622 research outputs found
Response properties of III-V dilute magnetic semiconductors: interplay of disorder, dynamical electron-electron interactions and band-structure effects
A theory of the electronic response in spin and charge disordered media is
developed with the particular aim to describe III-V dilute magnetic
semiconductors like GaMnAs. The theory combines a detailed k.p description of
the valence band, in which the itinerant carriers are assumed to reside, with
first-principles calculations of disorder contributions using an
equation-of-motion approach for the current response function. A fully dynamic
treatment of electron-electron interaction is achieved by means of
time-dependent density functional theory. It is found that collective
excitations within the valence band significantly increase the carrier
relaxation rate by providing effective channels for momentum relaxation. This
modification of the relaxation rate, however, only has a minor impact on the
infrared optical conductivity in GaMnAs, which is mostly determined by the
details of the valence band structure and found to be in agreement with
experiment.Comment: 15 pages, 9 figure
Predicted band structures of III-V semiconductors in wurtzite phase
While non-nitride III-V semiconductors typically have a zincblende structure,
they may also form wurtzite crystals under pressure or when grown as
nanowhiskers. This makes electronic structure calculation difficult since the
band structures of wurtzite III-V semiconductors are poorly characterized. We
have calculated the electronic band structure for nine III-V semiconductors in
the wurtzite phase using transferable empirical pseudopotentials including
spin-orbit coupling. We find that all the materials have direct gaps. Our
results differ significantly from earlier {\it ab initio} calculations, and
where experimental results are available (InP, InAs and GaAs) our calculated
band gaps are in good agreement. We tabulate energies, effective masses, and
linear and cubic Dresselhaus zero-field spin-splitting coefficients for the
zone-center states. The large zero-field spin-splitting coefficients we find
may lead to new functionalities for designing devices that manipulate spin
degrees of freedom
On-chip cavity quantum phonodynamics with an acceptor qubit in silicon
We describe a chip-based, solid-state analogue of cavity-QED utilizing
acoustic phonons instead of photons. We show how long-lived and tunable
acceptor impurity states in silicon nanomechanical cavities can play the role
of a matter non-linearity for coherent phonons just as, e.g., the Josephson
qubit plays in circuit-QED. Both strong coupling (number of Rabi oscillations ~
100) and strong dispersive coupling (0.1-2 MHz) regimes can be reached in
cavities in the 1-20 GHz range, enabling the control of single phonons,
phonon-phonon interactions, dispersive phonon readout of the acceptor qubit,
and compatibility with other optomechanical components such as phonon-photon
translators. We predict explicit experimental signatures of the acceptor-cavity
system.Comment: 6 pages, 2 figures, PDFLaTeX. New version improves clarit
Hole spin relaxation in intrinsic and -type bulk GaAs
We investigate hole spin relaxation in intrinsic and -type bulk GaAs from
a fully microscopic kinetic spin Bloch equation approach. In contrast to the
previous study on hole spin dynamics, we explicitly include the intraband
coherence and the nonpolar hole-optical-phonon interaction, both of which are
demonstrated to be of great importance to the hole spin relaxation. The
relative contributions of the D'yakonov-Perel' and Elliott-Yafet mechanisms on
hole spin relaxation are also analyzed. In our calculation, the screening
constant, playing an important role in the hole spin relaxation, is treated
with the random phase approximation. In intrinsic GaAs, our result shows good
agreement with the experiment data at room temperature, where the hole spin
relaxation is demonstrated to be dominated by the Elliott-Yafet mechanism. We
also find that the hole spin relaxation strongly depends on the temperature and
predict a valley in the density dependence of the hole spin relaxation time at
low temperature due to the hole-electron scattering. In -type GaAs, we
predict a peak in the spin relaxation time against the hole density at low
temperature, which originates from the distinct behaviors of the screening in
the degenerate and nondegenerate regimes. The competition between the screening
and the momentum exchange during scattering events can also lead to a valley in
the density dependence of the hole spin relaxation time in the low density
regime. At high temperature, the effect of the screening is suppressed due to
the small screening constant. Moreover, we predict a nonmonotonic dependence of
the hole spin relaxation time on temperature associated with the screening
together with the hole-phonon scattering. Finally, we find that the
D'yakonov-Perel' mechanism can markedly contribute to the .... (omitted due to
the limit of space)Comment: 11 pages, 7 figures, Phys. Rev. B, in pres
D'yakonov-Perel' spin relaxation for degenerate electrons in the electron-hole liquid
We present an analytical study of the D'yakonov-Perel' spin relaxation time
for degenerate electrons in a photo-excited electron-hole liquid in intrinsic
semiconductors exhibiting a spin-split band structure. The D'yakonov-Perel'
spin relaxation of electrons in these materials is controlled by electron-hole
scattering, with small corrections from electron-electron scattering and
virtually none from electron-impurity scattering. We derive simple expressions
(one-dimensional and two-dimensional integrals respectively) for the effective
electron-hole and electron-electron scattering rates which enter the spin
relaxation time calculation. The electron-hole scattering rate is found to be
comparable to the scattering rates from impurities in the electron liquid - a
common model for n-type doped semiconductors. As the density of electron-hole
pairs decreases (within the degenerate regime), a strong enhancement of the
scattering rates and a corresponding slowing down of spin relaxation is
predicted due to exchange and correlation effects in the electron-hole liquid.
