207 research outputs found
Do macroscopic properties dictate microscopic probabilities?
Aharonov and Reznik have recently (in quant-ph/0110093) argued that the form
of the probabilistic predictions of quantum theory can be seen to follow from
properties of macroscopic systems. An error in their argument is identified.Comment: LaTeX, 6 pages, no figure
Density-metric unimodular gravity: vacuum maximal symmetry
We have investigated the vacuum maximally symmetric solutions of recently
proposed density-metric unimodular gravity theory,the results are widely
different from inflationary senario.The exponential dependence on time in
deSitter space is substiuted by a power law. Open space-times with non-zero
cosmological constant are excluded in this theoryComment: 15 pages, no figures,stability section omitte
Magnetoresistance and dephasing in a two-dimensional electron gas at intermediate conductances
We study, both theoretically and experimentally, the negative
magnetoresistance (MR) of a two-dimensional (2D) electron gas in a weak
transverse magnetic field . The analysis is carried out in a wide range of
zero- conductances (measured in units of ), including the range
of intermediate conductances, . Interpretation of the experimental
results obtained for a 2D electron gas in GaAs/InGaAs/GaAs single
quantum well structures is based on the theory which takes into account terms
of higher orders in , stemming from both the interference contribution and
the mutual effect of weak localization (WL) and Coulomb interaction. We
demonstrate that at intermediate conductances the negative MR is described by
the standard WL "digamma-functions" expression, but with a reduced prefactor
. We also show that at not very high the second-loop corrections
dominate over the contribution of the interaction in the Cooper channel, and
therefore appear to be the main source of the lowering of the prefactor,
. We further analyze the regime of a "weak insulator",
when the zero- conductance is low due to the localization at low
, whereas the Drude conductance is high, In this regime, while the
MR still can be fitted by the digamma-functions formula, the experimentally
obtained value of the dephasing rate has nothing to do with the true one. The
corresponding fitting parameter in the low- limit is determined by the
localization length and may therefore saturate at , even though the
true dephasing rate vanishes.Comment: 36 pages, 16 figure
Magnetic fluctuations in 2D metals close to the Stoner instability
We consider the effect of potential disorder on magnetic properties of a
two-dimensional metallic system (with conductance ) when interaction in
the triplet channel is so strong that the system is close to the threshold of
the Stoner instability. We show, that under these conditions there is an
exponentially small probability for the system to form local spin droplets
which are local regions with non zero spin density. Using a non-local version
of the optimal fluctuation method we find analytically the probability
distribution and the typical spin of a local spin droplet (LSD). In particular,
we show that both the probability to form a LSD and its typical spin are
independent of the size of the droplet (within the exponential accuracy). The
LSDs manifest themselves in temperature dependence of observable quantities. We
show, that below certain cross-over temperature the paramagnetic susceptibility
acquires the Curie-like temperature dependence, while the dephasing time
(extracted from magneto-resistance measurements) saturates.Comment: 15 pages, 4 figure
Two-Component Scaling near the Metal-Insulator Bifurcation in Two-Dimensions
We consider a two-component scaling picture for the resistivity of
two-dimensional (2D) weakly disordered interacting electron systems at low
temperature with the aim of describing both the vicinity of the bifurcation and
the low resistance metallic regime in the same framework. We contrast the
essential features of one-component and two-component scaling theories. We
discuss why the conventional lowest order renormalization group equations do
not show a bifurcation in 2D, and a semi-empirical extension is proposed which
does lead to bifurcation. Parameters, including the product , are
determined by least squares fitting to experimental data. An excellent
description is obtained for the temperature and density dependence of the
resistance of silicon close to the separatrix. Implications of this
two-component scaling picture for a quantum critical point are discussed.Comment: 7 pages, 1 figur
Proximity effect in ultrathin Pb/Ag multilayers within the Cooper limit
We report on transport and tunneling measurements performed on ultra-thin
Pb/Ag (strong coupled superconductor/normal metal) multilayers evaporated by
