325 research outputs found
Algorithm for obtaining the gradient expansion of the local density of states and the free energy of a superconductor
We present an efficient algorithm for obtaining the gauge-invariant gradient
expansion of the local density of states and the free energy of a clean
superconductor. Our method is based on a new mapping of the semiclassical
linearized Gorkov equations onto a pseudo-Schroedinger equation for a
three-component wave-function psi(x), where one component is directly related
to the local density of states. Because psi(x) satisfies a linear equation of
motion, successive terms in the gradient expansion can be obtained by simple
linear iteration. Our method works equally well for real and complex order
parameter, and in the presence of arbitrary external fields. We confirm a
recent calculation of the fourth order correction to the free energy by
Kosztin, Kos, Stone and Leggett [Phys. Rev. B 58, 9365 (1998)], who obtained a
discrepancy with an earlier result by Tewordt [Z. Phys. 180, 385 (1964)]. We
also give the fourth order correction to the local density of states, which has
not been published before.Comment: 12 preprint pages, added remark concerning Eilenberger equation,
accepted for publication in Phys. Rev.
Magnetic field of Josephson vortices outside superconductors
We consider the structure of Josephson vortices approaching the junction
boundary with vacuum in large area Josephson junctions with the Josephson
length large relative to the London penetration depth .
Using the stability argument for one-dimentional solitons with respect to 2D
perturbations, it is shown that on the scale the Josephson vortices
do not spread near the boundary in the direction of the junction. %, which is
in a striking difference with behavior of Abrikosov vortices exiting
superconductors. The field distribution in vacuum due to the Josephson vortex
is evaluated, the information needed for the Scanning SQUID Microscopy.Comment: 5 RevTeX pages, 3 eps figures. The second version includes more
detailed explanations and corrections, and slightly modified figure
Thermodynamic Potential for Superfluid 3He in Aerogel
We present a free energy functional for superfluid 3He in the presence of
homogeneously distributed impurity disorder which extends the Ginzburg-Landau
free energy functional to all temperatures. We use the new free energy
functional to calculate the thermodynamic potential, entropy, heat capacity and
density of states for the B-phase of superfluid 3He in homogeneous, isotropic
aerogel.Comment: 10 pages, 4 figure
Theory of the Normal/Superfluid interface in population imbalanced Fermi gases
We present a series of theoretical studies of the boundary between a
superfluid and normal region in a partially polarized gas of strongly
interacting fermions. We present mean-field estimates of the surface energy in
this boundary as a function of temperature and scattering length. We discuss
the structure of the domain wall, and use a previously introduced
phenomonological model to study its influence on experimental observables.
Our microscopic mean-field calculations are not consistent with the magnitude
of the surface tension found from our phenomonological modelling of data from
the Rice experiments. We conclude that one must search for novel mechanisms to
explain the experiments.Comment: 15 pages, 9 figures (13 subfigures) -- v2: minor change
Quantum tunneling between paramagnetic and superconducting states of a nanometer-scale superconducting grain placed in a magnetic field
We consider the process of quantum tunneling between the superconducting and
paramagnetic states of a nanometer-scale superconducting grain placed in a
magnetic field. The grain is supposed to be coupled via tunneling junction to a
normal metallic contact that plays a role of the spin reservoir. Using the
instanton method we find the probability of the quantum tunneling process and
express it in terms of the applied magnetic field, order parameter of the
superconducting grain and conductance of the tunneling junction between the
grain and metallic contact
Anisotropy of the upper critical field in superconductors with anisotropic gaps. Anisotropy parameters of MgB2
The upper critical field Hc2 is evaluated for weakly-coupled two-band
superconductors. By modeling the actual bands and the gap distribution of MgB2
by two Fermi surface spheroids with average parameters of the real material, we
show that H_{c2,ab}/H_{c2,c} increases with decreasing temperature in agreement
with available data.Comment: 4 pages, 2 figure
Reentrant vortex lattice transformation in four-fold symmetric superconductors
The physics behind the rhombicsquarerhombic flux line lattice
transformation in increasing fields is clarified on the basis of Eilenberger
theory. We demonstrate that this reentrance observed in LuNiBC is due
to intrinsic competition between superconducting gap and Fermi surface
anisotropies. The calculations reproduce not only it but also predict yet not
found lock-in transition to a square lattice with different orientation in
higher field. In view of physical origin given, this sequence of transitions is
rather generic to occur in four-fold symmetric superconductors.Comment: 5 pages, 4 figures,submitted to Phys. Rev. Let
The Gradient Expansion for the Free-Energy of a Clean Superconductor
We describe a novel method for obtaining the gradient expansion for the free
energy of a clean BCS superconductor. We present explicit results up to fourth
order in the gradients of the order parameter.Comment: 33 pages, Late
Gauge Invariance and Hall Terms in the Quasiclassical Equations of Superconductivity
This paper presents a careful derivation of the quasiclassical equations of
superconductivity so that a manifest gauge invariance is retained with respect
to the space-time arguments of the quasiclassical Green's function .
The terms responsible for the Hall effect naturally appear from the derivation.
The equations are applicable to clean as well as dirty superconductors for an
arbitrary external frequency much smaller than the Fermi energy. Thus, they
will form a basis toward a complete microscopic understanding of the Hall
effect in type-II superconductors.Comment: 9 pages, 1 figur
A Two-dimensional Superconductor in a Tilted Magnetic Field - new states with finite Cooper-pair momentum
Varying the angle Theta between applied field and the conducting planes of a
layered superconductor in a small interval close to the plane-parallel field
direction, a large number of superconducting states with unusual properties may
be produced. For these states, the pair breaking effect of the magnetic field
affects both the orbital and the spin degree of freedom. This leads to pair
wave functions with finite momentum, which are labeled by Landau quantum
numbers 0<n<\infty. The stable order parameter structure and magnetic field
distribution for these states is found by minimizing the quasiclassical free
energy near H_{c2} including nonlinear terms. One finds states with coexisting
line-like and point-like order parameter zeros and states with coexisting
vortices and antivortices. The magnetic response may be diamagnetic or
paramagnetic depending on the position within the unit cell. The structure of
the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states at Theta=0 is reconsidered.
The transition n->\infty of the paramagnetic vortex states to the FFLO-limit is
analyzed and the physical reason for the occupation of higher Landau levels is
pointed out.Comment: 24 pages, 11 figure
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