1,133 research outputs found
Moving Detectors in Cavities
We consider two-level detectors, coupled to a quantum scalar field, moving
inside cavities. We highlight some pathological resonant effects due to abrupt
boundaries, and decide to describe the cavity by switching smoothly the
interaction by a time-dependent gate-like function. Considering uniformly
accelerated trajectories, we show that some specific choices of non-adiabatic
switching have led to hazardous interpretations about the enhancement of the
Unruh effect in cavities. More specifically, we show that the
emission/absorption ratio takes arbitrary high values according to the emitted
quanta properties and to the transients undergone at the entrance and the exit
of the cavity, {\it independently of the acceleration}. An explicit example is
provided where we show that inertial and uniformly accelerated world-lines can
even lead to the same ``pseudo-temperature''.Comment: 13 pages, 6 figures, version accepted in Phys.Rev.
Aging to Equilibrium Dynamics of SiO2
Molecular dynamics computer simulations are used to study the aging dynamics
of SiO2 (modeled by the BKS model). Starting from fully equilibrated
configurations at high temperatures T_i =5000K/3760K the system is quenched to
lower temperatures T_f=2500K, 2750K, 3000K, 3250K and observed after a waiting
time t_w. Since the simulation runs are long enough to reach equilibrium at
T_f, we are able to study the transition from out-of-equilibrium to equilibrium
dynamics. We present results for the partial structure factors, for the
generalized incoherent intermediate scattering function C_q(t_w, t_w+t), and
for the mean square displacement msd(t_w,t_w+t). We conclude that there are
three different t_w regions: (I) At very short waiting times, C_q(t_w, t_w+t)
decays very fast without forming a plateau. Similarly msd(t_w,t_w+t) increases
without forming a plateau. (II) With increasing t_w a plateau develops in
C_q(t_w, t_w+t) and msd(t_w,t_w+t). For intermediate waiting times the plateau
height is independent of t_w and T_i. Time superposition applies, i.e.
C_q=C_q(t/t_r) where t_r=t_r(t_w) is a waiting time dependent decay time.
Furthermore C_q=C(q,t_w,t_w+t) scales as C_q=C(q,z(t_w,t) where z is a function
of t_w and t only, i.e. independent of q. (III) At large t_w the system reaches
equilibrium, i.e. C_q(t_w,t_w+t) and msd(t_w,t_w+t) are independent of t_w and
T_i. For C_q(t_w,t_w+t) we find that the time superposition of intermediate
waiting times (II) includes the equilibrium curve (III).Comment: 9 pages, 11 figures, submission to PR
Time delay in thin slabs with self-focusing Kerr-type nonlinearity
Time delays for an intense transverse electric
(TE) wave propagating through a Kerr-type nonlinear slab are investigated.
The relation between the bidirectional group delay and the dwell time is
derived and it is shown that the difference between them can be separated
into three terms. The first one is the familiar self interference time, due
to the dispersion of the medium surrounding the slab. The other two terms
are caused by the nonlinearity and oblique incidence of the TE wave. It is
shown that the electric field distribution along the slab may be expressed
in terms of Jacobi elliptic functions while the phase difference introduced by the slab is given in terms of incomplete elliptic integrals. The expressions for the field intensity dependent complex reflection and transmission coefficients are derived and the multivalued oscillatory behavior of the delay times for the case of a thin slab is demonstrated
Multichannel demultiplexer/demodulator technologies for future satellite communication systems
NASA-Lewis' Space Electronics Div. supports ongoing research in advanced satellite communication architectures, onboard processing, and technology development. Recent studies indicate that meshed VSAT (very small aperture terminal) satellite communication networks using FDMA (frequency division multiple access) uplinks and TDMA (time division multiplexed) downlinks are required to meet future communication needs. One of the critical advancements in such a satellite communication network is the multichannel demultiplexer/demodulator (MCDD). The progress is described which was made in MCDD development using either acousto-optical, optical, or digital technologies
Theory of Nonlinear Matter Waves in Optical Lattices
We consider several effects of the matter wave dynamics which can be observed
in Bose-Einstein condensates embedded into optical lattices. For low-density
condensates we derive approximate evolution equations, the form of which
depends on relation among the main spatial scales of the system. Reduction of
the Gross-Pitaevskii equation to a lattice model (the tight-binding
approximation) is also presented. Within the framework of the obtained models
we consider modulational instability of the condensate, solitary and periodic
matter waves, paying special attention to different limits of the solutions,
i.e. to smooth movable gap solitons and to strongly localized discrete modes.
