2,355 research outputs found
Thermal dissociation of dipositronium: path integral Monte Carlo approach
Path integral Monte Carlo simulation of the dipositronium "molecule" Ps
reveals its surprising thermal instability. Although, the binding energy is
eV, due to the strong temperature dependence of its free energy
Ps dissociates, or does not form, above K, except for high
densities where a small fraction of molecules are in equilibrium with Ps atoms.
This prediction is consistent with the recently reported first observation of
stable Ps molecules by Cassidy & Mills Jr., Nature {\bf 449}, 195 (07), and
Phys.Rev.Lett. {\bf 100}, 013401 (08); at temperatures below 1000 K. The
relatively sharp transition from molecular to atomic equilibrium, that we find,
remains to be experimentally verified. To shed light on the origin of the large
entropy factor in free energy we analyze the nature of interatomic interactions
of these strongly correlated quantum particles. The conventional diatomic
potential curve is given by the van der Waals interaction at large distances,
but due to the correlations and high delocalization of constituent particles
the concept of potential curve becomes ambiguous at short atomic distances.Comment: Submitted to the Physical Review Letter
Finite temperature quantum statistics of H molecular ion
Full quantum statistical simulation of the five-particle system H
has been carried out using the path integral Monte Carlo method. Structure and
energetics is evaluated as a function of temperature up to the thermal
dissociation limit. The weakly density dependent dissociation temperature is
found to be around K. Contributions from the quantum dynamics and
thermal motion are sorted out by comparing differences between simulations with
quantum and classical nuclei. The essential role of the quantum description of
the protons is established.Comment: submitted to the Journal of Chemical Physic
Improved technique for design of perfect reconstruction FIR QMF banks with lossless polyphase matrices
A technique is developed for the design of analysis filters in an M-channel maximally decimated, perfect reconstruction, finite-impulse-response quadrature mirror filter (FIR QMF) bank that has a lossless polyphase-component matrix E(z). The aim is to optimize the parameters characterizing E(z) until the sum of the stopband energies of the analysis filters is minimized. There are four novel elements in the procedure reported here. The first is a technique for efficient initialization of one of the M analysis filters, as a spectral factor of an Mth band filter. The factorization itself is done in an efficient manner using the eigenfilters approach, without the need for root-finding techniques. The second element is the initialization of the internal parameters which characterize E(z), based on the above spectral factor. The third element is a modified characterization, mostly free from rotation angles, of the FIR E(z). The fourth is the incorporation of symmetry among the analysis filters, so as to minimize the number of unknown parameters being optimized. The resulting design procedure always gives better filter responses than earlier ones (for a given filter length) and converges much faste
Atomic diffraction in counter-propagating Gaussian pulses of laser light
We present an analysis of atomic diffraction due to the interaction of an
atomic beam with a pair of Gaussian light pulses. We derive a simple analytical
expression for the populations in different diffraction orders. The validity of
the obtained solution extends beyond the Raman-Nath regime, where the kinetic
energy associated with different diffraction peaks is neglected, into the
so-called channeling regime where accurate analytical expressions have not
previously been available for the diffraction. Comparison with experimental
results and exact numerical solutions demonstrate the validity of our
analytical formula.Comment: 6 pages, 5 figure
Few-body reference data for multicomponent formalisms: Light nuclei molecules
We present full quantum statistical energetics of some electron-light nuclei
systems. This is accomplished with the path integral Monte Carlo method. The
effects on energetics arising from the change in the nuclear mass are studied.
The obtained results may serve as reference data for the multicomponent density
functional theory calculations of light nuclei system. In addition, the results
reported here will enable better fitting of todays electron-nuclear energy
functionals, for which the description of light nuclei is most challenging, in
particular
Motion of vortices in inhomogeneous Bose-Einstein condensates
We derive a general and exact equation of motion for a quantised vortex in an
inhomogeneous two-dimensional Bose-Einstein condensate. This equation expresses
the velocity of a vortex as a sum of local ambient density and phase gradients
in the vicinity of the vortex. We perform Gross-Pitaevskii simulations of
single vortex dynamics in both harmonic and hard-walled disk-shaped traps, and
find excellent agreement in both cases with our analytical prediction. The
simulations reveal that, in a harmonic trap, the main contribution to the
vortex velocity is an induced ambient phase gradient, a finding that
contradicts the commonly quoted result that the local density gradient is the
only relevant effect in this scenario. We use our analytical vortex velocity
formula to derive a point-vortex model that accounts for both density and phase
contributions to the vortex velocity, suitable for use in inhomogeneous
condensates. Although good agreement is obtained between Gross-Pitaevskii and
point-vortex simulations for specific few-vortex configurations, the effects of
nonuniform condensate density are in general highly nontrivial, and are thus
difficult to efficiently and accurately model using a simplified point-vortex
description.Comment: 13 pages, 8 figure
Coherence vortices in one spatial dimension
Coherence vortices are screw-type topological defects in the phase of
Glauber's two-point degree of quantum coherence, associated with pairs of
spatial points at which an ensemble-averaged stochastic quantum field is
uncorrelated. Coherence vortices may be present in systems whose dimensionality
is too low to support spatial vortices. We exhibit lattices of such
quantum-coherence phase defects for a one-dimensional model quantum system. We
discuss the physical meaning of coherence vortices and propose how they may be
realized experimentally.Comment: 5 pages, 3 figure
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