459 research outputs found
Why magnesium diboride is not described by anisotropic Ginzburg-Landau theory
It is well established that the superconductivity in the recently discovered
superconducting compound MgB resides in the quasi-two-dimensional band
(-band) and three-dimensional band (-band). We demonstrate that,
due to such band structure, the anisotropic Ginzburg-Landau theory practically
does not have region of applicability, because gradient expansion in the
direction breaks down. In the case of dirty -band we derive the simplest
equations which describe properties of such superconductors near and
explore some consequences of these equations.Comment: 4 pages, 2 fugures, Subm. Phys. Rev. Let
Fictitious Level Dynamics: A Novel Approach to Spectral Statistics in Disordered Conductors
We establish a new approach to calculating spectral statistics in disordered
conductors, by considering how energy levels move in response to changes in the
impurity potential. We use this fictitious dynamics to calculate the spectral
form factor in two ways. First, describing the dynamics using a Fokker-Planck
equation, we make a physically motivated decoupling, obtaining the spectral
correlations in terms of the quantum return probability. Second, from an
identity which we derive between two- and three-particle correlation functions,
we make a mathematically controlled decoupling to obtain the same result. We
also calculate weak localization corrections to this result, and show for two
dimensional systems (which are of most interest) that corrections vanish to
three-loop order.Comment: 35 pages in REVTeX format including 10 postscript figures; to be
published in a special issue (on Topics in Mesoscopic Physics) of the Journal
of Mathematical Physics, October 199
Theory of fluctuations in a two-band superconductor
A theory of fluctuations in two-band superconductor MgB is developed.
Since the standard Ginzburg-Landau (GL) approach fails in description of its
properties, we generalize it basing on the microscopic theory of a two-band
superconductor. Calculating the microscopic fluctuation propagator, we build up
the nonlocal two-band GL functional and the corresponding time-dependent GL
equations. This allows us to calculate the main fluctuation observables such as
fluctuation specific heat and conductivity.Comment: 11 pages, 2 figures, subm. to Phys. Rev.
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
Gapless Fermi Surfaces in anisotropic multiband superconductors in magnetic field
We propose that a new state with a fully gapless Fermi surface appears in
quasi-2D multiband superconductors in magnetic field applied parallel to the
plane. It is characterized by a paramagnetic moment caused by a finite density
of states on the open Fermi surface. We calculate thermodynamic and magnetic
properties of the gapless state for both s-wave and d-wave cases, and discuss
the details of the 1-st order metamagnetic phase transition that accompanies
the appearance of the new phase in s-wave superconductors. We suggest possible
experiments to detect this state both in the s-wave (2-H NbSe2) and d-wave
(CeCoIn5) superconductors.Comment: 4 pages, 2 figure
Pseudo diamagnetism of four component exciton condensates
We analyze the spin structure of the ground state of four-component exciton
condensates in coupled quantum wells as a function of spin-dependent
interactions and applied magnetic field. The four components correspond to the
degenerate exciton states characterized by and spin projections
to the axis of the structure. We show that in a wide range of parameters, the
chemical potential of the system increases as a function of magnetic field,
which manifests a pseudo-diamagnetism of the system. The transitions to
polarized two- and one-component condensates can be of the first-order in this
case. The predicted effects are caused by energy conserving mixing of
and excitons.Comment: 4 pages, 2 figure
Magnetic properties of superconductors with strong spin-orbit coupling
We study the response of a superconductor with a strong spin-orbit coupling
on an external magnetic field. The Ginzburg-Landau free energy functional is
derived microscopically for a general crystal structure, both with and without
an inversion center, and for an arbitrary symmetry of the superconducting order
parameter. As a by-product, we obtain the general expressions for the intrinsic
magnetic moment of the Cooper pairs. It is shown that the Ginzburg-Landau
gradient energy in a superconductor lacking inversion symmetry has unusual
structure. The general formalism is illustrated using as an example CePtSi,
which is the first known heavy-fermion superconductor without an inversion
center.Comment: Published version, 14 pages, minor correction
Nonuniform mixed-parity superfluid state in Fermi gases
We study the effects of dipole interaction on the superfluidity in a
homogeneous Fermi gas with population imbalance. We show that the
Larkin-Ovchinnikov-Fulde-Ferrell phase is replaced by another nonuniform
superfluid phase, in which the order parameter has a nonzero triplet component
induced by the dipole interaction.Comment: 4 pages, 1 figur
Vortex core deformation and stepper motor behavior in a superconducting ratchet
We investigated experimentally the frequency dependence of a superconducting
vortex ratchet effect by means of electrical transport measurements and modeled
it theoretically using the time dependent Ginzburg-Landau formalism. We
demonstrate that the high frequency vortex behavior can be described as a
discrete motion of a particle in a periodic potential, i.e. the so called
stepper motor behavior. Strikingly, in the more conventional low frequency
response a transition takes place from an Abrikosov vortex rectifier to a phase
slip line rectifier. This transition is characterized by a strong increase in
the rectified voltage and the appearance of a pronounced hysteretic behavior.Comment: 4 pages, 4 figure
Comparison between a diagrammatic theory for the BCS-BEC crossover and Quantum Monte Carlo results
Predictions for the chemical potential and the excitation gap recently
obtained by our diagrammatic theory for the BCS-BEC crossover in the superfluid
phase are compared with novel Quantum Monte Carlo results at zero temperature
now available in the literature. A remarkable agreement is found between the
results obtained by the two approachesComment: 3 pages, 2 figure
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