447 research outputs found
The effect of Aharanov-Bohm phase on the magnetic-field dependence of two-pulse echos in glasses at low temperatures
The anomalous response of glasses in the echo amplitude experiment is
explained in the presence of a magnetic field. We have considered the low
energy excitations in terms of an effective two level system. The effective
model is constructed on the flip-flop configuration of two interacting two
level systems. The magnetic field affects the tunneling amplitude through the
Aharanov-Bohm effect. The effective model has a lower scale of energy in
addition to the new distribution of tunneling parameters which depend on the
interaction. We are able to explain some features of echo amplitude versus a
magnetic field, namely, the dephasing effect at low magnetic fields, dependence
on the strength of the electric field, pulse separation effect and the
influence of temperature. However this model fails to explain the isotope
effects which essentially can be explained by the nuclear quadrupole moment. We
will finally discuss the features of our results.Comment: 8 pages, 7 figure
Clausius inequality and optimality of quasi static transformations for nonequilibrium stationary states
Nonequilibrium stationary states of thermodynamic systems dissipate a
positive amount of energy per unit of time. If we consider transformations of
such states that are realized by letting the driving depend on time, the amount
of energy dissipated in an unbounded time window becomes then infinite.
Following the general proposal by Oono and Paniconi and using results of the
macroscopic fluctuation theory, we give a natural definition of a renormalized
work performed along any given transformation. We then show that the
renormalized work satisfies a Clausius inequality and prove that equality is
achieved for very slow transformations, that is in the quasi static limit. We
finally connect the renormalized work to the quasi potential of the macroscopic
fluctuation theory, that gives the probability of fluctuations in the
stationary nonequilibrium ensemble
Multiorbital effects on the transport and the superconducting fluctuations in LiFeAs
The resistivity, Hall effect and transverse magnetoresistance (MR) have been
measured in low residual resistivity single crystals of LiFeAs. A comparison
with angle resolved photoemission spectroscopy and quantum oscillation data
implies that four carrier bands unevenly contribute to the transport. However
the scattering rates of the carriers all display the T^2 behavior expected for
a Fermi liquid. Near Tc low field deviations of the MR with respect to a H^2
variation permit us to extract the superconducting fluctuation contribution to
the conductivity. Though below Tc the anisotropy of superconductivity is rather
small, the superconducting fluctuations display a quasi ideal two-dimensional
behavior which persists up to 1.4 Tc. These results call for a refined
theoretical understanding of the multiband behavior of superconductivity in
this pnictide.Comment: 8pages with supplementary material, 6 figure
A precursor state to unconventional superconductivity in CeIrIn
We present sensitive measurements of the Hall effect and magnetoresistance in
CeIrIn down to temperatures of 50 mK and magnetic fields up to 15 T. The
presence of a low temperature coherent Kondo state is established. Deviations
from Kohler's rule and a quadratic temperature dependence of the cotangent of
the Hall angle are reminiscent of properties observed in the high temperature
superconducting cuprates. The most striking observation pertains to the
presence of a \textit{precursor} state--characterized by a change in the Hall
mobility--that appears to precede the superconductivity in this material, in
similarity to the pseudogap in the cuprate high superconductors.Comment: 4 figure
Damping in high-frequency metallic nanomechanical resonators
We have studied damping in polycrystalline Al nanomechanical resonators by
measuring the temperature dependence of their resonance frequency and quality
factor over a temperature range of 0.1 - 4 K. Two regimes are clearly
distinguished with a crossover temperature of 1 K. Below 1 K we observe a
logarithmic temperature dependence of the frequency and linear dependence of
damping that cannot be explained by the existing standard models. We attribute
these phenomena to the effect of the two-level systems characterized by the
unexpectedly long (at least two orders of magnitude longer) relaxation times
and discuss possible microscopic models for such systems. We conclude that the
dynamics of the two-level systems is dominated by their interaction with
one-dimensional phonon modes of the resonators.Comment: 5 pages, 3 figure
Weak- to strong pinning crossover
Material defects in hard type II superconductors pin the flux lines and thus
establish the dissipation-free current transport in the presence of a finite
magnetic field. Depending on the density and pinning force of the defects and
the vortex density, pinning is either weak-collective or strong. We analyze the
weak- to strong pinning crossover of vortex matter in disordered
superconductors and discuss the peak effect appearing naturally in this
context.Comment: 4 pages, 2 figure
Non-ohmicity and energy relaxation in diffusive 2D metals
We analyze current-voltage characteristics taken on Au-doped indium-oxide
films. By fitting a scaling function to the data, we extract the
electron-phonon scattering rate as function of temperature, which yields a
quadratic dependence of the electron-phonon scattering rate on temperature from
1K down to 0.28K. The origin of this enhanced electron-phonon scattering rate
is ascribed to the mechanism proposed by Sergeev and Mitin.Comment: 7 pages, 6 figure
Comment on "On the importance of the free energy for elasticity under pressure"
Marcus et al. (Marcus P, Ma H and Qiu S L 2002 J. Phys.: Condens. Matter 14
L525) claim that thermodynamic properties of materials under pressure must be
computed using the Gibbs free energy , rather than the internal energy .
Marcus et al. state that ``The minima of , but not of , give the
equilibrium structure; the second derivatives of , but not of , with
respect to strains at the equilibrium structure give the equilibrium elastic
constants.'' Both statements are incorrect.Comment: Commen
Negative Magnetoresistance in the Nearest-neighbor Hopping Conduction
We propose a size effect which leads to the negative magnetoresistance in
granular metal-insulator materials in which the hopping between two nearest
neighbor clusters is the main transport mechanism. We show that the hopping
probability increases with magnetic field. This is originated from the level
crossing in a few-electron cluster. Thus, the overlap of electronic states of
two neighboring clusters increases, and the negative magnetoresistance is
resulted.Comment: Latex file, no figur
Localized Wavefunctions and Magnetic Band Structure for Lateral Semiconductor Superlattices
In this paper we present calculations on the electronic band structure of a
two-dimensional lateral superlattice subject to a perpendicular magnetic field
by employing a projection operator technique based on the ray-group of
magnetotranslation operators. We construct a new basis of appropriately
symmetrized Bloch-like wavefunctions as linear combination of well-localized
magnetic-Wannier functions. The magnetic field was consistently included in the
Wannier functions defined in terms of free-electron eigenfunctions in the
presence of external magnetic field in the symmetric gauge. Using the above
basis, we calculate the magnetic energy spectrum of electrons in a lateral
superlattice with bi-directional weak electrostatic modulation. Both a square
lattice and a triangular one are considered as special cases. Our approach
based on group theory handles the cases of integer and rational magnetic fluxes
in a uniform way and the provided basis could be convenient for further both
analytic and numerical calculations.Comment: 19 pages, 5 figures. accepted to Int. J. Mod. Phys. B (April 2006
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