1,989 research outputs found
Avenues and incentives for commercial use of a low-gravity environment
The scientific and commercial utilization of the low-g environments for materials research and for process and product development is considered. Any products of commercial interest which necessitate processing in space will probably be low volume, high value items. To encourage the commercialization of materials processing in low-g, NASA, in parallel with establishing and demonstrating the scientific/technological precepts for analyzing and using a low-g environment, is establishing the legal and management mechanisms to share in the cost and risk of early commercial ventures, and is now working with commercial firms on a case-by basis to explore applications of this new technology to specific needs of the company
Absorption in Ultra-Peripheral Nucleus-Atom Collisions in Crystal
The Glauber theory description of particle- and nucleus-crystal Coulomb
interactions at high-energy is developed. The allowance for the lattice thermal
vibrations is shown to produce strong absorption effect which is of prime
importance for quantitative understanding of the coherent Coulomb excitation of
ultra-relativistic particles and nuclei passing through the crystal.Comment: 8 pages, LaTe
Majorana Quasi-Particles Protected by Angular Momentum Conservation
We show how angular momentum conservation can stabilise a symmetry-protected
quasi-topological phase of matter supporting Majorana quasi-particles as edge
modes in one-dimensional cold atom gases. We investigate a number-conserving
four-species Hubbard model in the presence of spin-orbit coupling. The latter
reduces the global spin symmetry to an angular momentum parity symmetry, which
provides an extremely robust protection mechanism that does not rely on any
coupling to additional reservoirs. The emergence of Majorana edge modes is
elucidated using field theory techniques, and corroborated by
density-matrix-renormalization-group simulations. Our results pave the way
toward the observation of Majorana edge modes with alkaline-earth-like fermions
in optical lattices, where all basic ingredients for our recipe - spin-orbit
coupling and strong inter-orbital interactions - have been experimentally
realized over the last two years.Comment: 12 pages (6 + 6 supplementary material
Stochastic Gross-Pitaevsky Equation for BEC via Coarse-Grained Effective Action
We sketch the major steps in a functional integral derivation of a new set of
Stochastic Gross-Pitaevsky equations (GPE) for a Bose-Einstein condensate (BEC)
confined to a trap at zero temperature with the averaged effects of
non-condensate modes incorporated as stochastic sources. The closed-time-path
(CTP) coarse-grained effective action (CGEA) or the equivalent influence
functional method is particularly suitable because it can account for the full
back-reaction of the noncondensate modes on the condensate dynamics
self-consistently. The Langevin equations derived here containing nonlocal
dissipation together with colored and multiplicative noises are useful for a
stochastic (as distinguished from say, a kinetic) description of the
nonequilibrium dynamics of a BEC. This short paper contains original research
results not yet published anywhere.Comment: 6 page
Sonic analog of gravitational black holes in Bose-Einstein condensates
It is shown that, in dilute-gas Bose-Einstein condensates, there exist both
dynamically stable and unstable configurations which, in the hydrodynamic
limit, exhibit a behavior resembling that of gravitational black holes. The
dynamical instabilities involve creation of quasiparticle pairs in positive and
negative energy states, as in the well-known suggested mechanism for black hole
evaporation. We propose a scheme to generate a stable sonic black hole in a
ring trap.Comment: RevTeX 3.1, 1 figure, 4 page
Optical pumping of quantum dot nuclear spins
An all-optical scheme to polarize nuclear spins in a single quantum dot is
analyzed. The hyperfine interaction with randomly oriented nuclear spins
presents a fundamental limit for electron spin coherence in a quantum dot; by
cooling the nuclear spins, this decoherence mechanism could be suppressed. The
proposed scheme is inspired by laser cooling methods of atomic physics and
implements a "controlled Overhauser effect" in a zero-dimensional structure
Reconstructing the orbit type stratification of a torus action from its equivariant cohomology
We investigate what information on the orbit type stratification of a torus action on a compact space is contained in its rational equivariant cohomology algebra. Regarding the (labelled) poset structure of the stratification we show that equivariant cohomology encodes the subposet of ramified elements. For equivariantly formal actions, we also examine what cohomological information of the stratification is encoded. In the smooth setting we show that under certain conditions -- which in particular hold for a compact orientable manifold with discrete fixed point set -- the equivariant cohomologies of the strata are encoded in the equivariant cohomology of the manifold
Spatial and spectral shape of inhomogeneous non-equilibrium exciton-polariton condensates
We develop a mean-field theory of the spatial profile and the spectral
properties of polariton condensates in nonresonantly pumped semiconductor
microcavities in the strong coupling regime. Predictions are obtained for both
the continuous-wave and the pulsed excitation regimes and the specific
signatures of the non-equilibrium character of the condensation process are
pointed out. A striking sensitivity of the condensate shape on the optical pump
spot size is demonstrated by analytical and numerical calculations, in good
quantitative agreement with recent experimental observations.Comment: 5 pages, 3 figure
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