5,777 research outputs found
One hundred angstrom niobium wire
Composite of fine niobium wires in copper is used to study the size and proximity effects of a superconductor in a normal matrix. The niobium rod was drawn to a 100 angstrom diameter wire on a copper tubing
Nonabelian dark matter: models and constraints
Numerous experimental anomalies hint at the existence of a dark matter (DM)
multiplet chi_i with small mass splittings. We survey the simplest such models
which arise from DM in the low representations of a new SU(2) gauge symmetry,
whose gauge bosons have a small mass mu < 1 GeV. We identify preferred
parameters M_chi ~ 1 TeV, mu ~ 100 MeV, alpha_g ~ 0.04 and the chi chi -> 4e
annihilation channel, for explaining PAMELA, Fermi, and INTEGRAL/SPI lepton
excesses, while remaining consistent with constraints from relic density,
diffuse gamma rays and the CMB. This consistency is strengthened if DM
annihilations occur mainly in subhalos, while excitations (relevant to the
excited DM proposal to explain the 511 keV excess) occur in the galactic center
(GC), due to higher velocity dispersions in the GC, induced by baryons. We
derive new constraints and predictions which are generic to these models.
Notably, decays of excited DM states chi' -> chi gamma arise at one loop and
could provide a new signal for INTEGRAL/SPI; big bang nucleosynthesis (BBN)
constraints on the density of dark SU(2) gauge bosons imply a lower bound on
the mixing parameter epsilon between the SU(2) gauge bosons and photon. These
considerations rule out the possibility of the gauge bosons that decay into
e^+e^- being long-lived. We study in detail models of doublet, triplet and
quintuplet DM, showing that both normal and inverted mass hierarchies can
occur, with mass splittings that can be parametrically smaller, e.g., O(100)
keV, than the generic MeV scale of splittings. A systematic treatment of Z_2
symmetry which insures the stability of the intermediate DM state is given for
cases with inverted mass hierarchy, of interest for boosting the 511 keV signal
from the excited dark matter mechanism.Comment: 28 pages, 17 figures; v2. added brief comment, reference
Bounding the dimensions of rational cohomology groups
Let be an algebraically closed field of characteristic , and let
be a simple simply-connected algebraic group over that is defined and
split over the prime field . In this paper we investigate
situations where the dimension of a rational cohomology group for can be
bounded by a constant times the dimension of the coefficient module. We then
demonstrate how our results can be applied to obtain effective bounds on the
first cohomology of the symmetric group. We also show how, for finite Chevalley
groups, our methods permit significant improvements over previous estimates for
the dimensions of second cohomology groups.Comment: 13 page
On the origin of the March 5, 1979 gamma ray transient: A vibrating neutron star in the Large Magellanic Cloud
It is proposed that a vibrating neutron star in the Large Magellanic Cloud is the source of the March 5 transient. Neutron star vibrations transport energy rapidly to the surface, heat the atmosphere by wave dissipation, and decay by gravitational radiation reaction. The electromagnetic emission arises from e(+)-e(-) pairs which cool and annihilate in the strong magnetic field of the neutron star. The field also confines the pairs, and this allows the production of the redshifted annihilation feature observed in the data. The redshift implies a gravitational radiation damping time which agrees with the 0.15 second duration of the impulsive phase of the event. Thus, the March 5 transient may be both the first detection of a vibrating neutron star and indirect evidence for gravitational radiation
On the Nature of Trapped-Hole States in CdS Nanocrystals and the Mechanism of their Diffusion
Recent transient absorption experiments on CdS nanorods suggest that
photoexcited holes rapidly trap to the surface of these particles and then
undergo diffusion along the rod surface. In this paper, we present a
semiperiodic DFT model for the CdS nanocrystal surface, analyze it, and comment
on the nature of both the hole-trap states and the mechanism by which the holes
diffuse. Hole states near the top of the valence band form an energetic near
continuum with the bulk, and localize to the non-bonding sp orbitals on
surface sulfur atoms. After localization, the holes form nonadiabatic small
polarons that move between the sulfur orbitals on the surface of the particle
in a series of uncorrelated, incoherent, thermally-activated hops at room
temperature. The surface-trapped holes are deeply in the weak-electronic
coupling limit and, as a result, undergo slow diffusion.Comment: 4 figure
Shock Deformation in Zircon, a Comparison of Results from Shock-Reverberation and Single-Shock Experiments
