37,976 research outputs found
Monte Carlo Evaluation of Non-Abelian Statistics
We develop a general framework to (numerically) study adiabatic braiding of
quasiholes in fractional quantum Hall systems. Specifically, we investigate the
Moore-Read (MR) state at filling factor, a known candidate for
non-Abelian statistics, which appears to actually occur in nature. The
non-Abelian statistics of MR quasiholes is demonstrated explicitly for the
first time, confirming the results predicted by conformal field theories.Comment: 4 pages, 4 figure
Phase diagram of the frustrated, spatially anisotropic S=1 antiferromagnet on a square lattice
We study the S=1 square lattice Heisenberg antiferromagnet with spatially
anisotropic nearest neighbor couplings , frustrated by a
next-nearest neighbor coupling numerically using the density-matrix
renormalization group (DMRG) method and analytically employing the
Schwinger-Boson mean-field theory (SBMFT). Up to relatively strong values of
the anisotropy, within both methods we find quantum fluctuations to stabilize
the N\'{e}el ordered state above the classically stable region. Whereas SBMFT
suggests a fluctuation-induced first order transition between the N\'{e}el
state and a stripe antiferromagnet for and an
intermediate paramagnetic region opening only for very strong anisotropy, the
DMRG results clearly demonstrate that the two magnetically ordered phases are
separated by a quantum disordered region for all values of the anisotropy with
the remarkable implication that the quantum paramagnetic phase of the spatially
isotropic - model is continuously connected to the limit of
decoupled Haldane spin chains. Our findings indicate that for S=1 quantum
fluctuations in strongly frustrated antiferromagnets are crucial and not
correctly treated on the semiclassical level.Comment: 10 pages, 10 figure
Scaling Behaviour of Developing and Decaying Networks
We find that a wide class of developing and decaying networks has scaling
properties similar to those that were recently observed by Barab\'{a}si and
Albert in the particular case of growing networks. The networks considered here
evolve according to the following rules: (i) Each instant a new site is added,
the probability of its connection to old sites is proportional to their
connectivities. (ii) In addition, (a) new links between some old sites appear
with probability proportional to the product of their connectivities or (b)
some links between old sites are removed with equal probability.Comment: 7 pages (revtex
Multi-Instanton Calculus and Equivariant Cohomology
We present a systematic derivation of multi-instanton amplitudes in terms of
ADHM equivariant cohomology. The results rely on a supersymmetric formulation
of the localization formula for equivariant forms. We examine the cases of N=4
and N=2 gauge theories with adjoint and fundamental matter.Comment: 29 pages, one more reference adde
Stability of 1-D Excitons in Carbon Nanotubes under High Laser Excitations
Through ultrafast pump-probe spectroscopy with intense pump pulses and a wide
continuum probe, we show that interband exciton peaks in single-walled carbon
nanotubes (SWNTs) are extremely stable under high laser excitations. Estimates
of the initial densities of excitons from the excitation conditions, combined
with recent theoretical calculations of exciton Bohr radii for SWNTs, suggest
that their positions do not change at all even near the Mott density. In
addition, we found that the presence of lowest-subband excitons broadens all
absorption peaks, including those in the second-subband range, which provides a
consistent explanation for the complex spectral dependence of pump-probe
signals reported for SWNTs.Comment: 4 pages, 4 figure
Condensation of `composite bosons' in a rotating BEC
We provide evidence for several novel phases in the dilute limit of rotating
BECs. By exact calculation of wavefunctions and energies for small numbers of
particles, we show that the states near integer angular momentum per particle
are best considered condensates of composite entities, involving vortices and
atoms. We are led to this result by explicit comparison with a description
purely in terms of vortices. Several parallels with the fractional quantum Hall
effect emerge, including the presence of the Pfaffian state.Comment: 4 pages, Latex, 3 figure
A Process for Producing Highly Wettable Aluminum 6061 Surfaces Compatible with Hydrazine
