9,820 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
Optimized design of universal two-qubit gates
We construct optimized implementations of the CNOT and other universal
two-qubit gates that, unlike many of the previously proposed protocols, are
carried out in a single step. The new protocols require tunable inter-qubit
couplings but, in return, show a significant improvements in the quality of
gate operations. Our optimization procedure can be further extended to the
combinations of elementary two-qubit as well as irreducible many-qubit gates.Comment: 6 pages, 2 figure
Control of electron spin and orbital resonance in quantum dots through spin-orbit interactions
Influence of resonant oscillating electromagnetic field on a single electron
in coupled lateral quantum dots in the presence of phonon-induced relaxation
and decoherence is investigated. Using symmetry arguments it is shown that spin
and orbital resonance can be efficiently controlled by spin-orbit interactions.
The control is possible due to the strong sensitivity of Rabi frequency to the
dot configuration (orientation of the dot and a static magnetic field) as a
result of the anisotropy of the spin-orbit interactions. The so called easy
passage configuration is shown to be particularly suitable for magnetic
manipulation of spin qubits, ensuring long spin relaxation time and protecting
the spin qubit from electric field disturbances accompanying on-chip
manipulations.Comment: 11 pages, 5 figures; v2: introduction and conclusions broadened,
moderate structure and content change
Electron Spin Dynamics in Impure Quantum Wells for Arbitrary Spin-Orbit Coupling
Strong interest has arisen recently on low-dimensional systems with strong
spin-orbit interaction due to their peculiar properties of interest for some
spintronic applications. Here, the time evolution of the electron spin
polarization of a disordered two-dimensional electron gas is calculated exactly
within the Boltzmann formalism for arbitrary couplings to a Rashba spin-orbit
field. The classical Dyakonov-Perel mechanism of spin relaxation is shown to
fail for sufficiently strong Rashba fields, in which case new regimes of spin
decay are identified. These results suggest that spin manipulation can be
greatly improved in strong spin-orbit interaction materials.Comment: 5 pages, 2 figures -revised versio
Hints of a Therapeutic Vaccine for Alzheimer's?
AbstractIn this issue of Neuron, Hock et al. report the cognitive functions of a small number of Alzheimer's disease patients that participated in a clinical trial of immunization with β-amyloid. Patients with serum antibodies against β-amyloid plaques showed a diminished cognitive decline and slowed disease progression. Dangerous meningoencephalitis was present in some patients, as has been previously reported
Crossing barriers in planetesimal formation: The growth of mm-dust aggregates with large constituent grains
Collisions of mm-size dust aggregates play a crucial role in the early phases
of planet formation. We developed a laboratory setup to observe collisions of
dust aggregates levitating at mbar pressures and elevated temperatures of 800
K. We report on collisions between basalt dust aggregates of from 0.3 to 5 mm
in size at velocities between 0.1 and 15 cm/s. Individual grains are smaller
than 25 \mum in size. We find that for all impact energies in the studied range
sticking occurs at a probability of 32.1 \pm 2.5% on average. In general, the
sticking probability decreases with increasing impact parameter. The sticking
probability increases with energy density (impact energy per contact area). We
also observe collisions of aggregates that were formed by a previous sticking
of two larger aggregates. Partners of these aggregates can be detached by a
second collision with a probability of on average 19.8 \pm 4.0%. The measured
accretion efficiencies are remarkably high compared to other experimental
results. We attribute this to the rel. large dust grains used in our
experiments, which make aggregates more susceptible to restructuring and energy
dissipation. Collisional hardening by compaction might not occur as the
aggregates are already very compact with only 54 \pm 1% porosity. The
disassembly of previously grown aggregates in collisions might stall further
aggregate growth. However, owing to the levitation technique and the limited
data statistics, no conclusive statement about this aspect can yet be given. We
find that the detachment efficiency decreases with increasing velocities and
accretion dominates in the higher velocity range. For high accretion
efficiencies, our experiments suggest that continued growth in the mm-range
with larger constituent grains would be a viable way to produce larger
aggregates, which might in turn form the seeds to proceed to growing
planetesimals.Comment: 9 pages, 20 figure
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