222 research outputs found
Vortex instability in molybdenum-germanium superconducting films
We studied the high driving force regime of the current-voltage transport
response in the mixed state of amorphous molybdenum-germanium superconducting
films to the point where the flux flow becomes unstable. The observed nonlinear
response conforms with the classic Larkin-Ovchinikov (LO) picture with a
quasiparticle-energy-relaxation rate dominated by the quasiparticle
recombination process. The measured energy relaxation rate was found to have a
magnitude and temperature dependence in agreement with theory.
PACS: 74.40.Gh, 74.25.Uv, 72.15.Lh, 73.50.Gr, 73.50.Fq
Keywords: fluxon, vortices, TDGL, FFF, negative differential conductivity,
NDC, non-monotonic IV curveComment: 5 pages, 3 figure
Evaluating free flux flow in low-pinning molybdenum-germanium superconducting films
Vortex dynamics in molybdenum-germanium superconducting films were found to
well approximate the unpinned free limit even at low driving forces. This
provided an opportunity to empirically establish the intrinsic character of
free flux flow and to test in detail the validity of theories for this regime
beyond the Bardeen-Stephen approximation. Our observations are in good
agreement with the mean-field result of time dependent Ginzburg-Landau theory.
PACS: 74.25.Sv,74.25.Wx,74.25.Uv,74.25.Op,74.25.F- Keywords: vortices,
fluxon, Larkin, Ovchinnikov, upper critical magnetic fieldComment: This is the final revised version of a paper that is currently in
press. It is expected to appear in Phys. Rev. B in 201
Optical conductivity of a granular metal at not very low temperatures
We study the finite-temperature optical conductivity, sigma(omega,T), of a
granular metal using a simple model consisting of a array of spherical metallic
grains. It is necessary to include quantum tunneling and Coulomb blockade
effects to obtain the correct temperature dependence of sigma(omega, T), and to
consider polarization oscillations to obtain the correct frequency dependence.
We have therefore generalized the Ambegaokar-Eckern-Schoen (AES) model for
granular metals to obtain an effective field theory incorporating the
polarization fluctuations of the individual metallic grains. In contrast to the
DC conductivity, which is determined by inter-grain charge transfer and obeys
an Arrhenius law at low temperature, the AC conductivity is dominated by a
resonance peak for intra-grain polarization oscillations, which has a power-law
tail at low frequencies. More importantly, although the resonance frequency
agrees with the classical prediction, the resonance width depends on intergrain
quantum tunneling and Coulomb blockade parameters, in addition to the classical
Drude relaxation within the grain. This additional damping is due to inelastic
cotunneling of polarization fluctuations to neighbouring grains and it
qualitatively differs from the DC conductivity in its temperature dependence
quite unlike the expectation from Drude theory.Comment: Added figures, published version, 16 pages, REVTe
Conductance of Aharonov--Bohm Rings: From the Discrete to the Continuous Spectrum Limit
The dissipative conductance of an array of mesoscopic rings, subject to an
a.c. magnetic flux is investigated. The magneto--conductance may change sign
between canonical and grand-canonical statistical ensembles, as function of the
inelastic level broadening and as function of the temperature. Differences
between canonical and grand-canonical ensembles persist up to temperature of
the order of the Thouless energy.Comment: 13 pages, 2 figures, REVTeX v2.1, WIS--93/121/Dec.--P
Finite Temperature Phase Diagram of Quasi-Two-Dimensional Imbalanced Fermi Gases Beyond Mean-Field
We investigate the superfluid transition temperature of quasi-two-dimensional
imbalanced Fermi gases beyond the mean-field approximation, through the
second-order (or induced) interaction effects. For a balanced Fermi system the
transition temperature is suppressed by a factor . For imbalanced
Fermi systems, the polarization and transition temperature of the tricritical
point are significantly reduced as the two-body binding energy
increases.Comment: 6 pages, 3 figure
Interaction correction to the conductance of a ballistic conductor
In disordered metals, electron-electron interactions are the origin of a
small correction to the conductivity, the "Altshuler-Aronov correction". Here
we investigate the Altshuler-Aronov correction of a conductor in which the
electron motion is ballistic and chaotic. We consider the case of a double
quantum dot, which is the simplest example of a ballistic conductor in which
the Altshuler-Aronov correction is nonzero. The fact that the electron motion
is ballistic leads to an exponential suppression of the correction if the
Ehrenfest time is larger than the mean dwell time or the inverse temperature.Comment: 4 pages, 2 figure
Electron transport in disordered graphene
We study electron transport properties of a monoatomic graphite layer
(graphene) with different types of disorder. We show that the transport
properties of the system depend strongly on the character of disorder. Away
from half filling, the concentration dependence of conductivity is linear in
the case of strong scatterers, in line with recent experimental observations,
and logarithmic for weak scatterers. At half filling the conductivity is of the
order of e^2/h if the randomness preserves one of the chiral symmetries of the
clean Hamiltonian; otherwise, the conductivity is strongly affected by
localization effects.Comment: 21 pages, 9 figure
From Trapped Atoms to Liberated Quarks
We discuss some aspects of cold atomic gases in the unitarity limit that are
of interest in connection with the physics of dense hadronic matter. We
consider, in particular, the equation of state at zero temperature, the
magnitude of the pairing gap, and the phase diagram at non-zero polarization.Comment: 13 pages, 5 figures; to appear in the proceedings of the
International Symposium on Heavy Ion Physics 2006, Frankfurt, Germany;
International Journal of Modern Physics E, in pres
Ehrenfest time in the weak dynamical localization
The quantum kicked rotor (QKR) is known to exhibit dynamical localization in
the space of its angular momentum. The present paper is devoted to the
systematic first--principal (without a regularizer) diagrammatic calculations
of the weak--localization corrections for QKR. Our particular emphasis is on
the Ehrenfest time regime -- the phenomena characteristic for the
classical--to--quantum crossover of classically chaotic systems.Comment: 27 pages, 9 figure
Universal Conductance Distribution in the Quantum Size Regime
We study the conductance (g) distribution function of an ensemble of isolated
conducting rings, with an Aharonov--Bohm flux. This is done in the discrete
spectrum limit, i.e., when the inelastic rate, frequency and temperature are
all smaller than the mean level spacing. Over a wide range of g the
distribution function exhibits universal behavior P(g)\sim g^{-(4+\beta)/3},
where \beta=1 (2) for systems with (without) a time reversal symmetry. The
nonuniversal large g tail of this distribution determines the values of high
moments.Comment: 13 pages+1 figure, RevTEX
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