The utility of the mineral zircon, ZrSiO4, as a shock-metamorphic geobarometer and geochronometer, has been steadily growing within the planetary science community. Zircon is an accessory phase found in many terrestrial rock types, lunar samples, lunar meteorites, martian meteorites and various other achondrites. Because zircon is refractory and has a high closure temperature for Pb diffusion, it has been used to determine the ages of some of the oldest material on Earth and elsewhere in the Solar System. Furthermore, major (O) and trace-element (REE, Ti, Hf) abundances and isotope compositions of zircon help characterize the petrogenetic environments and sources from which they crystallized. The response of zircon to impact-induced shock deformation is predominantly crystallographic, including dislocation creep and the formation of planar and sub-planar, low-angle grain boundaries; the formation of mechanical {112} twins; transformation to the high pressure polymorph reidite; the development of polycrystalline microtextures; and dissociation to the oxide constituents SiO2 and ZrO2. Shock microstructures can also variably affect the U- Pb isotope systematics of zircon and, in some instances, be used to constrain the impact age. While numerous studies have characterized shock deformation in zircon recovered from a variety of terrestrial impact craters and ejecta deposits and Apollo samples, experimental studies of shock deformation in zircon are limited to a handful of examples in the literature. In addition, the formation conditions (e.g., P, T) of various shock microstructures, such as planar-deformation bands, twins, and reidite lamellae, remain poorly con-strained. Furthermore, previous shocked-zircon experimental charges have not been analyzed using modern analytical equipment. This study will therefore under-take an new set of zircon shock experiments, which will then be microstructurally characterized using state-of-the-art instrumentation within the Astromaterials Research and Exploration Science Division (ARES), NASA Johnson Space Center
The quick and the dead: when reaction beats intention
Everyday behaviour involves a trade-off between planned actions and reaction to environmental events.Evidence from neurophysiology, neurology and functional brain imaging suggests different neural bases for the control of different movement types. Here we develop a behavioural paradigm to test movement dynamics for intentional versus reaction movements and provide evidence for a ‘reactive advantage’ in movement execution, whereby the same action is executed faster in reaction to an opponent. We placed pairs of participants in competition with each other to make a series of button presses. Within subject analysis of movement times revealed a 10 per cent benefit for reactive actions. This was maintained when opponents performed dissimilar actions, and when participants competed against a computer, suggesting that the effect is not related to facilitation produced by action observation. Rather, faster ballistic movements may be a general property of reactive motor control, potentially providing a useful means of promoting survival
A giant, periodic flare from the soft gamma repeater SGR1900+14
Soft gamma repeaters are high-energy transient sources associated with
neutron stars in young supernova remnants. They emit sporadic, short (~ 0.1 s)
bursts with soft energy spectra during periods of intense activity. The event
of March 5, 1979 was the most intense and the only clearly periodic one to
date. Here we report on an even more intense burst on August 27, 1998, from a
different soft gamma repeater, which displayed a hard energy spectrum at its
peak, and was followed by a ~300 s long tail with a soft energy spectrum and a
dramatic 5.16 s period. Its peak and time integrated energy fluxes at Earth are
the largest yet observed from any cosmic source. This event was probably
initiated by a massive disruption of the neutron star crust, followed by an
outflow of energetic particles rotating with the period of the star. Comparison
of these two bursts supports the idea that magnetic energy plays an important
role, and that such giant flares, while rare, are not unique, and may occur at
any time in the neutron star's activity cycle.Comment: Accepted for publication in Natur
Gamma-Ray Burster Counterparts: HST Blue and Ultraviolet Data
The surest solution of the Gamma Ray Burst (GRB) mystery is to find an
unambiguous low-energy quiescent counterpart. However, to date no reasonable
candidates have been identified in the x-ray, optical, infrared, or radio
ranges. The Hubble Space Telescope (HST) has now allowed for the first deep
ultraviolet searches for quiescent counterparts. This paper reports on
multiepoch ultraviolet searches of five GRB positions with HST. We found no
sources with significant ultraviolet excesses, variability, parallax, or proper
motion in any of the burst error regions. In particular, we see no sources
similar to that proposed as a counterpart to the GRB970228. While this negative
result is disappointing, it still has good utility for its strict limits on the
no-host-galaxy problem in cosmological models of GRBs. For most cosmological
models (with peak luminosity 6X10^50 erg/s), the absolute B magnitude of any
possible host galaxy must be fainter than -15.5 to -17.4. These smallest boxes
for some of the brightest bursts provide the most critical test, and our limits
are a severe problem for all published cosmological burst models.Comment: 15 pages, 2 ps figures, accepted for publication in the Astrophysical
Journa
The protein import apparatus of chloroplasts
Routing of cytosolically synthesized precursor proteins into chloroplasts is a specific process which involves a multitude of soluble and membrane components. In this review we wil1 focus on early events of the translocation pathway of nuclear coded plastidic precursor proteins and compare import routes for polypeptide of the outer chloroplast envelope to that of internal chloroplast compartments. A number of proteins housed in the chloroplast envelopes have been implied to be involved in the translocation process, but so far a certain function has not been assigned to any of these proteins. The only exception could be an envelope localized hsc 70 homologue which could retain the import competence of a precursor protein in transit into the organelle
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