary material, 9 eps figures, revised
version as published, some text moved to the supplemental materia
Effect of magnetic pair breaking on Andreev bound states and resonant supercurrent in quantum dot Josephson junctions
We propose a model for resonant Josephson tunneling through quantum dots that
accounts for Cooper pair-breaking processes in the superconducting leads caused
by a magnetic field or spin-flip scattering. The pair-breaking effect on the
critical supercurrent and the Josephson current-phase relation
is largely due to the modification of the spectrum of Andreev bound states
below the reduced (Abrikosov-Gorkov) quasiparticle gap. For a quantum dot
formed in a quasi-one-dimensional channel, both and can show a
significant magnetic field dependence induced by pair breaking despite the
suppression of the orbital magnetic field effect in the channel. This case is
relevant to recent experiments on quantum dot Josephson junctions in carbon
nanotubes. Pair-breaking processes are taken into account via the relation
between the Andreev scattering matrix and the quasiclassical Green functions of
the superconductors in the Usadel limit.Comment: 5 pages, 6 eps figures, new results adde
The Amplitude of Non-Equilibrium Quantum Interference in Metallic Mesoscopic Systems
We study the influence of a DC bias voltage V on quantum interference
corrections to the measured differential conductance in metallic mesoscopic
wires and rings. The amplitude of both universal conductance fluctuations (UCF)
and Aharonov-Bohm effect (ABE) is enhanced several times for voltages larger
than the Thouless energy. The enhancement persists even in the presence of
inelastic electron-electron scattering up to V ~ 1 mV. For larger voltages
electron-phonon collisions lead to the amplitude decaying as a power law for
the UCF and exponentially for the ABE. We obtain good agreement of the
experimental data with a model which takes into account the decrease of the
electron phase-coherence length due to electron-electron and electron-phonon
scattering.Comment: New title, refined analysis. 7 pages, 3 figures, to be published in
Europhysics Letter
Nonuniform Self-Organized Dynamical States in Superconductors with Periodic Pinning
We consider magnetic flux moving in superconductors with periodic pinning
arrays. We show that sample heating by moving vortices produces negative
differential resistivity (NDR) of both N and S type (i.e., N- and S-shaped) in
the voltage-current characteristic (VI curve). The uniform flux flow state is
unstable in the NDR region of the VI curve. Domain structures appear during the
NDR part of the VI curve of an N type, while a filamentary instability is
observed for the NDR of an S type. The simultaneous existence of the NDR of
both types gives rise to the appearance of striking self-organized (both
stationary and non-stationary) two-dimensional dynamical structures.Comment: 4 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
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