2,952 research outputs found
Transient heat and mass transfer analysis of supercritical cryogenic storage systems with spherical static heaters Final report
Transient heat and mass transfer analysis of supercritical cryogenic storage systems with spherical static heaters by computer progra
BCS pairing in Fermi systems with several flavors
Motivated by the prospect of Bardeen-Cooper-Schrieffer (BCS) pairing in cold
fermionic gases we analyze the superfluid phase of 3 fermionic flavors in the
attractive Hubbard model. We show that there are several low--lying collective
pairing modes and investigate their damping due to the partially gapless nature
of the single-particle spectrum. Furthermore we analyze how these modes show up
in the density response of the system. Apart from the Anderson-Bogoliubov phase
mode of the pairing between two flavors, the dynamical structure factor
contains signatures of the gapless third flavor. This picture is found to be
robust against perturbations that break the global SU(3)-symmetry of the
Hamiltonian.Comment: 13 pages, 6 figure
Two-scale localization in disordered wires in a magnetic field
Calculating the density-density correlation function for disordered wires, we
study localization properties of wave functions in a magnetic field. The
supersymmetry technique combined with the transfer matrix method is used. It is
demonstrated that at arbitrarily weak magnetic field the far tail of the wave
functions decays with the length , where and are the localization lengths in the absence of a
magnetic field and in a strong magnetic field, respectively. At shorter
distances, the decay of the wave functions is characterized by the length
. Increasing the magnetic field broadens the region of the decay
with the length , leading finally to the decay with at all distances. In other words, the crossover between the orthogonal
and unitary ensembles in disordered wires is characterized by two localization
lengths. This peculiar behavior must result in two different temperature
regimes in the hopping conductivity with the boundary between them depending on
the magnetic field.Comment: 4 page
Considerable enhancement of the critical current in a superconducting film by magnetized magnetic strip
We show that a magnetic strip on top of a superconducting strip magnetized in
a specified direction may considerably enhance the critical current in the
sample. At fixed magnetization of the magnet we observed diode effect - the
value of the critical current depends on the direction of the transport
current. We explain these effects by a influence of the nonuniform magnetic
field induced by the magnet on the current distribution in the superconducting
strip. The experiment on a hybrid Nb/Co structure confirmed the predicted
variation of the critical current with a changing value of magnetization and
direction of the transport current.Comment: 6 pages, 7 figure
Identifying spin-triplet pairing in spin-orbit coupled multi-band superconductors
We investigate the combined effect of Hund's and spin-orbit (SO) coupling on
superconductivity in multi-orbital systems. Hund's interaction leads to
orbital-singlet spin-triplet superconductivity, where the Cooper pair wave
function is antisymmetric under the exchange of two orbitals. We identify three
d-vectors describing even-parity orbital-singlet spin-triplet pairings among
t2g-orbitals, and find that the three d-vectors are mutually orthogonal to each
other. SO coupling further assists pair formation, pins the orientation of the
d-vector triad, and induces spin-singlet pairings with a relative phase
difference of \pi/2. In the band basis the pseudospin d-vectors are aligned
along the z-axis and correspond to momentum-dependent inter- and intra-band
pairings. We discuss quasiparticle dispersion, magnetic response, collective
modes, and experimental consequences in light of the superconductor Sr2RuO4.Comment: 6 pages, 5 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
Reentrant behavior of superconducting alloys
A dirty BCS superconductor with magnetic impurities is studied. Asymptotic
solution of the thermodynamics of such superconductor with spin and
magnetic impurities, is found. To this end, the system's free energy is bounded from above and below by mean-field type bounds, which are
shown to coalesce almost exactly in the thermodynamic limit, provided the
impurity concentration is sufficiently small. The resulting mean-field
equations for the gap and a parameter , characterizing the
impurity subsystem, are solved and the solution minimizing is found for
various values of magnetic coupling constant and impurity concentration
. The phase diagrams of the system are depicted with five distinct phases:
the normal phase, unperturbed superconducting phase, perturbed superconducting
phase with nonzero gap in the excitation spectrum, perturbed gapless
superconducting phase and impurity phase with completely suppressed
superconductivity. Furthermore, evidence of reentrant superconductivity and
Jaccarino-Peter compensation is found. The credibility of the theory is
verified by testing the dependence of the superconducting transition
temperature on . Very good quantitative agreement with
experimental data is obtained for several alloys: (LaCe)Al,
(LaGd)Al and (LaY)Ce. The theory
presented improves earlier developments in this field.Comment: 15 pages, 8 figures, full length articl
Upper critical field divergence induced by mesoscopic phase separation in the organic superconductor (TMTSF)2ReO4
Due to the competition of two anion orders, (TMTSF)2ReO4, presents a phase
coexistence between semiconducting and metallic (superconducting) regions
(filaments or droplets) in a wide range of pressure. In this regime, the
superconducting upper critical field for H parallel to both c* and b' axes
present a linear part at low fields followed by a divergence above a cross-over
field. This cross-over corresponds to the 3D-2D decoupling transition expected
in filamentary or granular superconductors. The sharpness of the transition
also demonstrates that all filaments are of similar sizes and self organize in
a very ordered way. The distance between the filaments and their cross-section
are estimated.Comment: 4 pages, 4 figure
Hall Coefficient of Equilibrium Supercurrents Flowing inside Superconductors
We study augmented quasiclassical equations of superconductivity with the
Lorentz force, which is missing from the standard Ginzburg-Landau and
Eilenberger equations. It is shown that the magnetic Lorentz force on
equilibrium supercurrents induces finite charge distribution and the resulting
electric field to balance the Lorentz force. An analytic expression is obtained
for the corresponding Hall coefficient of clean type-II superconductors with
simultaneously incorporating the Fermi-surface and gap anisotropies. It has the
same sign and magnitude at zero temperature as the normal state for an
arbitrary pairing, having no temperature dependence specifically for the s-wave
pairing. The gap anisotropy may bring a considerable temperature dependence in
the Hall coefficient and can lead to its sign change as a function of
temperature, as exemplified for a model d-wave pairing with a two-dimensional
Fermi surface. The sign change may be observed in some high-
superconductors.Comment: 7 pages, 3 figure
Hall Effect in the mixed state of moderately clean superconductors
The Hall conductivity in the mixed state of a clean () type-II
s-wave superconductor is determined from a microscopic calculation within a
quasiclassical approximation. We find that below the superconducting transition
the contribution to the transverse conductivity due to dynamical fluctuations
of the order parameter is compensated by the modification of the quasiparticle
contribution. In this regime the nonlinear behaviour of the Hall angle is
governed by the change in the effective quasiparticle scattering rate due to
the reduction in the density of states at the Fermi level. The connection with
experimental results is discussed
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