195 research outputs found
Localization properties of driven disordered one-dimensional systems
We generalize the definition of localization length to disordered systems
driven by a time-periodic potential using a Floquet-Green function formalism.
We study its dependence on the amplitude and frequency of the driving field in
a one-dimensional tight-binding model with different amounts of disorder in the
lattice. As compared to the autonomous system, the localization length for the
driven system can increase or decrease depending on the frequency of the
driving. We investigate the dependence of the localization length with the
particle's energy and prove that it is always periodic. Its maximum is not
necessarily at the band center as in the non-driven case. We study the
adiabatic limit by introducing a phenomenological inelastic scattering rate
which limits the delocalizing effect of low-frequency fields.Comment: Accepted for publication in European Physical Journal
Ed O’Bannon v. NCAA: Do Former NCAA Athletes Have a Case Against the NCAA for Its Use of Their Likenesses?
Spheroidal nanoparticles as thermal near-field sensors
We suggest to exploit the shape-dependence of the near-field heat transfer
for nanoscale thermal imaging. By utilizing strongly prolate or oblate
nanoparticles as sensors one can assess individual components of the
correlation tensors characterizing the thermal near field close to a
nanostructured surface, and thus obtain directional information beyond the
local density of states. Our theoretical considerations are backed by idealized
numerical model calculations
Ac Stark Effects and Harmonic Generation in Periodic Potentials
The ac Stark effect can shift initially nonresonant minibands in
semiconductor superlattices into multiphoton resonances. This effect can result
in strongly enhanced generation of a particular desired harmonic of the driving
laser frequency, at isolated values of the amplitude.Comment: RevTeX, 10 pages (4 figures available on request), Preprint
UCSBTH-93-2
Bogoliubov speed of sound for a dilute Bose-Einstein condensate in a 3d optical lattice
We point out that the velocity of propagation of sound wavepackets in a Bose-Einstein condensate filling a three-dimensional cubic optical lattice undergoes a maximum with increasing lattice depth. For a realistic choice of parameters, the maximum sound velocity in a lattice condensate can exceed the sound velocity in a homogeneous condensate with the same average density by 30%. The maximum falls into the superfluid regime, and should be observable under currently achievable laboratory conditions
Level crossings in a cavity QED model
In this paper I study the dynamics of a two-level atom interacting with a
standing wave field. When the atom is subjected to a weak linear force, the
problem can be turned into a time dependent one, and the evolution is
understood from the band structure of the spectrum. The presence of level
crossings in the spectrum gives rise to Bloch oscillations of the atomic
motion. Here I investigate the effects of the atom-field detuning parameter. A
variety of different level crossings are obtained by changing the magnitude of
the detuning, and the behaviour of the atomic motion is strongly affected due
to this. I also consider the situation in which the detuning is oscillating in
time and its impact on the atomic motion. Wave packet simulations of the full
problem are treated numerically and the results are compared with analytical
solutions given by the standard Landau-Zener and the three-level Landau-Zener
models.Comment: 12 pages, 10 figure
Theory of Coherent Time-dependent Transport in One-dimensional Multiband Semiconductor Superlattices
We present an analytical study of one-dimensional semiconductor superlattices
in external electric fields, which may be time-dependent. A number of general
results for the (quasi)energies and eigenstates are derived. An equation of
motion for the density matrix is obtained for a two-band model, and the
properties of the solutions are analyzed. An expression for the current is
obtained. Finally, Zener-tunneling in a two-band tight-binding model is
considered. The present work gives the background and an extension of the
theoretical framework underlying our recent Letter [J. Rotvig {\it et al.},
Phys. Rev. Lett. {\bf 74}, 1831 (1995)], where a set of numerical simulations
were presented.Comment: 15 pages, Revtex 3.0, uses epsf, 2 ps figures attache
Perturbation theory for plasmonic eigenvalues
We develop a perturbative approach for calculating, within the quasistatic
approximation, the shift of surface resonances in response to a deformation of
a dielectric volume. Our strategy is based on the conversion of the homogeneous
system for the potential which determines the plasmonic eigenvalues into an
inhomogeneous system for the potential's derivative with respect to the
deformation strength, and on the exploitation of the corresponding
compatibility condition. The resulting general expression for the first-order
shift is verified for two explicitly solvable cases, and for a realistic
example of a deformed nanosphere. It can be used for scanning the huge
parameter space of possible shape fluctuations with only quite small
computational effort
Dressed matter waves
We suggest to view ultracold atoms in a time-periodically shifted optical
lattice as a "dressed matter wave", analogous to a dressed atom in an
electromagnetic field. A possible effect lending support to this concept is a
transition of ultracold bosonic atoms from a superfluid to a Mott-insulating
state in response to appropriate "dressing" achieved through time-periodic
lattice modulation. In order to observe this effect in a laboratory experiment,
one has to identify conditions allowing for effectively adiabatic motion of a
many-body Floquet state.Comment: 9 pages, 4 figures, to be published in: J. Phys.: Conference Serie
Dynamical control of correlated states in a square quantum dot
In the limit of low particle density, electrons confined to a quantum dot
form strongly correlated states termed Wigner molecules, in which the Coulomb
interaction causes the electrons to become highly localized in space. By using
an effective model of Hubbard-type to describe these states, we investigate how
an oscillatory electric field can drive the dynamics of a two-electron Wigner
molecule held in a square quantum dot. We find that, for certain combinations
of frequency and strength of the applied field, the tunneling between various
charge configurations can be strongly quenched, and we relate this phenomenon
to the presence of anti-crossings in the Floquet quasi-energy spectrum. We
further obtain simple analytic expressions for the location of these
anti-crossings, which allows the effective parameters for a given quantum dot
to be directly measured in experiment, and suggests the exciting possibility of
using ac-fields to control the time evolution of entangled states in mesoscopic
devices.Comment: Replaced with version to be published in Phys. Rev.
- …