341 research outputs found
Zero-energy peak of the density of states and localization properties of a one-dimensional Frenkel exciton: Off-diagonal disorder
We study a one-dimensional Frenkel Hamiltonian with off-diagonal disorder,
focusing our attention on the physical nature of the zero-energy peak of the
density of states. The character of excitonic states (localized or delocalized)
is also examined in the vicinity of this peak. It is shown that the state being
responsible for the peak is localized. A detailed comparison of the
nearest-neighbor approach with the long-range dipole-dipole coupling is
performed.Comment: 15 pages with 7 figures (REVTeX). To appear in Physical Review
Understanding delocalization in the Continuous Random Dimer model
We propose an explanation of the bands of extended states appearing in random
one dimensional models with correlated disorder, focusing on the Continuous
Random Dimer model [A.\ S\'{a}nchez, E.\ Maci\'a, and F.\ Dom\'\i nguez-Adame,
Phys.\ Rev.\ B {\bf 49}, 147 (1994)]. We show exactly that the transmission
coefficient at the resonant energy is independent of the number of host sites
between two consecutive dimers. This allows us to understand why are there
bands of extended states for every realization of the model as well as the
dependence of the bandwidths on the concentration. We carry out a perturbative
calculation that sheds more light on the above results. In the conclusion we
discuss generalizations of our results to other models and possible
applications which arise from our new insight of this problem.Comment: REVTeX 3.0, 4 pages, 4 figures (hard copy on request from
[email protected]), Submitted to Phys Rev
Finite temperature dynamics of vortices in the two dimensional anisotropic Heisenberg model
We study the effects of finite temperature on the dynamics of non-planar
vortices in the classical, two-dimensional anisotropic Heisenberg model with
XY- or easy-plane symmetry. To this end, we analyze a generalized
Landau-Lifshitz equation including additive white noise and Gilbert damping.
Using a collective variable theory with no adjustable parameters we derive an
equation of motion for the vortices with stochastic forces which are shown to
represent white noise with an effective diffusion constant linearly dependent
on temperature. We solve these stochastic equations of motion by means of a
Green's function formalism and obtain the mean vortex trajectory and its
variance. We find a non-standard time dependence for the variance of the
components perpendicular to the driving force. We compare the analytical
results with Langevin dynamics simulations and find a good agreement up to
temperatures of the order of 25% of the Kosterlitz-Thouless transition
temperature. Finally, we discuss the reasons why our approach is not
appropriate for higher temperatures as well as the discreteness effects
observed in the numerical simulations.Comment: 12 pages, 8 figures, accepted for publication in European Physical
Journal B (uses EPJ LaTeX
FIBONACCI SUPERLATTICES OF NARROW-GAP III-V SEMICONDUCTORS
We report theoretical electronic structure of Fibonacci superlattices of
narrow-gap III-V semiconductors. Electron dynamics is accurately described
within the envelope-function approximation in a two-band model.
Quasiperiodicity is introduced by considering two different III-V semiconductor
layers and arranging them according to the Fibonacci series along the growth
direction. The resulting energy spectrum is then found by solving exactly the
corresponding effective-mass (Dirac-like) wave equation using tranfer-matrix
techniques. We find that a self-similar electronic spectrum can be seen in the
band structure. Electronic transport properties of samples are also studied and
related to the degree of spatial localization of electronic envelope-functions
via Landauer resistance and Lyapunov coefficient. As a working example, we
consider type II InAs/GaSb superlattices and discuss in detail our results in
this system.Comment: REVTeX 3.0, 16 pages, 8 figures available upon request. To appear in
Semiconductor Science and Technolog
Stability and decay of Bloch oscillations in presence of time-dependent nonlinearity
We consider Bloch oscillations of Bose-Einstein condensates in presence of a
time-modulated s-wave scattering length. Generically, interaction leads to
dephasing and decay of the wave packet. Based on a cyclic-time argument, we
find---additionally to the linear Bloch oscillation and a rigid soliton
solution---an infinite family of modulations that lead to a periodic time
evolution of the wave packet. In order to quantitatively describe the dynamics
of Bloch oscillations in presence of time-modulated interactions, we employ two
complementary methods: collective-coordinates and the linear stability analysis
of an extended wave packet. We provide instructive examples and address the
question of robustness against external perturbations.Comment: 15 pages, 8 figures. Slightly amended final versio
Dynamics and stability of Bose-Einstein solitons in tilted optical lattices
Bloch oscillations of Bose-Einstein condensates realize sensitive matter-wave
interferometers. We investigate the dynamics and stability of bright-soliton
wave packets in one-dimensional tilted optical lattices with a modulated
mean-field interaction . By means of a time-reversal argument, we prove
the stability of Bloch oscillations of breathing solitons that would be
quasistatically unstable. Floquet theory shows that these breathing solitons
can be more stable against certain experimental perturbations than rigid
solitons or even non-interacting wave packets.Comment: final, published versio
Dynamical phenomena in Fibonacci Semiconductor Superlattices
We present a detailed study of the dynamics of electronic wavepackets in
Fibonacci semiconductor superlattices, both in flat band conditions and subject
to homogeneous electric fields perpendicular to the layers. Coherent
propagation of electrons is described by means of a scalar Hamiltonian using
the effective-mass approximation. We have found that an initial Gaussian
wavepacket is filtered selectively when passing through the superlattice. This
means that only those components of the wavepacket whose wavenumber belong to
allowed subminibands of the fractal-like energy spectrum can propagate over the
entire superlattice. The Fourier pattern of the transmitted part of the
wavepacket presents clear evidences of fractality reproducing those of the
underlying energy spectrum. This phenomenon persists even in the presence of
unintentional disorder due to growth imperfections. Finally, we have
demonstrated that periodic coherent-field induced oscillations (Bloch
oscillations), which we are able to observe in our simulations of periodic
superlattices, are replaced in Fibonacci superlattices by more complex
oscillations displaying quasiperiodic signatures, thus sheding more light onto
the very peculiar nature of the electronic states in these systems.Comment: 7 pagex, RevTex, 5 Postscript figures. Physical Review B (in press
Electron dynamics in intentionally disordered semiconductor superlattices
We study the dynamical behavior of disordered quantum-well-based
semiconductor superlattices where the disorder is intentional and short-range
correlated. We show that, whereas the transmission time of a particle grows
exponentially with the number of wells in an usual disordered superlattice for
any value of the incident particle energy, for specific values of the incident
energy this time increases linearly when correlated disorder is included. As
expected, those values of the energy coincide with a narrow subband of extended
states predicted by the static calculations of Dom\'{\i}nguez-Adame {\em et
al.} [Phys. Rev. B {\bf 51}, 14 ,359 (1994)]; such states are seen in our
dynamical results to exhibit a ballistic regime, very close to the WKB
approximation of a perfect superlattice. Fourier transform of the output signal
for an incident Gaussian wave packet reveals a dramatic filtering of the
original signal, which makes us confident that devices based on this property
may be designed and used for nanotechnological applications. This is more so in
view of the possibility of controllingthe outp ut band using a dc electric
field, which we also discuss. In the conclusion we summarize our results and
present an outlook for future developments arising from this work.Comment: 10 pagex, RevTex, 13 Postscript figures. Physical Review B (in press
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