In the opposite limit of high density, the original D'yakonov-Perel' model
fails due to decreasing scattering rates and is eventually superseded by free
precession of individual quasiparticle spins.Comment: 16 pages, 5 figure
Spin relaxation in -type ZnO quantum wells
We perform an investigation on the spin relaxation for -type ZnO (0001)
quantum wells by numerically solving the kinetic spin Bloch equations with all
the relevant scattering explicitly included. We show the temperature and
electron density dependence of the spin relaxation time under various
conditions such as impurity density, well width, and external electric field.
We find a peak in the temperature dependence of the spin relaxation time at low
impurity density. This peak can survive even at 100 K, much higher than the
prediction and measurement value in GaAs. There also exhibits a peak in the
electron density dependence at low temperature. These two peaks originate from
the nonmonotonic temperature and electron density dependence of the Coulomb
scattering. The spin relaxation time can reach the order of nanosecond at low
temperature and high impurity density.Comment: 6 pages, 4 figure
Pauli blockade of the electron spin flip in bulk GaAs
By means of time-resolved optical orientation under strong optical pumping,
the k-dependence of the electron spin-flip time (t_sf) in undoped GaAs is
experimentally determined. t_sf monotonically decreases by more than one order
of magnitude when the electron kinetic energy varies from 2 to 30 meV. At the
high excitation densities and low temperatures of the reported experiments the
main spin-flip mechanism of the conduction band electrons is the
Bir-Aronov-Pikus. By means of Monte-Carlo simulations we evidence that
phase-space filling effects result in the blocking of the spin flip, yielding
an increase of t_sf with excitation density. These effects obtain values of
t_sf up to 30 ns at k=0, the longest reported spin-relaxation time in undoped
GaAs in the absence of a magnetic field.Comment: new author added, major changes in section IV (phenomenological
model), minor changes throughout the entire manuscrip
Hole spin dephasing time associated to hyperfine interaction in quantum dots
The spin interaction of a hole confined in a quantum dot with the surrounding
nuclei is described in terms of an effective magnetic field. We show that, in
contrast to the Fermi contact hyperfine interaction for conduction electrons,
the dipole-dipole hyperfine interaction is anisotropic for a hole, for both
pure or mixed hole states. We evaluate the coupling constants of the
hole-nuclear interaction and demonstrate that they are only one order of
magnitude smaller than the coupling constants of the electron-nuclear
interaction. We also study, theoretically, the hole spin dephasing of an
ensemble of quantum dots via the hyperfine interaction in the framework of
frozen fluctuations of the nuclear field, in absence or in presence of an
applied magnetic field. We also discuss experiments which could evidence the
dipole-dipole hyperfine interaction and give information on hole mixing.Comment: 35 pages, 7 figures and 2 table
Effective Hamiltonian of Strained Graphene
Based on the symmetry properties of graphene lattice, we derive the effective
Hamiltonian of graphene under spatially non-uniform acoustic and optical
strains. We show that with the proper selection of the parameters, the obtained
Hamiltonian reproduces the results of first-principles spectrum calculations
for acoustic strain up to 10%. The results are generalized for the case of
graphene with broken plane reflection symmetry, which corresponds, for example,
to the case of graphene placed at a substrate. Here, essential modifications to
the Hamiltonian give rise, in particular, to the gap opening in the spectrum in
the presence of the out of plane component of optical strain, which is shown to
be due to the lifting of the sublattice symmetry. The developed effective
Hamiltonian can be used as a convenient tool for analysis of a variety of
strain-related effects, including electron-phonon interaction or
pseudo-magnetic fields induced by the non-uniform strain
Soil Baiting, Rapid PCR Assay and Quantitative Real TIME PCR to Diagnose Late Blight of Potato in Quarantine Programs
Phytophthora infestans (mont) de Bary is a pathogen of great concern across the globe, and accurate detection is an important component in responding to the outbreaks of potential disease. Although the molecular diagnostic protocol used in regulatory programs has been evaluated but till date methods implying direct comparison has rarely used. In this study, a known area soil samples from potato fields where light blight appear every year (both A1 and A2 mating type) was assayed by soil bait method, PCR assay detection and quantification of the inoculums. Suspected disease symptoms appeared on bait tubers were further confirmed by rapid PCR, inoculums were quantified through Real Time PCR, which confirms presence of P. infestans. These diagnostic methods can be highly correlated with one another. Potato tuber baiting increased the sensitivity of the assay compared with direct extraction of DNA from tuber and soil samples. Our study determines diagnostic sensitivity and specificity of the assays to determine the performance of each method. Overall, molecular techniques based on different types of PCR amplification and Real-time PCR can lead to high throughput, faster and more accurate detection method which can be used in quarantine programmes in potato industry and diagnostic laboratory
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