8,357 research outputs found
On empirical models of the upper atmosphere in the polar regions
Modified expression for exospheric temperature in Jacchia static diffusion models of upper atmosphere in polar region
Brane Supersymmetry Breaking
We show how to construct chiral tachyon-free perturbative orientifold models,
where supersymmetry is broken at the string scale on a collection of branes
while, to lowest order, the bulk and the other branes are supersymmetric. In
higher orders, supersymmetry breaking is mediated to the remaining sectors, but
is suppressed by the size of the transverse space or by the distance from the
brane where supersymmetry breaking primarily occurred. This setting is of
interest for orbifold models with discrete torsion, and is of direct relevance
for low-scale string models. It can guarantee the stability of the gauge
hierarchy against gravitational radiative corrections, allowing an almost exact
supergravity a millimeter away from a non-supersymmetric world.Comment: 15 pages, LaTe
Polarization dependent photoionization cross-sections and radiative lifetimes of atomic states in Ba
The photoionization cross-sections of two even-parity excited states, and , of atomic Ba at the ionization-laser wavelength of
556.6 nm were measured. We found that the total cross-section depends on the
relative polarization of the atoms and the ionization-laser light. With
density-matrix algebra, we show that, in general, there are at most three
parameters in the photoionization cross-section. Some of these parameters are
determined in this work. We also present the measurement of the radiative
lifetime of five even-parity excited states of barium.Comment: 11 pages, 7 figure
Spin relaxation of "upstream" electrons in quantum wires: Failure of the drift diffusion model
The classical drift diffusion (DD) model of spin transport treats spin
relaxation via an empirical parameter known as the ``spin diffusion length''.
According to this model, the ensemble averaged spin of electrons drifting and
diffusing in a solid decays exponentially with distance due to spin dephasing
interactions. The characteristic length scale associated with this decay is the
spin diffusion length. The DD model also predicts that this length is different
for ``upstream'' electrons traveling in a decelerating electric field than for
``downstream'' electrons traveling in an accelerating field. However this
picture ignores energy quantization in confined systems (e.g. quantum wires)
and therefore fails to capture the non-trivial influence of subband structure
on spin relaxation. Here we highlight this influence by simulating upstream
spin transport in a multi-subband quantum wire, in the presence of
D'yakonov-Perel' spin relaxation, using a semi-classical model that accounts
for the subband structure rigorously.
We find that upstream spin transport has a complex dynamics that defies the
simplistic definition of a ``spin diffusion length''.
In fact, spin does not decay exponentially or even monotonically with
distance, and the drift diffusion picture fails to explain the qualitative
behavior, let alone predict quantitative features accurately. Unrelated to spin
transport, we also find that upstream electrons undergo a ``population
inversion'' as a consequence of the energy dependence of the density of states
in a quasi one-dimensional structure.Comment: 13 figures. To appear in Phys. Rev.
Dynamical fermion algorithm for variable actions
A new version of the two-step multi-boson algorithm is developed with
different fermion actions in the multi-boson and noisy Metropolis steps.Comment: 8 pages, 2 figures, more discussion and references include
Genetic and serological heterogeneity of the supertypic HLA-B locus specificities Bw4 and Bw6
Gene cloning and sequencing of the HLA-B
locus split antigens B38 (B16.1) and B39 (B16.2) allowed
localization of their subtypic as well as their public
specificities HLA-Bw4 or -Bw6 to the c~-helical region of
the c~ 1 domain flanked by the amino acid positions 74-83.
Comparison of their amino acid sequences with those of
other HLA-B-locus alleles established HLA-Bw6 to be
distinguished by Ser at residue 77 and Asn at residue 80.
In contrast, HLA-Bw4 is characterized by at least seven
different patterns of amino acid exchanges at positions 77
and 80-83. Reactivity patterns of Bw4- or Bw6-specific
monoclonal antibodies reveal two alloantigenic epitopes
contributing to the HLA-Bw4 or -Bw6 specificity residing
next to the region of highest diversity of the cr domain
Long-Lived Spin Coherence States
We study evolution of electron spin coherence having non-homogeneous
direction of spin polarization vector in semiconductor heterostructures. It is
found that the electron spin relaxation time due to the D'yakonov-Perel'
relaxation mechanism essentially depends on the initial spin polarization
distribution. This effect has its origin in the coherent spin precession of
electrons diffusing in the same direction. We predict a long spin relaxation
time of a novel structure: a spin coherence standing wave and discuss its
experimental realization
Completely positive covariant two-qubit quantum processes and optimal quantum NOT operations for entangled qubit pairs
The structure of all completely positive quantum operations is investigated
which transform pure two-qubit input states of a given degree of entanglement
in a covariant way. Special cases thereof are quantum NOT operations which
transform entangled pure two-qubit input states of a given degree of
entanglement into orthogonal states in an optimal way. Based on our general
analysis all covariant optimal two-qubit quantum NOT operations are determined.
In particular, it is demonstrated that only in the case of maximally entangled
input states these quantum NOT operations can be performed perfectly.Comment: 14 pages, 2 figure
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