1,177 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
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
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
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
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