We also discuss how the Feshbach resonance, a linear force, and lattice defects
affect the nonlinear matter waves.Comment: Modern Physics Letters B (invited brief review), 25 pages, 9 figure
Two-electron photoionization of endohedral atoms
Using as an example, we demonstrate that static potential of the
fullerene core essentially alters the cross section of the two-electron
ionization differential in one-electron energy . We found that at high photon energy prominent oscillations
appear in it due to reflection of the second, slow electron wave on the shell, which "dies out" at relatively high values, of about
23 two-electron ionization potentials. The results were presented for
ratios , where is the two-electron differential
photoionization cross section. We have calculated the ratio , that accounts for
reflection of both photoelectrons by the shell. We have calculated
also the value of two-electron photoionization cross section and found that this value is close to that of an isolated
atom.Comment: 13 pages, 4 figure
On the hyperbolicity and causality of the relativistic Euler system under the kinetic equation of state
We show that a pair of conjectures raised in [11] concerning the construction
of normal solutions to the relativistic Boltzmann equation are valid. This
ensures that the results in [11] hold for any range of positive temperatures
and that the relativistic Euler system under the kinetic equation of state is
hyperbolic and the speed of sound cannot overcome .Comment: 6 pages. Abridged version; full version to appear in Commun. Pure
Appl. Ana
Local dissipation effects in two-dimensional quantum Josephson junction arrays with magnetic field
We study the quantum phase transitions in two-dimensional arrays of
Josephson-couples junctions with short range Josephson couplings (given by the
Josephson energy) and the charging energy. We map the problem onto the solvable
quantum generalization of the spherical model that improves over the mean-field
theory method. The arrays are placed on the top of a two-dimensional electron
gas separated by an insulator. We include effects of the local dissipation in
the presence of an external magnetic flux f in square lattice for several
rational fluxes f=0,1/2,1/3,1/4 and 1/6. We also have examined the T=0
superconducting-insulator phase boundary as function of a dissipation alpha for
two different geometry of the lattice: square and triangular. We have found
critical value of the dissipation parameter independent on geometry of the
lattice and presence magnetic field.Comment: accepted to PR
Spectral flow and level spacing of edge states for quantum Hall hamiltonians
We consider a non relativistic particle on the surface of a semi-infinite
cylinder of circumference submitted to a perpendicular magnetic field of
strength and to the potential of impurities of maximal amplitude . This
model is of importance in the context of the integer quantum Hall effect. In
the regime of strong magnetic field or weak disorder it is known that
there are chiral edge states, which are localised within a few magnetic lengths
close to, and extended along the boundary of the cylinder, and whose energy
levels lie in the gaps of the bulk system. These energy levels have a spectral
flow, uniform in , as a function of a magnetic flux which threads the
cylinder along its axis. Through a detailed study of this spectral flow we
prove that the spacing between two consecutive levels of edge states is bounded
below by with , independent of , and of the
configuration of impurities. This implies that the level repulsion of the
chiral edge states is much stronger than that of extended states in the usual
Anderson model and their statistics cannot obey one of the Gaussian ensembles.
Our analysis uses the notion of relative index between two projections and
indicates that the level repulsion is connected to topological aspects of
quantum Hall systems.Comment: 22 pages, no figure
Strong-coupling approach to the Mott--Hubbard insulator on a Bethe lattice in Dynamical Mean-Field Theory
We calculate the Hubbard bands for the half-filled Hubbard model on a Bethe
lattice with infinite coordination number up to and including third order in
the inverse Hubbard interaction. We employ the Kato--Takahashi perturbation
theory to solve the self-consistency equation of the Dynamical Mean-Field
Theory analytically for the single-impurity Anderson model in multi-chain
geometry. The weight of the secondary Hubbard sub-bands is of fourth order so
that the two-chain geometry is sufficient for our study. Even close to the
Mott--Hubbard transition, our results for the Mott--Hubbard gap agree very well
with those from numerical Dynamical Density-Matrix Renormalization Group
(DDMRG) calculations. The density of states of the lower Hubbard band also
agrees very well with DDMRG data, apart from a resonance contribution at the
upper band edge which cannot be reproduced in low-order perturbation theory.Comment: 40 pages, 7 figure
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