956 research outputs found

### Comment on "Magnetic response of Disordered Metallic Rings: Large Contributions of Far Levels"

Comment on cond-mat/0205390; PRL 90, 026805 (2003

### Effective descriptions of complex quantum systems: path integrals and operator ordering problems

We study certain aspects of the effective, occasionally called collective,
description of complex quantum systems within the framework of the path
integral formalism, in which the environment is integrated out. Generalising
the standard Feynman-Vernon Caldeira-Leggett model to include a non-linear
coupling between ``particle'' and environment, and considering a particular
spectral density of the coupling, a coordinate-dependent mass (or
velocity-dependent potential) is obtained. The related effective quantum
theory, which depends on the proper discretisation of the path integral, is
derived and discussed. As a result, we find that in general a simple effective
low-energy Hamiltonian, in which only the coordinate-dependent mass enters,
cannot be formulated. The quantum theory of weakly coupled superconductors and
the quantum dynamics of vortices in Josephson junction arrays are physical
examples where these considerations, in principle, are of relevance.Comment: 13 pages, no figure

### Decoherence without dissipation?

In a recent article, Ford, Lewis and O'Connell (PRA 64, 032101 (2001))
discuss a thought experiment in which a Brownian particle is subjected to a
double-slit measurement. Analyzing the decay of the emerging interference
pattern, they derive a decoherence rate that is much faster than previous
results and even persists in the limit of vanishing dissipation. This result is
based on the definition of a certain attenuation factor, which they analyze for
short times. In this note, we point out that this attenuation factor captures
the physics of decoherence only for times larger than a certain time t_mix,
which is the time it takes until the two emerging wave packets begin to
overlap. Therefore, the strategy of Ford et al of extracting the decoherence
time from the regime t < t_mix is in our opinion not meaningful. If one
analyzes the attenuation factor for t > t_mix, one recovers familiar behaviour
for the decoherence time; in particular, no decoherence is seen in the absence
of dissipation. The latter conclusion is confirmed with a simple calculation of
the off-diagonal elements of the reduced density matrix.Comment: 8 pages, 4 figure

### Entropy and Time

The emergence of a direction of time in statistical mechanics from an
underlying time-reversal-invariant dynamics is explained by examining a simple
model. The manner in which time-reversal symmetry is preserved and the role of
initial conditions are emphasized. An extension of the model to finite
temperatures is also discussed.Comment: 9 pages, 8eps figures. To appear in the theme issue of the American
Journal of Physics on Statistical Physic

### Critical Current in the High-T_c Glass model

The high-T_c glass model can be combined with the repulsive tt'--Hubbard
model as microscopic description of the striped domains found in the high-T_c
materials. In this picture the finite Hubbard clusters are the origin of the
d-wave pairing. In this paper we show, that the glass model can also explain
the critical currents usually observed in the high-T_c materials. We use two
different approaches to calculate the critical current densities of the
high-T_c glass model. Both lead to a strongly anisotropic critical current.
Finally we give an explanation, why we expect nonetheless a nearly perfect
isotropic critical current in the high-T_c superconductors.Comment: 8 pages with 5 eps-figures, LaTeX using RevTeX, accepted by
Int.J.Mod.Phys.

### Width and Magnetic Field Dependence of Transition Temperature in Ultranarrow Superconducting Wires

We calculate the transition temperature in ultranarrow superconducting wires
as a function of wire width, resistance and applied magnetic field. We compare
the results of first-order perturbation theory and the non-perturbative
resummation technique developed by Oreg and Finkel'stein. The latter technique
is found to be superior as it is valid even in the strong disorder limit. In
both cases the predicted additional suppression of the transition temperature
due to the reduced dimensionality is strongly dependent upon the boundary
conditions used. When we use the correct (zero-gradient) boundary conditions,
we find that theory and experiment are consistent, although more experimental
data is required to verify this systematically. We calculate the magnetic field
dependence of the transition temperature for different wire widths and
resistances in the hope that this will be measured in future experiments. The
predicted results have a rich structure - in particular we find a dimensional
crossover which can be tuned by varying either the width of the wire or its
resistance per square.Comment: 12 pages, 1 table, 7 figures. The changes made to the paper are ones
of emphasis. The comparison between theory and experiment has been altered,
and detailed comparisons of various approximations have been omitted,
although the results are summarised in the paper. Much more emphasis has been
placed on the new predictions of the effect of an applied magnetic field on
transition temperature in wires (Figs. 5-7

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