620 research outputs found
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
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
Superfluidity and excitations at unitarity
We present lattice results for spin-1/2 fermions at unitarity, where the
effective range of the interaction is zero and the scattering length is
infinite. We measure the spatial coherence of difermion pairs for a system of
6, 10, 14, 18, 22, 26 particles with equal numbers of up and down spins in a
periodic cube. Using Euclidean time projection, we analyze ground state
properties and transient behavior due to low-energy excitations. At
asymptotically large values of t we see long-range order consistent with
spontaneously broken U(1) fermion-number symmetry and a superfluid ground
state. At intermediate times we see exponential decay in the t-dependent signal
due to an unknown low-energy excitation. We probe this low-energy excitation
further by calculating two-particle correlation functions. We find that the
excitation has the properties of a chain of particles extending across the
periodic lattice.Comment: 40 pages, 19 figures, revised version includes new data on
two-particle density correlation
Dissipation and coherent effects in narrow superconducting channels
We apply the time dependent Ginzburg-Landau equations (TDGL) to study small
ac currents of frequency in superconducting channels narrow on the
scale of London penetration depth. We show that TDGL have -dependent and
spatially uniform solutions that describe the order parameter with an
oscillating part of the double frequency coexisting with an ac electric field.
We evaluate the Ohmic losses (related neither to the flux flow nor to the phase
slips) and show that the resistivity reduction on cooling through the critical
temperature should behave as . If the channel is cut
out of an anisotropic material in a direction other than the principal axes,
the transverse phase difference and the Josephson voltage between the channel
sides are generated.Comment: 5 pages, 1 figures, Accepted for publication in Phys. Rev.
Motion and homogenization of vortices in anisotropic Type II superconductors
The motion of vortices in an anisotropic superconductor is considered. For a system of well-separated vortices, each vortex is found to obey a law of motion analogous to the local induction approximation, in which velocity of the vortex depends upon the local curvature and orientation. A system of closely packed vortices is then considered, and a mean field model is formulated in which the individual vortex lines are replaced by a vortex density
Multiple phase slips phenomena in mesoscopic superconducting rings
We investigate the behavior of a mesoscopic one-dimensional ring in an
external magnetic field by simulating the time dependent Ginzburg-Landau
equations with periodic boundary conditions. We analyze the stability and the
different possible evolutions for the phase slip phenomena starting from a
metastable state. We find a stability condition relating the winding number of
the initial solution and the number of flux quanta penetrating the ring. The
analysis of multiple phase slips solutions is based on analytical results and
simulations. The role of the ratio of two characteristic times u is studied for
the case of a multiple phase slips transition. We found out that if u>>1,
consecutive multiple phase slips will be more favorable than simultaneous ones.
If u>1 is often a necessary
condition to reach the ground state. The influence of the Langevin noise on the
kinetics of the phase transition is discussed.Comment: 8 pages, 6 figure
Coherent backscattering of Bose-Einstein condensates in two-dimensional disorder potentials
We study quantum transport of an interacting Bose-Einstein condensate in a
two-dimensional disorder potential. In the limit of vanishing atom-atom
interaction, a sharp cone in the angle-resolved density of the scattered matter
wave is observed, arising from constructive interference between amplitudes
propagating along reversed scattering paths. Weak interaction transforms this
coherent backscattering peak into a pronounced dip, indicating destructive
instead of constructive interference. We reproduce this result, obtained from
the numerical integration of the Gross-Pitaevskii equation, by a diagrammatic
theory of weak localization in presence of a nonlinearity.Comment: 4 pages, 4 figure
Free energy and torque for superconductors with different anisotropies of H_{c2} and lambda
The free energy is evaluated for a uniaxial superconductor with the
anisotropy of the upper critical field, gamma_H = H_{c2,ab}/H_{c2,c}, different
from the anisotropy of the penetration depth gamma_{lambda} =
lambda_c/lambda_{ab}. With increasing difference between gamma_H and
gamma_{lambda}, the equilibrium orientation of the crystal relative to the
applied field may shift from theta = pi/2 (theta is the angle between the field
and the c axis) to lower angles and reach theta = 0 for large enough gamma_H.
These effects are expected to take place in MgB_2.Comment: 4 pages, 3 fig
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