330 research outputs found
CPT and Lorentz violation effects in hydrogen-like atoms
Within the framework of Lorentz-violating extended electrodynamics, the Dirac
equation for a bound electron in an external electromagnetic field is
considered assuming the interaction with a CPT-odd axial vector background
. The quasi-relativistic Hamiltonian is obtained using a -series
expansion. Relativistic Dirac eigenstates in a spherically-symmetric potential
are found accurate up to the second order in . -induced CPT-odd
corrections to the electromagnetic dipole moment operators of a bound electron
are calculated that contribute to the anapole moment of the atomic orbital and
may cause a specific asymmetry of the angular distribution of the radiation of
a hydrogen atom.Comment: 13 pages, 1 figure; (5.14) is corrected to conform to the
normalization convention for Laguerre polynomials adopted at present; minor
grammatical change
On de Sitter radiation via quantum tunneling
We discuss why the tunneling picture does not necessarily lead to Hawking
radiation from the de Sitter horizon. The experience with the condensed matter
analogs of event horizon suggests that the de Sitter vacuum is stable against
the Hawking radiation. On the other hand the detector immersed into the de
Sitter background will detect the radiation which looks as thermal with the
effective temperature twice larger than the Hawking temperature associated with
the cosmological horizon.Comment: 15 pages, no figures, IJMPD style, version submitted to IJMP
First Passage Time Densities in Non-Markovian Models with Subthreshold Oscillations
Motivated by the dynamics of resonant neurons we consider a differentiable,
non-Markovian random process and particularly the time after which it
will reach a certain level . The probability density of this first passage
time is expressed as infinite series of integrals over joint probability
densities of and its velocity . Approximating higher order terms
of this series through the lower order ones leads to closed expressions in the
cases of vanishing and moderate correlations between subsequent crossings of
. For a linear oscillator driven by white or coloured Gaussian noise,
which models a resonant neuron, we show that these approximations reproduce the
complex structures of the first passage time densities characteristic for the
underdamped dynamics, where Markovian approximations (giving monotonous first
passage time distribution) fail
Probing momentum-dependent scattering in uniaxially stressed SrRuO through the Hall effect
Under in-plane uniaxial stress, the largest Fermi surface sheet of the
correlated metal SrRuO undergoes a Lifshitz transition from an
electron-like to an open geometry. We investigate the effects of this
transition on transport through measurement of the longitudinal resistivity
and the Hall coefficient . At temperatures where
scattering is dominated by electron-electron scattering, becomes
more negative across the Lifshitz transition, opposite to expectations from the
change in Fermi surface topology. We show that this change in is
explainable only if scattering changes throughout the Brillouin zone, not just
at the point in -space where the Lifshitz transition occurs. In a model of
orbital-dependent scattering, the electron-electron scattering rate on sections
of Fermi surface with orbital weight decreases dramatically. On the other
hand, at temperatures where defect scattering dominates and
are essentially constant across the Lifshitz transition.Comment: 9 pages, 10 figure
First Passage Time Densities in Resonate-and-Fire Models
Motivated by the dynamics of resonant neurons we discuss the properties of
the first passage time (FPT) densities for nonmarkovian differentiable random
processes. We start from an exact expression for the FPT density in terms of an
infinite series of integrals over joint densities of level crossings, and
consider different approximations based on truncation or on approximate
summation of this series. Thus, the first few terms of the series give good
approximations for the FPT density on short times. For rapidly decaying
correlations the decoupling approximations perform well in the whole time
domain.