quench condensation. The critical temperature and energy gap of the
heterostructures oscillate with addition of each layer, demonstrating the
validity of the Cooper limit model in the case of multilayers. We observe
excellent agreement with a simple theory for samples with layer thickness
larger than 30\AA . Samples with single layers thinner than 30\AA deviate from
the Cooper limit theory. We suggest that this is due to the "inverse proximity
effect" where the normal metal electrons improve screening in the
superconducting ultrathin layer and thus enhance the critical temperature.Comment: 4 pages, 4 figure
Superconductive proximity effect in interacting disordered conductors
We present a general theory of the superconductive proximity effect in
disordered normal--superconducting (N-S) structures, based on the recently
developed Keldysh action approach. In the case of the absence of interaction in
the normal conductor we reproduce known results for the Andreev conductance G_A
at arbitrary relation between the interface resistance R_T and the diffusive
resistance R_D. In two-dimensional N-S systems, electron-electron interaction
in the Cooper channel of normal conductor is shown to strongly affect the value
of G_A as well as its dependence on temperature, voltage and magnetic field. In
particular, an unusual maximum of G_A as a function of temperature and/or
magnetic field is predicted for some range of parameters R_D and R_T. The
Keldysh action approach makes it possible to calculate the full statistics of
charge transfer in such structures. As an application of this method, we
calculate the noise power of an N-S contact as a function of voltage,
temperature, magnetic field and frequency for arbitrary Cooper repulsion in the
normal metal and arbitrary values of the ratio R_D/R_T.Comment: RevTeX, 28 pages, 18 PostScript figures; added and updated reference
Classical and quantum q-deformed physical systems
On the basis of the non-commutative q-calculus, we investigate a
q-deformation of the classical Poisson bracket in order to formulate a
generalized q-deformed dynamics in the classical regime. The obtained
q-deformed Poisson bracket appears invariant under the action of the
q-symplectic group of transformations. In this framework we introduce the
q-deformed Hamilton's equations and we derive the evolution equation for some
simple q-deformed mechanical systems governed by a scalar potential dependent
only on the coordinate variable. It appears that the q-deformed Hamiltonian,
which is the generator of the equation of motion, is generally not conserved in
time but, in correspondence, a new constant of motion is generated. Finally, by
following the standard canonical quantization rule, we compare the well known
q-deformed Heisenberg algebra with the algebra generated by the q-deformed
Poisson bracket.Comment: 9 pages, accepted for publication in "The European Physical Journal
C
Electrons in an annealed environment: A special case of the interacting electron problem
The problem of noninteracting electrons in the presence of annealed magnetic
disorder, in addition to nonmagnetic quenched disorder, is considered. It is
shown that the proper physical interpretation of this model is one of electrons
interacting via a potential that is long-ranged in time, and that its technical
analysis by means of renormalization group techniques must also be done in
analogy to the interacting problem. As a result, and contrary to previous
claims, the model does not simply describe a metal-insulator transition in
() dimensions. Rather, it describes a transition
to a ferromagnetic state that, as a function of the disorder, precedes the
metal-insulator transition close to . In , a transition from a
paramagnetic metal to a paramagnetic insulator is possible.Comment: 13 pp., LaTeX, 2 eps figs; final version as publishe
Schwinger-Keldysh Approach to Disordered and Interacting Electron Systems: Derivation of Finkelstein's Renormalization Group Equations
We develop a dynamical approach based on the Schwinger-Keldysh formalism to
derive a field-theoretic description of disordered and interacting electron
systems. We calculate within this formalism the perturbative RG equations for
interacting electrons expanded around a diffusive Fermi liquid fixed point, as
obtained originally by Finkelstein using replicas. The major simplifying
feature of this approach, as compared to Finkelstein's is that instead of replicas, we only need to consider N=2 species. We compare the dynamical
Schwinger-Keldysh approach and the replica methods, and we present a simple and
pedagogical RG procedure to obtain Finkelstein's RG equations.Comment: 22 pages, 14 figure
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