222 research outputs found
Loschmidt echo for a chaotic oscillator
Chaotic dynamics of a nonlinear oscillator is considered in the semiclassical
approximation. The Loschmidt echo is calculated for a time scale which is of
the power law in semiclassical parameter. It is shown that an exponential decay
of the Loschmidt echo is due to a Lyapunov exponent and it has a pure classical
nature.Comment: Submit to PR
Thermodynamic properties in the normal and superconducting states of Na(x)CoO(2)*yH(2)O powder measured by heat capacity experiments
The heat capacity of superconducting Na(x)CoO(2)*yH(2)O was measured and the
data are discussed based on three different models: The thermodynamic
Ginzburg-Landau model, the BCS theory, and a model including the effects of
line nodes in the superconducting gap function. The electronic heat capacity is
separated from the lattice contribution in a thermodynamically consistent way
maintaining the entropy balance of superconducting and normal states at the
critical temperature. It is shown that for a fully gapped superconductor the
data can only be explained by a reduced (about 55 %) superconducting volume
fraction. The data are compatible with 100 % superconductivity in the case
where line nodes are present in the superconducting gap function.Comment: Revised, 19 pages, 3 figure
Shape-induced magnetic anisotropy in dilute magnetic alloys
We extend the theory of the surface-induced magnetic anisotropy to mesoscopic
samples with arbitrary geometry. The shape-induced anisotropy of impurity spins
in small brick-shaped grains of dilute magnetic alloys is studied in detail.
The surface-induced blocking of a magnetic-impurity spin is shown to be very
sensitive to geometric parameters of a grain. This implies that the apparent
discrepancy between the experimental data of different groups on the size
dependence of the Kondo resistivity can result from different microstructure of
the used samples. In order to interpret recent experimental data on the
anomalous Hall effect in thin polycrystalline Fe doped Au films, we analyse the
magnetisation of impurity spins as a function of the impurity position and of
the grain shape.Comment: 10 pages, 6 figures, E-mail addresses: [email protected],
[email protected], [email protected]
Effect of hyperon bulk viscosity on neutron-star r-modes
Neutron stars are expected to contain a significant number of hyperons in
addition to protons and neutrons in the highest density portions of their
cores. Following the work of Jones, we calculate the coefficient of bulk
viscosity due to nonleptonic weak interactions involving hyperons in
neutron-star cores, including new relativistic and superfluid effects. We
evaluate the influence of this new bulk viscosity on the gravitational
radiation driven instability in the r-modes. We find that the instability is
completely suppressed in stars with cores cooler than a few times 10^9 K, but
that stars rotating more rapidly than 10-30% of maximum are unstable for
temperatures around 10^10 K. Since neutron-star cores are expected to cool to a
few times 10^9 K within seconds (much shorter than the r-mode instability
growth time) due to direct Urca processes, we conclude that the gravitational
radiation instability will be suppressed in young neutron stars before it can
significantly change the angular momentum of the star.Comment: final PRD version, minor typos etc correcte
Gor'kov and Eliashberg Linear Response Theory: Rigorous Derivation and Limits of Applicability
A rigorous microscopic calculation of the polarizability of disordered
mesoscopic particles within the grand canonical ensemble is given in terms of
the supersymmetry method. The phenomenological result of Gor'kov and Eliashberg
is confirmed. Thus the underlying assumptions of their method are justified.
This encourages application of RMT in the Gor'kov--Eliashberg style to more
complicated situations.Comment: Final published versio
Fixed-N Superconductivity: The Crossover from the Bulk to the Few-Electron Limit
We present a truly canonical theory of superconductivity in ultrasmall
metallic grains by variationally optimizing fixed-N projected BCS
wave-functions, which yields the first full description of the entire crossover
from the bulk BCS regime (mean level spacing bulk gap )
to the ``fluctuation-dominated'' few-electron regime (). A
wave-function analysis shows in detail how the BCS limit is recovered for , and how for pairing correlations become
delocalized in energy space. An earlier grand-canonical prediction for an
observable parity effect in the spectral gaps is found to survive the fixed-N
projection.Comment: 4 pages, 3 figures, RevTeX, V2: minor charges to mach final printed
versio
Strong Electron-Phonon Coupling in Superconducting MgB: A Specific Heat Study
We report on measurements of the specific heat of the recently discovered
superconductor MgB in the temperature range between 3 and 220 K. Based on a
modified Debye-Einstein model, we have achieved a rather accurate account of
the lattice contribution to the specific heat, which allows us to separate the
electronic contribution from the total measured specific heat. From our result
for the electronic specific heat, we estimate the electron-phonon coupling
constant to be of the order of 2, significantly enhanced compared to
common weak-coupling values . Our data also indicate that the
electronic specific heat in the superconducting state of MgB can be
accounted for by a conventional, s-wave type BCS-model.Comment: 4 pages, 4 figure
Parity-Affected Superconductivity in Ultrasmall Metallic Grains
We investigate the breakdown of BCS superconductivity in {\em ultra}\/small
metallic grains as a function of particle size (characterized by the mean
spacing between discrete electronic eigenstates), and the parity ( =
even/odd) of the number of electrons on the island. Assuming equally spaced
levels, we solve the parity-dependent BCS gap equation for the order parameter
. Both the critical level spacing and the
critical temperature at which are parity
dependent, and both are so much smaller in the odd than the even case that
these differences should be measurable in current experiments.Comment: 4 pages RevTeX, 1 encapsulated postscript figure, submitted to
Physical Review Letter
A small superconducting grain in the canonical ensemble
By means of the Lanczos method we analyze superconducting correlations in
ultrasmall grains at fixed particle number. We compute the ground state
properties and the excitation gap of the pairing Hamiltonian as a function of
the level spacing . Both quantities turn out to be parity dependent and
universal functions of the ratio ( is the BCS gap). We
then characterize superconductivity in the canonical ensemble from the scaling
behavior of correlation functions in energy space.Comment: 11 pages Revtex, 5 figures .ep
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