37,716 research outputs found
Dynamics of thermalisation in small Hubbard-model systems
We study numerically the thermalisation and temporal evolution of the reduced
density matrix for a two-site subsystem of a fermionic Hubbard model prepared
far from equilibrium at a definite energy. Even for very small systems near
quantum degeneracy, the subsystem can reach a steady state resembling
equilibrium. This occurs for a non-perturbative coupling between the subsystem
and the rest of the lattice where relaxation to equilibrium is Gaussian in
time, in sharp contrast to perturbative results. We find similar results for
random couplings, suggesting such behaviour is generic for small systems.Comment: 4 pages, 5 figure
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SLS Processing Studies of Nylon 11 Nanocomposites
Selective Laser Sintering (SLS) is widely used for rapid prototyping/manufacturing of
nylon 11 and nylon 12 parts. This processing technique has not been explored for
nylon nanocomposites. This study investigates the technicalities of processing nylon
11-clay and nylon-carbon nanofiber nanocomposites with SLS. Microstructural
analyses of the SLS powders and parts were conducted under SEM. Results suggest
that SLS processing is possible with the new nylon 11 nanocomposites. Yet the SLS
parts built have inferior properties relative to those of injection molding, suggesting
that more fine tuning for the processing is required.Mechanical Engineerin
Two--Electron Atoms in Short Intense Laser Pulses
We discuss a method of solving the time dependent Schrodinger equation for
atoms with two active electrons in a strong laser field, which we used in a
previous paper [A. Scrinzi and B. Piraux, Phys. Rev. A 56, R13 (1997)] to
calculate ionization, double excitation and harmonic generation in Helium by
short laser pulses. The method employs complex scaling and an expansion in an
explicitly correlated basis. Convergence of the calculations is documented and
error estimates are provided. The results for Helium at peak intensities up to
10^15 W/cm^2 and wave length 248 nm are accurate to at least 10 %. Similarly
accurate calculations are presented for electron detachment and double
excitation of the negative hydrogen ion.Comment: 14 pages, including figure
Thermalisation of Local Observables in Small Hubbard Lattices
We present a study of thermalisation of a small isolated Hubbard lattice
cluster prepared in a pure state with a well-defined energy. We examine how a
two-site subsystem of the lattice thermalises with the rest of the system as
its environment. We explore numerically the existence of thermalisation over a
range of system parameters, such as the interaction strength, system size and
the strength of the coupling between the subsystem and the rest of the lattice.
We find thermalisation over a wide range of parameters and that interactions
are crucial for efficient thermalisation of small systems. We relate this
thermalisation behaviour to the eigenstate thermalisation hypothesis and
quantify numerically the extent to which eigenstate thermalisation holds. We
also verify our numerical results theoretically with the help of previously
established results from random matrix theory for the local density of states,
particularly the finite-size scaling for the onset of thermalisation.Comment: 22 pages, 23 figure
An equations-of-motion approach to quantum mechanics: application to a model phase transition
We present a generalized equations-of-motion method that efficiently
calculates energy spectra and matrix elements for algebraic models. The method
is applied to a 5-dimensional quartic oscillator that exhibits a quantum phase
transition between vibrational and rotational phases. For certain parameters,
10 by 10 matrices give better results than obtained by diagonalising 1000 by
1000 matrices.Comment: 4 pages, 1 figur
Systematic effects from an ambient-temperature, continuously-rotating half-wave plate
We present an evaluation of systematic effects associated with a
continuously-rotating, ambient-temperature half-wave plate (HWP) based on two
seasons of data from the Atacama B-Mode Search (ABS) experiment located in the
Atacama Desert of Chile. The ABS experiment is a microwave telescope sensitive
at 145 GHz. Here we present our in-field evaluation of celestial (CMB plus
galactic foreground) temperature-to-polarization leakage. We decompose the
leakage into scalar, dipole, and quadrupole leakage terms. We report a scalar
leakage of ~0.01%, consistent with model expectations and an order of magnitude
smaller than other CMB experiments have reported. No significant dipole or
quadrupole terms are detected; we constrain each to be <0.07% (95% confidence),
limited by statistical uncertainty in our measurement. Dipole and quadrupole
leakage at this level lead to systematic error on r<0.01 before any mitigation
due to scan cross-linking or boresight rotation. The measured scalar leakage
and the theoretical level of dipole and quadrupole leakage produce systematic
error of r<0.001 for the ABS survey and focal-plane layout before any data
correction such as so-called deprojection. This demonstrates that ABS achieves
significant beam systematic error mitigation from its HWP and shows the promise
of continuously-rotating HWPs for future experiments.Comment: 11 pages, 8 figures; revision to submitted version, Fig. 5 and Eqs.