NASA's Global Precipitation Measurement (GPM) mission is an ongoing Goddard Space Flight Center (GSFC) project whose basic objective is to improve global precipitation measurements. The space-based portion of the mission architecture consists of a primary or core spacecraft and a constellation of NASA and contributed spacecrafts. The efforts described in this paper refer to the core spacecraft (hereafter referred to as simply GPM) which is to be fabricated at GSFC. It has been decided that the GPM spacecraft is to be a "design-for-demise-spacecraft." This requirement resulted in the need for a propellant tank that would also demise or ablate to an appropriate degree upon re-entry. Composite overwrapped aluminum lined propellant tanks with aluminum propellant management devices (PMD) were shown by analyses to demise and thus became the baseline configuration for GPM. As part of the GPM tank development effort, long term compatibility and wettability testing with hydrazine was performed on Al6061 and 2219 coupons fabricated and cleaned by conventional processes. Long term compatibility was confirmed. However, the wettability of the aluminum as measured by contact angle produced higher than desired angles (greater than 30 deg.) with excessive scatter. The availability of PMD materials exhibiting consistently low contact angles aids in the design of simple PMDs. Two efforts performed by Angeles Crest Engineering and funded by GSFC were undertaken to reduce the risk of using aluminum for the GPM PMD. The goal of the first effort was to develop a cleaning or treatment process to produce consistently low contact angles. The goal of the second effort was to prove via testing that the processed aluminum would retain compatibility with hydrazine and retain low contact angle after long term exposure to hydrazine. Both goals were achieved. This paper describes both efforts and the results achieved
Pre-Maximum Spectropolarimetry of the Type Ia SN 2004dt
We report observations of SN 2004dt obtained with the Very Large Telescope of
the European Southern Observatory on August 13.30, 2004 when the supernova was
more than a week before optical maximum. SN 2004dt showed strong lines of
\ion{O}{1}, \ion{Mg}{2}, \ion{Si}{2}, and \ion{Ca}{2} with typical velocities
of absorption minimum around 17,000 \kms. The line profiles show material
moving at velocities as high as 25,000 \kms in these lines. The observations
also reveal absorption lines from \ion{S}{2} and \ion{Si}{3} with a velocity of
only 11,000 \kms. The highest velocity in the \ion{S}{2} features can be traced
no higher than 15,000 \kms, much lower than those of O, Mg, Si, and Ca. SN
2004dt has a polarization spectrum unlike any previously observed. The
variation of the polarization across some \ion{Si}{2} lines approaches 2%,
making SN 2004dt the most highly polarized SN Ia ever observed. In contrast,
the strong line of O I at 777.4 nm shows little or no polarization signature.
The degree of polarization points to a richly-structured partially burned
silicon layer with substantial departure from spherical symmetry. A geometry
that would account for the observations is one in which the distribution of
oxygen is essentially spherically symmetric, but with bubbles of
intermediate-mass elements with significant opacity within the oxygen
substrate.Comment: Submitted to Ap
Unitarity Bounds and the Cuspy Halo Problem
Conventional Cold Dark Matter cosmological models predict small scale
structures, such as cuspy halos, which are in apparent conflict with
observations. Several alternative scenarios based on modifying fundamental
properties of the dark matter have been proposed. We show that general
principles of quantum mechanics, in particular unitarity, imply interesting
constraints on two proposals: collisional dark matter proposed by Spergel &
Steinhardt, and strongly annihilating dark matter proposed by Kaplinghat, Knox
& Turner. Efficient scattering required in both implies m < 12 GeV and m < 25
GeV respectively. The same arguments show that the strong annihilation in the
second scenario implies the presence of significant elastic scattering,
particularly for large enough masses. Recently, a variant of the collisional
scenario has been advocated to satisfy simultaneously constraints from dwarf
galaxies to clusters, with a cross section that scales inversely with velocity.
We show that this scenario likely involves super-elastic processes, and the
associated kinetic energy change must be taken into account when making
predictions. Exceptions and implications for experimental searches are
discussed.Comment: 4 pages, references adde
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