As an example we consider resonate-and-fire neurons representing stochastic
underdamped or moderately damped harmonic oscillators driven by white Gaussian
or by Ornstein-Uhlenbeck noise. We show, that approximations reproduce all
qualitatively different structures of the FPT densities: from monomodal to
multimodal densities with decaying peaks. The approximations work for the
systems of whatever dimension and are especially effective for the processes
with narrow spectral density, exactly when markovian approximations fail.Comment: 11 pages, 8 figure
Statistics of S-matrix poles in Few-Channel Chaotic Scattering: Crossover from Isolated to Overlapping Resonances
We derive the explicit expression for the distribution of resonance widths in
a chaotic quantum system coupled to continua via M equivalent open channels. It
describes a crossover from the distribution (regime of isolated
resonances) to a broad power-like distribution typical for the regime of
overlapping resonances. The first moment is found to reproduce exactly the
Moldauer-Simonius relation between the mean resonance width and the
transmission coefficient. This fact may serve as another manifestation of
equivalence between the spectral and the ensemble averaging.Comment: 4 two-column pages, RevTex. text is slightly modified; some misprints
are correcte
The non-equilibrium steady state of sparse systems with nontrivial topology
We study the steady state of a multiply-connected system that is driven out
of equilibrium by a sparse perturbation. The prototype example is an -site
ring coupled to a thermal bath, driven by a stationary source that induces
transitions with log-wide distributed rates. An induced current arises, which
is controlled by the strength of the driving, and an associated topological
term appears in the expression for the energy absorption rate. Due to the
sparsity, the crossover from linear response to saturation is mediated by an
intermediate regime, where the current is exponentially small in ,
which is related to the work of Sinai on "random walk in a random environment".Comment: 6 pages, 4 figure
Dynamical Casimir Effect with Semi-Transparent Mirrors, and Cosmology
After reviewing some essential features of the Casimir effect and,
specifically, of its regularization by zeta function and Hadamard methods, we
consider the dynamical Casimir effect (or Fulling-Davis theory), where related
regularization problems appear, with a view to an experimental verification of
this theory. We finish with a discussion of the possible contribution of vacuum
fluctuations to dark energy, in a Casimir like fashion, that might involve the
dynamical version.Comment: 11 pages, Talk given in the Workshop ``Quantum Field Theory under the
Influence of External Conditions (QFEXT07)'', Leipzig (Germany), September 17
- 21, 200
Dialectics and Implications of Natural Neurotropic Autoantibodies in Neurological Disease and Rehabilitation
The role of natural idiotypic (Id-Abs) and anti-idiotypic (AId-Abs) autoantibodies
against neuro-antigens observed in different neurological disorders is not fully
understood. In particular, limited experimental evidence has been provided
concerning the qualitative and quantitative serological response after acute injuries
of the central nervous system or during chronic mental diseases. In this study, we
analyzed the specific Id-Abs and AId-Abs serological reactivities against 4
neuro-antigens in a large population of patients with ischemic stroke, schizophrenia,
as well as healthy individuals. Patients with ischemic stroke were tested at different
time points following the acute stroke episode and a correlation was attempted
between autoantibodies response and different patterns of functional recovery.
Results showed variable and detectable Id-Abs and AId-Abs in different proportions
of all three populations of subjects. Among patients with different functional
recovery after ischemic stroke, a difference in time-related trends of Id-Abs and
AId-Abs was encountered. Our observations suggest that changes in
the production of natural neurotropic Abs may engender a positive homeostatic,
beside a possible
pathogenic effect, in specific neurological disorders
Weakly-nonlocal Symplectic Structures, Whitham method, and weakly-nonlocal Symplectic Structures of Hydrodynamic Type
We consider the special type of the field-theoretical Symplectic structures
called weakly nonlocal. The structures of this type are in particular very
common for the integrable systems like KdV or NLS. We introduce here the
special class of the weakly nonlocal Symplectic structures which we call the
weakly nonlocal Symplectic structures of Hydrodynamic Type. We investigate then
the connection of such structures with the Whitham averaging method and propose
the procedure of "averaging" of the weakly nonlocal Symplectic structures. The
averaging procedure gives the weakly nonlocal Symplectic Structure of
Hydrodynamic Type for the corresponding Whitham system. The procedure gives
also the "action variables" corresponding to the wave numbers of -phase
solutions of initial system which give the additional conservation laws for the
Whitham system.Comment: 64 pages, Late
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