307 research outputs found
Multiquantum well spin oscillator
A dc voltage biased II-VI semiconductor multiquantum well structure attached
to normal contacts exhibits self-sustained spin-polarized current oscillations
if one or more of its wells are doped with Mn. Without magnetic impurities, the
only configurations appearing in these structures are stationary. Analysis and
numerical solution of a nonlinear spin transport model yield the minimal number
of wells (four) and the ranges of doping density and spin splitting needed to
find oscillations.Comment: 11 pages, 2 figures, shortened and updated versio
Microscopic Model for Sequential Tunneling in Semiconductor Multiple Quantum Wells
We propose a selfconsistent microscopic model of vertical sequential
tunneling through a multi-quantum well.The model includes a detailed
description of the contacts,uses the Transfer Hamiltonian for expressions of
the current and it treats the Coulomb interaction within a mean field
approximation. We analyze the current density through a double well and a
superlattice and study the formation of electric field domains and
multistability coming from the Coulomb interaction. Phase diagrams of parameter
regions (bias, doping in the heterostructure and in the contacts,etc) where the
different solutions exist are given.Comment: 4 pages, 8 Postscript Figure
Current self-oscillations, spikes and crossover between charge monopole and dipole waves in semiconductor superlattices
Self-sustained current oscillations in weakly-coupled superlattices are
studied by means of a self-consistent microscopic model of sequential tunneling
including boundary conditions naturally. Well-to-well hopping and recycling of
charge monopole domain walls produce current spikes (high frequency modulation)
superimposed on the oscillation. For highly doped injecting contacts, the
self-oscillations are due to dynamics of monopoles. As the contact doping
decreases, a lower-frequency oscillatory mode due to recycling and motion of
charge dipoles is predicted. For low contact doping, this mode dominates and
monopole oscillations disappear. At intermediate doping, both oscillation modes
coexist as stable solutions and hysteresis between them is possible.Comment: 4 pages, 4 figure
Magnetoswitching of current oscillations in diluted magnetic semiconductor nanostructures
Strongly nonlinear transport through Diluted Magnetic Semiconductor
multiquantum wells occurs due to the interplay between confinement, Coulomb and
exchange interaction. Nonlinear effects include the appearance of spin
polarized stationary states and self-sustained current oscillations as possible
stable states of the nanostructure, depending on its configuration and control
parameters such as voltage bias and level splitting due to an external magnetic
field. Oscillatory regions grow in size with well number and level splitting. A
systematic analysis of the charge and spin response to voltage and magnetic
field switching of II-VI Diluted Magnetic Semiconductor multiquantum wells is
carried out. The description of stationary and time-periodic spin polarized
states, the transitions between them and the responses to voltage or magnetic
field switching have great importance due to the potential implementation of
spintronic devices based on these nanostructures.Comment: 14 pages, 4 figures, Revtex, to appear in PR
Temperature effects on microwave-induced resistivity oscillations and zero resistance states in 2D electron systems
In this work we address theoretically a key issue concerning
microwave-induced longitudinal resistivity oscillations and zero resistance
states, as is tempoerature. In order to explain the strong temperature
dependence of the longitudinal resistivity and the thermally activated
transport in 2DEG, we have developed a microscopic model based on the damping
suffered by the microwave-driven electronic orbit dynamics by interactions with
the lattice ions yielding acoustic phonons. Recent experimental results show a
reduction in the amplitude of the longitudinal resistivity oscillations and a
breakdown of zero resistance states as the radiation intensity increases. In
order to explain it we have included in our model the electron heating due to
large microwave intensities and its effect on the longitudinal resistivity.Comment: 4 pages and 4 figures. Accepted in Phys Rev
Temperature-dependent dynamical nuclear polarization bistabilities in double quantum dots in the spin-blockade regime
The interplay of dynamical nuclear polarization (DNP) and leakage current
through a double quantum dot in the spin-blockade regime is analyzed. A finite
DNP is built up due to a competition between hyperfine (HF) spin-flip
transitions and another inelastic escape mechanism from the triplets, which
block transport. We focus on the temperature dependence of the DNP for zero
energy-detuning (i.e. equal electrostatic energy of one electron in each dot
and a singlet in the right dot). Our main result is the existence of a
transition temperature, below which the DNP is bistable, so a hysteretic
leakage current versus external magnetic field B appears. This is studied in
two cases: (i) Close to the crossing of the three triplet energy levels near
B=0, where spin-blockade is lifted due to the inhomogeneity of the effective
magnetic field from the nuclei. (ii) At higher B-fields, where the two
spin-polarized triplets simultaneously cross two different singlet energy
levels. We develop simplified models leading to different transition
temperatures T_TT and T_ST for the crossing of the triplet levels and the
singlet-triplet level crossings, respectively. We find T_TT analytically to be
given solely by the HF couplings, whereas T_ST depends on various parameters
and T_ST>T_TT. The key idea behind the existence of the transition temperatures
at zero energy-detuning is the suppression of energy absorption compared to
emission in the inelastic HF transitions. Finally, by comparing the rate
equation results with Monte Carlo simulations, we discuss the importance of
having both HF interaction and another escape mechanism from the triplets to
induce a finite DNP.Comment: 26 pages, 17 figure
Effects of noise on hysteresis and resonance width in graphene and nanotubes resonators
We investigate the role that noise plays in the hysteretic dynamics of a
suspended nanotube or a graphene sheet subject to an oscillating force. We find
that not only the size but also the position of the hysteresis region in these
systems can be controlled by noise. We also find that nano-resonators act as
noise rectifiers: by increasing the noise in the setup, the resonance width of
the characteristic peak in these systems is reduced and, as a result, the
quality factor is increased.Comment: 15 pages, 6 figures. Sent to PRB (in revision
Coarse-graining the vertex model and its response to shear
Tissue dynamics and collective cell motion are crucial biological processes.
Their biological machinery is mostly known, and simulation models such as the
"active vertex model" (AVM) exist and yield reasonable agreement with
experimental observations like tissue fluidization or fingering. However, a
good and well-founded continuum description for tissues remains to be
developed. In this work we derive a macroscopic description for a
two-dimensional cell monolayer by coarse-graining the vertex model through the
Poisson bracket approach. We obtain equations for cell density, velocity and
the cellular shape tensor. We then study the homogeneous steady states, their
stability (which coincides with thermodynamic stability), and especially their
behavior under an externally applied shear. Our results contribute to elucidate
the interplay between flow and cellular shape. The obtained macroscopic
equations present a good starting point for adding cell motion, morphogenetic
and other biologically relevant processes.Comment: 14 pages, 11 figure
Photoassisted sequential resonant tunneling through superlattices
We have analyzed theoretically the photoassisted tunneling current through a
superlattice in the presence of an AC potential. For that purpose we have
developed a new model to calculate the sequential resonant currrent trhough a
superlattice based in the TRansfer Hamiltonian Method. The tunneling current
presents new features due to new effective tunneling chanels coming from the
photoside bands induced by the AC field. Our theoretical results are in good
agreement with the available experimental evidence.Comment: Revtex 3.0 4 pages, 4 figures uuencoded compressed tar-fil
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