288 research outputs found
Spontaneous magnetization and Hall effect in superconductors with broken time-reversal symmetry
Broken time reversal symmetry (BTRS) in d wave superconductors is studied and
is shown to yield current carrying surface states. The corresponding
spontaneous magnetization is temperature independent near the critical
temperature Tc for weak BTRS, in accord with recent data. For strong BTRS and
thin films we expect a temperature dependent spontaneous magnetization with a
paramagnetic anomaly near Tc. The Hall conductance is found to vanish at zero
wavevector q and finite frequency w, however at finite q,w it has an unusual
structure.Comment: 7 pages, 1 eps figure, Europhysics Letters (in press
A Classification of random Dirac fermions
We present a detailed classification of random Dirac hamiltonians in two
spatial dimensions based on the implementation of discrete symmetries. Our
classification is slightly finer than that of random matrices, and contains
thirteen classes. We also extend this classification to non-hermitian
hamiltonians with and without Dirac structure.Comment: 15 pages, version2: typos in the table of classes are correcte
Winding of planar gaussian processes
We consider a smooth, rotationally invariant, centered gaussian process in
the plane, with arbitrary correlation matrix . We study the winding
angle around its center. We obtain a closed formula for the variance
of the winding angle as a function of the matrix . For most stationary
processes the winding angle exhibits diffusion at large time
with diffusion coefficient .
Correlations of with integer , the distribution of the
angular velocity , and the variance of the algebraic area are also
obtained. For smooth processes with stationary increments (random walks) the
variance of the winding angle grows as , with proper
generalizations to the various classes of fractional Brownian motion. These
results are tested numerically. Non integer is studied numerically.Comment: 12 pages, 6 figure
Abrupt Change of Josephson Plasma Frequency at the Phase Boundary of the Bragg Glass in Bi_2Sr_2CaCu_2O_{8+\delta}
We report the first detailed and quantitative study of the Josephson coupling
energy in the vortex liquid, Bragg glass and vortex glass phases of
Bi_2Sr_2CaCu_2O_{8+\delta} by the Josephson plasma resonance. The measurements
revealed distinct features in the T- and H-dependencies of the plasma frequency
for each of these three vortex phases. When going across either
the Bragg-to-vortex glass or the Bragg-to-liquid transition line,
shows a dramatic change. We provide a quantitative discussion on the properties
of these phase transitions, including the first order nature of the
Bragg-to-vortex glass transition.Comment: 5pages, 4figure
On the Decoupling of Layered Superconducting Films in Parallel Magnetic Field
The issue of the decoupling of extreme type-II superconducting thin films
() with weakly Josephson-coupled layers in magnetic
field parallel to the layers is considered via the corresponding frustrated
model used to describe the mixed phase in the critical regime. For the
general case of arbitrary field orientations such that the perpendicular
magnetic field component is larger than the decoupling cross-over scale
characteristic of layered superconductors, we obtain independent parallel and
perpendicular vortex lattices. Specializing to the double-layer case, we
compute the parallel lower-critical field with entropic effects included, and
find that it vanishes exponentially as temperature approaches the layer
decoupling transition in zero-field. The parallel reversible magnetization is
also calculated in this case, where we find that it shows a cross-over
phenomenon as a function of parallel field in the intermediate regime of the
mixed phase in lieu of a true layer-decoupling transition. It is argued that
such is the case for any finite number of layers, since the isolated double
layer represents the weakest link.Comment: 29 pages of plain TeX, 2 postscript figures, improved discussio
Insulating and Conducting Phases of RbC60
Optical measurements were performed on thin films of RbC,
identified by X-ray diffraction as mostly material. The samples were
subjected to various heat treatments, including quenching and slow cooling from
400K. The dramatic increase in the transmission of the quenched samples, and
the relaxation towards the transmission observed in slow cooled samples
provides direct evidence for the existence of a metastable insulating phase.
Slow cooling results in a phase transition between two electrically conducting
phases.Comment: Minor revisions. Submitted to PRB, RevTeX 3.0 file, 2 postscript
figures included, ir_dop
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