203 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
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
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
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
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
Bipolar spin blockade and coherent state superpositions in a triple quantum dot
Spin qubits based on interacting spins in double quantum dots have been
successfully demonstrated. Readout of the qubit state involves a conversion of
spin to charge information, universally achieved by taking advantage of a spin
blockade phenomenon resulting from Pauli's exclusion principle. The archetypal
spin blockade transport signature in double quantum dots takes the form of a
rectified current. Currently more complex spin qubit circuits including triple
quantum dots are being developed. Here we show both experimentally and
theoretically (a) that in a linear triple quantum dot circuit, the spin
blockade becomes bipolar with current strongly suppressed in both bias
directions and (b) that a new quantum coherent mechanism becomes relevant.
Within this mechanism charge is transferred non-intuitively via coherent states
from one end of the linear triple dot circuit to the other without involving
the centre site. Our results have implications in future complex
nano-spintronic circuits.Comment: 21 pages, 7 figure
Canted phase in double quantum dots
We perform a Hartree-Fock calculation in order to describe the ground state
of a vertical double quantum dot in the absence of magnetic fields parallel to
the growth direction. Intra- and interdot exchange interactions determine the
singlet or triplet character of the system as the tunneling is tuned. At finite
Zeeman splittings due to in-plane magnetic fields, we observe the continuous
quantum phase transition from ferromagnetic to symmetric phase through a canted
antiferromagnetic state. The latter is obtained even at zero Zeeman energy for
an odd electron number.Comment: 5 pages, 3 figure
Spin correlations in spin blockade
We investigate spin currents and spin-current correlations for double quantum
dots in the spin blockade regime. By analysing the time evolution of the
density matrix, we obtain the spin resolved currents and derive from a
generating function an expression for the fluctuations and correlations. Both
the charge current and the spin current turn out to be generally
super-Poissonian. Moreover, we study the influence of ac fields acting upon the
transported electrons. In particular, we focus on fields that cause spin
rotation or photon-assisted tunnelling.Comment: 14 pages, 9 figure
Gain in quantum cascade lasers and superlattices: A quantum transport theory
Gain in current-driven semiconductor heterostructure devices is calculated
within the theory of nonequilibrium Green functions. In order to treat the
nonequilibrium distribution self-consistently the full two-time structure of
the theory is employed without relying on any sort of Kadanoff-Baym Ansatz. The
results are independent of the choice of the electromagnetic field if the
variation of the self-energy is taken into account. Excellent quantitative
agreement is obtained with the experimental gain spectrum of a quantum cascade
laser. Calculations for semiconductor superlattices show that the simple 2-time
miniband transport model gives reliable results for large miniband widths at
room temperatureComment: 8 Pages, 4 Figures directly included, to appear in Physical Review
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