677 research outputs found
vaccination in a patient with Behcet's disease
Case report: A 25-year-old man with Behcet's disease was admitted because of weakness of the lower limbs and difficulty in urination. He had received a rabies vaccination 2 months previous because he had been bitten by a dog.Findings: Clinical and laboratory findings supported acute transverse myelitis. A hyperintense lesion and expansion at the level of conus medullaris was detected on spinal magnetic resonance imaging.Conclusion: Although neurologic involvement is one of the main causes of mortality and morbidity in Behcet's disease, the factors that aggravate the involvement of the nervous system are still unclear. Vaccination may have been the factor that had activated autoimmune mechanisms in this case. To our knowledge, involvement of the conus medullaris in Behcet's disease after rabies vaccination has not been reported
3-D model building for computer vision
technical reportThis paper presents a Computer-Aided Geometric Design (CAGD) based approach for building 3-D models which can be used for the recognition of 3-D objects for industrial machine vision applications. The objects are designed using the Alpha_1 CAGD system developed at the University of Utah. A new method is given which uses the CAGD design and allows the points on the surface of the object to be sampled at the desired resolution, thus allowing the construction of multiresolution 3-D models. The resulting data structure of points includes coordinates of the points in 3-D space, surface normals and information about the neighboring points
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
Origin of bulk uniaxial anisotropy in zinc-blende dilute magnetic semiconductors
It is demonstrated that the nearest neighbor Mn pair on the GaAs (001)
surface has a lower energy for the [-110] direction comparing to the [110]
case. According to the group theory and the Luttinger's method of invariants,
this specific Mn distribution results in bulk uniaxial in-plane and
out-of-plane anisotropies. The sign and magnitude of the corresponding
anisotropy energies determined by a perturbation method and ab initio
computations are consistent with experimental results.Comment: 5 pages, 1 figur
Recognition of 2-D occluded objects and their manipulation by PUMA 560 robot
Journal ArticleA new method based on a cluster-structure paradigm is presented for the recognition of 2-D partially occluded objects. This method uses the line segments which comprise the boundary of an object in the recognition process. The length of each of these segments as well as the angle between successive segments comprise the only information needed by the program to find an object's position. The technique is applied in several steps which include segment clustering, finding all sequences in one pass over the data, and final clustering of sequences so as to obtain the desired rotational and translational information. The amount of computational effort decreases as the recognition algorithm progresses. As compared to earlier methods, which identify an object based on only one sequence of matched segments, the new technique allows the identification of all parts of the model which match with the apparent image. These parts need not be adjacent to each other. Also the method is able to tolerate a moderate change in scale and a significant amount of shape distortion arising as a result of segmentation or the polygonal approximation of the boundary of the object. The method has been evaluated with respect to a large number of examples where several objects partially occlude one another. A summary of the results is presented
Hole spin relaxation in semiconductor quantum dots
Hole spin relaxation time due to the hole-acoustic phonon scattering in GaAs
quantum dots confined in quantum wells along (001) and (111) directions is
studied after the exact diagonalization of Luttinger Hamiltonian. Different
effects such as strain, magnetic field, quantum dot diameter, quantum well
width and the temperature on the spin relaxation time are investigated
thoroughly. Many features which are quite different from the electron spin
relaxation in quantum dots and quantum wells are presented with the underlying
physics elaborated.Comment: 10 pages, 10 figure
Effect of initial spin polarization on spin dephasing and electron g factor in a high-mobility two-dimensional electron system
We have investigated the spin dynamics of a high-mobility two-dimensional
electron system (2DES) in a GaAs--AlGaAs single quantum well by
time-resolved Faraday rotation (TRFR) in dependence on the initial degree of
spin polarization, , of the 2DES. From to %, we observe
an increase of the spin dephasing time, , by an order of magnitude,
from about 20 ps to 200 ps, in good agreement with theoretical predictions by
Weng and Wu [Phys. Rev. B {\bf 68}, 075312 (2003)]. Furthermore, by applying an
external magnetic field in the Voigt configuration, also the electron
factor is found to decrease for increasing . Fully microscopic calculations,
by numerically solving the kinetic spin Bloch equations considering the
D'yakonov-Perel' and the Bir-Aronov-Pikus mechanisms, reproduce the most
salient features of the experiments, {\em i.e}., a dramatic decrease of spin
dephasing and a moderate decrease of the electron factor with increasing
. We show that both results are determined dominantly by the Hartree-Fock
contribution of the Coulomb interaction.Comment: 4 pages, 4 figures, to be published in PR
Spin flip from dark to bright states in InP quantum dots
We report measurements of the time for spin flip from dark (non-light
emitting) exciton states in quantum dots to bright (light emitting) exciton
states in InP quantum dots. Dark excitons are created by two-photon excitation
by an ultrafast laser. The time for spin flip between dark and bright states is
found to be approximately 200 ps, independent of density and temperature below
70 K. This is much shorter than observed in other quantum dot systems. The rate
of decay of the luminescence intensity, approximately 300 ps, is not simply
equal to the radiative decay rate from the bright states, because the rate of
decay is limited by the rate of conversion from dark excitons into bright
excitons. The dependence of the luminescence decay time on the spin flip time
is a general effect that applies to many experiments.Comment: 3 figure
Mixing of two-electron spin states in a semiconductor quantum dot
We show that the low lying spin states of two electrons in a semiconductor
quantum dot can be strongly mixed by electron-electron asymmetric exchange.
This mixing is generated by the coupling of electron spin to its orbital motion
and to the relative orbital motion of the two electrons. The asymmetric
exchange can be as large as 50% of the isotropic exchange, even for cylindrical
quantum dots. The resulting spin mixing contributes to understanding spin
dynamics in quantum dots, including light polarization reversal
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
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