93,201 research outputs found
Hole tunneling times in GaAs/AlAs double-barrier structures
We have calculated hole tunneling times in GaAs/AlAs double-barrier structures taking quantum well band-mixing effects into account. Our results indicate that for sufficiently high hole temperatures and concentrations, band-mixing effects reduce average hole tunneling times from the pure heavy hole value to values comparable to electron tunneling times in the same structure. For very low hole temperatures and concentrations, band mixing is less important and average hole tunneling times should approach the pure heavy hole value. These results provide an explanation for previously reported experimental results in which electrons and holes were found to be characterized by very similar tunneling times
Manifestation of superfluidity in an evolving Bose-condensed gas
We study the generation of excitations due to an ''impurity''(static
perturbation) placed into an oscillating Bose-condensed gas in the
time-dependent trapping field. It is shown that there are two regions for the
position of the local perturbation. In the first region the condensate flows
around the ''impurity'' without generation of excitations demonstrating
superfluid properties. In the second region the creation of excitations occurs,
at least within a limited time interval, revealing destruction of
superfluidity. The phenomenon can be studied by measuring the damping of
condensate oscillations at different positions of the ''impurity''
Localized ferromagnetic resonance force microscopy in permalloy-cobalt films
We report Ferromagnetic Resonance Force Microscopy (FMRFM) experiments on a
justaposed continuous films of permalloy and cobalt. Our studies demonstrate
the capability of FMRFM to perform local spectroscopy of different
ferromagnetic materials. Theoretical analysis of the uniform resonance mode
near the edge of the film agrees quantitatively with experimental data. Our
experiments demonstrate the micron scale lateral resolution in determining
local magnetic properties in continuous ferromagnetic samples.Comment: 7 pages, 3 figure
Nonlinear ac response of anisotropic composites
When a suspension consisting of dielectric particles having nonlinear
characteristics is subjected to a sinusoidal (ac) field, the electrical
response will in general consist of ac fields at frequencies of the
higher-order harmonics. These ac responses will also be anisotropic. In this
work, a self-consistent formalism has been employed to compute the induced
dipole moment for suspensions in which the suspended particles have nonlinear
characteristics, in an attempt to investigate the anisotropy in the ac
response. The results showed that the harmonics of the induced dipole moment
and the local electric field are both increased as the anisotropy increases for
the longitudinal field case, while the harmonics are decreased as the
anisotropy increases for the transverse field case. These results are
qualitatively understood with the spectral representation. Thus, by measuring
the ac responses both parallel and perpendicular to the uniaxial anisotropic
axis of the field-induced structures, it is possible to perform a real-time
monitoring of the field-induced aggregation process.Comment: 14 pages and 4 eps figure
The Crossover from Impurity to Valence Band in Diluted Magnetic Semiconductors: The Role of the Coulomb Attraction by Acceptor
The crossover between an impurity band (IB) and a valence band (VB) regime as
a function of the magnetic impurity concentration in models for diluted
magnetic semiconductors (DMS) is studied systematically by taking into
consideration the Coulomb attraction between the carriers and the magnetic
impurities. The density of states and the ferromagnetic transition temperature
of a Spin-Fermion model applied to DMS are evaluated using Dynamical Mean-Field
Theory (DMFT) and Monte Carlo (MC) calculations. It is shown that the addition
of a square-well-like attractive potential can generate an IB at small enough
Mn doping for values of the exchange that are not strong enough
to generate one by themselves. We observe that the IB merges with the VB when
where is a function of and the Coulomb attraction strength
. Using MC calculations, we demonstrate that the range of the Coulomb
attraction plays an important role. While the on-site attraction, that has been
used in previous numerical simulations, effectively renormalizes for all
values of , an unphysical result, a nearest-neighbor range attraction
renormalizes only at very low dopings, i.e., until the bound holes wave
functions start to overlap. Thus, our results indicate that the Coulomb
attraction can be neglected to study Mn doped GaSb, GaAs, and GaP in the
relevant doping regimes, but it should be included in the case of Mn doped GaN
that is expected to be in the IB regime.Comment: 8 pages, 4 Postscript figures, RevTex
Landau-Zener Interference in Multilevel Superconducting Flux Qubits Driven by Large Amplitude Fields
We proposed an analytical model to analyze the Landau-Zener interference in a
multilevel superconducting flux qubit driven by large amplitude external
fields. Our analytical results agree remarkably with those of the experiment
[Nature 455, 51 (2008)]. Moreover, we studied the effect of driving-frequency
and dephasing rate on the interference. The dephasing generally destroys the
interference while increasing frequency rebuilds the interference at large
dephasing rate. At certain driving frequency and dephasing rate, the
interference shows some anomalous features as observed in recent experiments.Comment: 7 pages, 6 figure
Dielectric Behavior of Nonspherical Cell Suspensions
Recent experiments revealed that the dielectric dispersion spectrum of
fission yeast cells in a suspension was mainly composed of two sub-dispersions.
The low-frequency sub-dispersion depended on the cell length, whereas the
high-frequency one was independent of it. The cell shape effect was
qualitatively simulated by an ellipsoidal cell model. However, the comparison
between theory and experiment was far from being satisfactory. In an attempt to
close up the gap between theory and experiment, we considered the more
realistic cells of spherocylinders, i.e., circular cylinders with two
hemispherical caps at both ends. We have formulated a Green function formalism
for calculating the spectral representation of cells of finite length. The
Green function can be reduced because of the azimuthal symmetry of the cell.
This simplification enables us to calculate the dispersion spectrum and hence
access the effect of cell structure on the dielectric behavior of cell
suspensions.Comment: Preliminary results have been reported in the 2001 March Meeting of
the American Physical Society. Accepted for publications in J. Phys.:
Condens. Matte
Two-band modeling of narrow band gap and interband tunneling devices
A two-band transfer matrix method has been developed to study tunneling currents in narrow gap and interband tunnel structures. This relatively simple model gives good agreement with recently reported experimental results for InAs/AlSb/InAs/AlSb/InAs double-barrier heterostructures and InAs/AlSb/GaSb/AlSb/InAs resonant interband tunneling devices, and should be useful in the design of new interband tunneling devices
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