(14) and (15) corrected; added Fig. 9 and description, text revisions for
clarification, Fig. 5 revised for better calibration, corrected labeling
errors and plotting bugs in Fig. 3, 4, and Eq. (14) and (15
"Low-state" Black Hole Accretion in Nearby Galaxies
I summarize the main observational properties of low-luminosity AGNs in
nearby galaxies to argue that they are the high-mass analogs of black hole
X-ray binaries in the "low/hard" state. The principal characteristics of
low-state AGNs can be accommodated with a scenario in which the central engine
is comprised of three components: an optically thick, geometrically accretion
disk with a truncated inner radius, a radiatively inefficient flow, and a
compact jet.Comment: 8 pages. To appear in From X-ray Binaries to Quasars: Black Hole
Accretion on All Mass Scales, ed. T. J. Maccarone, R. P. Fender, and L. C. Ho
(Dordrecht: Kluwer
Lowest Open Channels, Bound States, and Narrow Resonances of Dipositronium
The constraints imposed by symmetry on the open channels of dipositronium has
been studied, and the symmetry-adapted lowest open channel of each quantum
state has been identified. Based on this study, the existence of two more 0^+
bound states has been theoretically confirmed, and a 0^+ narrow resonance has
been predicted. A variational calculation has been performed to evaluate the
critical strength of the repulsive interaction . Two 0^- states are found to
have their critical strengths very close to 1, they are considered as
candidates of new narrow resonances or loosely bound states .Comment: 10 pages, 0 figure
Characterizing Atacama B-mode Search Detectors with a Half-Wave Plate
The Atacama B-Mode Search (ABS) instrument is a cryogenic (10 K)
crossed-Dragone telescope located at an elevation of 5190 m in the Atacama
Desert in Chile that observed for three seasons between February 2012 and
October 2014. ABS observed the Cosmic Microwave Background (CMB) at large
angular scales () to limit the B-mode polarization spectrum around
the primordial B-mode peak from inflationary gravity waves at .
The ABS focal plane consists of 480 transition-edge sensor (TES) bolometers.
They are coupled to orthogonal polarizations from a planar ortho-mode
transducer (OMT) and observe at 145 GHz. ABS employs an ambient-temperature,
rapidly rotating half-wave plate (HWP) to mitigate systematic effects and move
the signal band away from atmospheric noise, allowing for the recovery of
large angular scales. We discuss how the signal at the second harmonic of the
HWP rotation frequency can be used for data selection and for monitoring the
detector responsivities.Comment: 7 pages, 3 figures, conference proceedings submitted to the Journal
of Low Temperature Detector
Supergalactic winds driven by multiple superstar clusters
We present two dimensional hydrodynamic calculations of free expanding
supergalactic winds, taking into consideration strong radiative cooling. Our
main premise is that supergalactic winds are powered by collections of
superstar clusters. Every individual superstar cluster is a source of a high
metallicity radiative supersonic outflow (paper I, 2003, ApJ, 590, 791). The
interaction of winds from neighboring knots of star formation is shown to lead
to a collection of stationary oblique shocks and crossing shocks, able to
structure the general outflow into a network of dense and cold, kpc long
filaments that originate near the base of the outflow. The shocks also lead to
extended regions of diffuse soft X-ray emission and furthermore, to channel the
outflow with a high degree of collimation into the intergalactic medium.Comment: 10 pages, 5 figures, Accepted for publication in The Astrophysical
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