1,379 research outputs found
Analytic treatment of the precessional (ballistic) contribution to the conventional magnetic switching
We consider a switching of the magnetic moment with an easy axis anisotropy
from an "up" to a "down" direction under the influence of an external magnetic
field. The driving field is applied parallel to the easy axis and is
continuously swept from a positive to a negative value. In addition, a small
constant perpendicular bias field is present. It is shown that while the
driving field switches the moment in a conventional way, the perpendicular
field creates an admixture of the precessional (ballistic) switching that
speeds up the switching process. Precessional contribution produces a
non-monotonic dependence of the switching time on the field sweep time with a
minimum at a particular sweep time value. We derive an analytic expressions for
the optimal point, and for the entire dependence of the switching time on the
field sweep time. Our approximation is valid in a wide parameter range and can
be used to engineer and optimize of the magnetic memory devices.Comment: 13 pages, 7 figure
On the Third Critical Speed for Rotating Bose-Einstein Condensates
We study a two-dimensional rotating Bose-Einstein condensate confined by an
anharmonic trap in the framework of the Gross-Pitaevksii theory. We consider a
rapid rotation regime close to the transition to a giant vortex state. It was
proven in [M. Correggi {\it et al}, {\it J. Math. Phys. \textbf{53}(2012)] that
such a transition occurs when the angular velocity is of order , with denoting the coefficient of the nonlinear
term in the Gross-Pitaevskii functional and (Thomas-Fermi
regime). In this paper we identify a finite value such
that, if with , the condensate is in the giant vortex phase. Under the
same condition we prove a refined energy asymptotics and an estimate of the
winding number of any Gross-Pitaevskii minimizer.Comment: pdfLaTeX, 39 pages, minor changes, to appear in J. Math. Phy
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.
Enhancement of Persistent Current in Metal Rings by Correlated Disorder
We study analytically the effect of a correlated random potential on the
persistent current in a one-dimensional ring threaded by a magnetic flux
, using an Anderson tight-binding model. In our model, the system of
atomic sites of the ring is assumed to be partitioned into pairs of
identical nearest-neighbour sites (dimers). The site energies for different
dimers are taken to be uncorrelated gaussian variables. For this system we
obtain the exact flux-dependent energy levels to second order in the random
site energies, using an earlier exact transfer matrix perturbation theory.
These results are used to study the mean persistent current generated by
spinless electrons occupying the lowest levels of the
flux-dependent energy band at zero temperature. Detailed analyses are carried
out in the limit and for a half-filled band (), for
magnetic fluxes . While the uncorrelated disorder leads
to a reduction of the persistent current, the disorder correlation acts to
enhance it. In particular, in the half-filled band case the correlated disorder
leads to a global flux-dependent enhancement of persistent current which has
the same form for even and odd . At low filling of the energy band the
effect of the disorder on the persistent current is found to depend on the
parity of : the correlated disorder yields a reduction of the current for
odd and an enhancement of the current for even .Comment: 1
Beats of the Magnetocapacitance Oscillations in Lateral Semiconductor Superlattices
We present calculations on the magnetocapacitance of the two-dimensional
electron gas in a lateral semiconductor superlattice under two-dimensional weak
periodic potential modulation in the presence of a perpendicular magnetic
field. Adopting a Gaussian broadening of magnetic-field-dependent width in the
density of states, we present explicit and simple expressions for the
magnetocapacitance, valid for the relevant weak magnetic fields and modulation
strengths. As the modulation strength in both directions increase, beats of the
magnetocapacitance oscillations are observed, in the low magnetic field range
(Weiss-oscillations regime), which are absent in the one-dimensional weak
modulation case.Comment: 11 pages, 7 figures, accepted by Mod. Phys. Lett. B (March 2007
Thermodynamics of Rotating Black Branes in Gauss-Bonnet-Born-Infeld Gravity
Considering both the Gauss-Bonnet and the Born-Infeld terms, which are on
similar footing with regard to string corrections on the gravity side and
electrodynamic side, we present a new class of rotating solutions in
Gauss-Bonnet gravity with rotation parameters in the presence of a
nonlinear electromagnetic field. These solutions, which are asymptotically
anti-de Sitter in the presence of cosmological constant, may be interpreted as
black brane solutions with inner and outer event horizons, an extreme black
brane or naked singularity provided the metric parameters are chosen suitably.
We calculate the finite action and conserved quantities of the solutions by
using the counterterm method, and find that these quantities do not depend on
the Gauss-Bonnet parameter. We also compute the temperature, the angular
velocities, the electric charge and the electric potential. Then, we calculate
the entropy of the black brane through the use of Gibbs-Duhem relation and show
that it obeys the area law of entropy. We obtain a Smarr-type formula for the
mass as a function of the entropy, the angular momenta and the charge, and show
that the conserved and thermodynamic quantities satisfy the first law of
thermodynamics. Finally, we perform a stability analysis in both the canonical
and grand-canonical ensemble and show that the presence of a nonlinear
electromagnetic field has no effect on the stability of the black branes, and
they are stable in the whole phase space.Comment: 17 pages, one figur
Coherent properties of nano-electromechanical systems
We study the properties of a nano-electromechanical system in the coherent
regime, where the electronic and vibrational time scales are of the same order.
Employing a master equation approach, we obtain the stationary reduced density
matrix retaining the coherences between vibrational states. Depending on the
system parameters, two regimes are identified, characterized by either () an
{\em effective} thermal state with a temperature {\em lower} than that of the
environment or () strong coherent effects. A marked cooling of the
vibrational degree of freedom is observed with a suppression of the vibron Fano
factor down to sub-Poissonian values and a reduction of the position and
momentum quadratures.Comment: 12 pages, 11 figure
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
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