8,179 research outputs found
Relaxation to quantum equilibrium for Dirac fermions in the de Broglie-Bohm pilot-wave theory
Numerical simulations indicate that the Born rule does not need to be
postulated in the de Broglie-Bohm pilot-wave theory, but arises dynamically
(relaxation to quantum equilibrium). These simulations were done for a particle
in a two-dimensional box whose wave-function obeys the non-relativistic
Schroedinger equation and is therefore scalar. The chaotic nature of the de
Broglie-Bohm trajectories, thanks to the nodes of the wave-function which act
as vortices, is crucial for a fast relaxation to quantum equilibrium. For
spinors, we typically do not expect any node. However, in the case of the Dirac
equation, the de Broglie-Bohm velocity field has vorticity even in the absence
of nodes. This observation raises the question of the origin of relaxation to
quantum equilibrium for fermions. In this article, we provide numerical
evidence to show that Dirac particles also undergo relaxation, by simulating
the evolution of various non-equilibrium distributions for two-dimensional
systems (the 2D Dirac oscillator and the Dirac particle in a spherical 2D box).Comment: 11 pages, 9 figure
Fluids confined in wedges and by edges: Virial series for the line-thermodynamic properties of hard spheres
This work is devoted to analyze the relation between the thermodynamic properties of a confined fluid and the shape of its confining vessel. Recently, new insights in this topic were found through the study of cluster integrals for inhomogeneous fluids that revealed the dependence on the vessel shape of the low density behavior of the system. Here, the statistical mechanics and thermodynamics of fluids confined in wedges or by edges is revisited, focusing on their cluster integrals. In particular, the well known hard sphere fluid, which was not studied in this framework so far, is analyzed under confinement and its thermodynamic properties are analytically studied up to order two in the density. Furthermore, the analysis is extended to the confinement produced by a corrugated wall. These results rely on the obtained analytic expression for the second cluster integral of the confined hard sphere system as a function of the opening dihedral angle 0 < β < 2Ï€. It enables a unified approach to both wedges and edges.Fil: Urrutia, Ignacio. Comisión Nacional de EnergÃa Atómica. Centro Atómico Constituyentes; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas; Argentin
LISA Science Results in the Presence of Data Disturbances
Each spacecraft in the Laser Interferometer Space Antenna houses a proof mass
which follows a geodesic through spacetime. Disturbances which change the proof
mass position, momentum, and/or acceleration will appear in the LISA data
stream as additive quadratic functions. These data disturbances inhibit signal
extraction and must be removed. In this paper we discuss the identification and
fitting of monochromatic signals in the data set in the presence of data
disturbances. We also present a preliminary analysis of the extent of science
result limitations with respect to the frequency of data disturbances
Fluctuations Do Matter: Large Noise-Enhanced Halos in Charged-Particle Beams
The formation of beam halos has customarily been described in terms of a
particle-core model in which the space-charge field of the oscillating core
drives particles to large amplitudes. This model involves parametric resonance
and predicts a hard upper bound to the orbital amplitude of the halo particles.
We show that the presence of colored noise due to space-charge fluctuations
and/or machine imperfections can eject particles to much larger amplitudes than
would be inferred from parametric resonance alone.Comment: 13 pages total, including 5 figure
Energy and variance optimization of many body wave functions
We present a simple, robust and efficient method for varying the parameters
in a many-body wave function to optimize the expectation value of the energy.
The effectiveness of the method is demonstrated by optimizing the parameters in
flexible Jastrow factors, that include 3-body electron-electron-nucleus
correlation terms, for the NO and decapentaene (CH)
molecules. The basic idea is to add terms to the straightforward expression for
the Hessian that are zero when the integrals are performed exactly, but that
cancel much of the statistical fluctuations for a finite Monte Carlo sample.
The method is compared to what is currently the most popular method for
optimizing many-body wave functions, namely minimization of the variance of the
local energy. The most efficient wave function is obtained by optimizing a
linear combination of the energy and the variance.Comment: 4 pages, 4 figures, minor corrections of inexact statements, missing
Boundary hopping and the mobility edge in the Anderson model in three dimensions
It is shown, using high-precision numerical simulations, that the mobility
edge of the 3d Anderson model depends on the boundary hopping term t in the
infinite size limit. The critical exponent is independent of it. The
renormalized localization length at the critical point is also found to depend
on t but not on the distribution of on-site energies for box and Lorentzian
distributions. Implications of results for the description of the transition in
terms of a local order-parameter are discussed
A Model for the Propagation of Sound in Granular Materials
This paper presents a simple ball-and-spring model for the propagation of
small amplitude vibrations in a granular material. In this model, the
positional disorder in the sample is ignored and the particles are placed on
the vertices of a square lattice. The inter-particle forces are modeled as
linear springs, with the only disorder in the system coming from a random
distribution of spring constants. Despite its apparent simplicity, this model
is able to reproduce the complex frequency response seen in measurements of
sound propagation in a granular system. In order to understand this behavior,
the role of the resonance modes of the system is investigated. Finally, this
simple model is generalized to include relaxation behavior in the force network
-- a behavior which is also seen in real granular materials. This model gives
quantitative agreement with experimental observations of relaxation.Comment: 21 pages, requires Harvard macros (9/91), 12 postscript figures not
included, HLRZ preprint 6/93, (replacement has proper references included
Linear and Non Linear Effects on the Newtonian Gravitational Constant as deduced from the Torsion Balance
The Newtonian gravitational constant has still 150 parts per million of
uncertainty. This paper examines the linear and nonlinear equations governing
the rotational dynamics of the torsion gravitational balance. A nonlinear
effect modifying the oscillation period of the torsion gravitational balance is
carefully explored.Comment: 11 pages, 2 figure
Quantum kinetic theory model of a continuous atom laser
We investigate the feasible limits for realising a continuously evaporated
atom laser with high-temperature sources. A plausible scheme for realising a
truly continuous atom laser is to outcouple atoms from a partially condensed
Bose gas, whilst continuously reloading the system with non-condensed thermal
atoms and performing evaporative cooling. Here we use quantum kinetic theory to
model this system and estimate feasible limits for the operation of such a
scheme. For sufficiently high temperatures, the figure of merit for the source
is shown to be the phase-space flux. The dominant process limiting the usage of
sources with low phase-space flux is the three-body loss of the condensed gas.
We conclude that certain double-magneto-optical trap (MOT) sources may produce
substantial mean condensate numbers through continuous evaporation, and provide
an atom laser source with a narrow linewidth and reasonable flux.Comment: 28 pages, 5 figure
New set of measures to analyze non-equilibrium structures
We introduce a set of statistical measures that can be used to quantify
non-equilibrium surface growth. They are used to deduce new information about
spatiotemporal dynamics of model systems for spinodal decomposition and surface
deposition. Patterns growth in the Cahn-Hilliard Equation (used to model
spinodal decomposition) are shown to exhibit three distinct stages. Two models
of surface growth, namely the continuous Kardar-Parisi-Zhang (KPZ) model and
the discrete Restricted-Solid-On-Solid (RSOS) model are shown to have different
saturation exponents
- …