222 research outputs found
Remarks on the Extended Characteristic Uncertainty Relations
Three remarks concerning the form and the range of validity of the
state-extended characteristic uncertainty relations (URs) are presented. A more
general definition of the uncertainty matrix for pure and mixed states is
suggested. Some new URs are provided.Comment: LaTex, 4 pages, no figure
An \emph{ab initio} method for locating characteristic potential energy minima of liquids
It is possible in principle to probe the many--atom potential surface using
density functional theory (DFT). This will allow us to apply DFT to the
Hamiltonian formulation of atomic motion in monatomic liquids [\textit{Phys.
Rev. E} {\bf 56}, 4179 (1997)]. For a monatomic system, analysis of the
potential surface is facilitated by the random and symmetric classification of
potential energy valleys. Since the random valleys are numerically dominant and
uniform in their macroscopic potential properties, only a few quenches are
necessary to establish these properties. Here we describe an efficient
technique for doing this. Quenches are done from easily generated "stochastic"
configurations, in which the nuclei are distributed uniformly within a
constraint limiting the closeness of approach. For metallic Na with atomic pair
potential interactions, it is shown that quenches from stochastic
configurations and quenches from equilibrium liquid Molecular Dynamics (MD)
configurations produce statistically identical distributions of the structural
potential energy. Again for metallic Na, it is shown that DFT quenches from
stochastic configurations provide the parameters which calibrate the
Hamiltonian. A statistical mechanical analysis shows how the underlying
potential properties can be extracted from the distributions found in quenches
from stochastic configurations
Observation of Single Transits in Supercooled Monatomic Liquids
A transit is the motion of a system from one many-particle potential energy
valley to another. We report the observation of transits in molecular dynamics
(MD) calculations of supercooled liquid argon and sodium. Each transit is a
correlated simultaneous shift in the equilibrium positions of a small local
group of particles, as revealed in the fluctuating graphs of the particle
coordinates versus time. This is the first reported direct observation of
transit motion in a monatomic liquid in thermal equilibrium. We found transits
involving 2 to 11 particles, having mean shift in equilibrium position on the
order of 0.4 R_1 in argon and 0.25 R_1 in sodium, where R_1 is the nearest
neighbor distance. The time it takes for a transit to occur is approximately
one mean vibrational period, confirming that transits are fast.Comment: 19 pages, 8 figure
Dynamics of monatomic liquids
We present a theory of the dynamics of monatomic liquids built on two basic
ideas: (1) The potential surface of the liquid contains three classes of
intersecting nearly-harmonic valleys, one of which (the ``random'' class)
vastly outnumbers the others and all whose members have the same depth and
normal mode spectrum; and (2) the motion of particles in the liquid can be
decomposed into oscillations in a single many-body valley, and nearly
instantaneous inter-valley transitions called transits. We review the
thermodynamic data which led to the theory, and we discuss the results of
molecular dynamics (MD) simulations of sodium and Lennard-Jones argon which
support the theory in more detail. Then we apply the theory to problems in
equilibrium and nonequilibrium statistical mechanics, and we compare the
results to experimental data and MD simulations. We also discuss our work in
comparison with the QNM and INM research programs and suggest directions for
future research.Comment: 53 pages, 16 figures. Differs from published version in using
American English spelling and grammar (published version uses British
English
Uncertainty relations in curved spaces
Uncertainty relations for particle motion in curved spaces are discussed. The
relations are shown to be topologically invariant. New coordinate system on a
sphere appropriate to the problem is proposed. The case of a sphere is
considered in details. The investigation can be of interest for string and
brane theory, solid state physics (quantum wires) and quantum optics.Comment: published version; phase space structure discussion adde
V-T Theory of Self Dynamic Response in a Monatomic Liquid
A new theoretical model for self dynamic response is developed using
Vibration-Transit (V-T) theory, and is applied to liquid sodium at all
wavevectors q from the hydrodynamic regime to the free particle limit. In this
theory the zeroth-order Hamiltonian describes the vibrational motion in a
single random valley harmonically extended to infinity. This Hamiltonian is
tractable, is evaluated a priori for monatomic liquids, and the same
Hamiltonian (the same set of eigenvalues and eigenvectors) is used for
equilibrium and nonequlibrium theory. Here, for the self intermediate
scattering function Fself(q,t) we find the vibrational contribution is in near
perfect agreement with molecular dynamics (MD) through short and intermediate
times, at all q. This is direct confirmation that normal mode vibrational
correlations are present in the motion of the liquid state. The primary transit
effect is diffusive motion of the vibrational equilibrium positions, as the
liquid transits rapidly among random valleys. This motion is modeled as a
standard random walk, and the resulting theoretical Fself(q,t) is in excellent
agreement with MD results at all q and t. In the limit for q to infinity, the
theory automatically exhibits the correct approach to the free-particle limit.
Also in the limit for q to zero, the hydrodynamic limit emerges as well. In
contrast to the benchmark theories of generalized hydrodynamics and mode
coupling, the present theory is near a priori, while achieving modestly better
accuracy. Therefore, in our view, it constitutes an improvement over the
traditional theories.Comment: 16 pages, 11 figures, Journal Paper. Following referee's comments,
Section IID has been completely rewritten and a new Section IIE has been
adde
Mean-atom-trajectory model for the velocity autocorrelation function of monatomic liquids
We present a model for the motion of an average atom in a liquid or
supercooled liquid state and apply it to calculations of the velocity
autocorrelation function and diffusion coefficient . The model
trajectory consists of oscillations at a distribution of frequencies
characteristic of the normal modes of a single potential valley, interspersed
with position- and velocity-conserving transits to similar adjacent valleys.
The resulting predictions for and agree remarkably well with MD
simulations of Na at up to almost three times its melting temperature. Two
independent processes in the model relax velocity autocorrelations: (a)
dephasing due to the presence of many frequency components, which operates at
all temperatures but which produces no diffusion, and (b) the transit process,
which increases with increasing temperature and which produces diffusion.
Because the model provides a single-atom trajectory in real space and time,
including transits, it may be used to calculate all single-atom correlation
functions.Comment: LaTeX, 8 figs. This is an updated version of cond-mat/0002057 and
cond-mat/0002058 combined Minor changes made to coincide with published
versio
Linked randomised controlled trials of face-to-face and electronic brief intervention methods to prevent alcohol related harm in young people aged 14–17 years presenting to Emergency Departments (SIPS junior)
Background: Alcohol is a major global threat to public health. Although the main burden of chronic alcohol-related disease is in adults, its foundations often lie in adolescence. Alcohol consumption and related harm increase steeply from the age of 12 until 20 years. Several trials focusing upon young people have reported significant positive effects of brief interventions on a range of alcohol consumption outcomes. A recent review of reviews also suggests that electronic brief interventions (eBIs) using internet and smartphone technologies may markedly reduce alcohol consumption compared with minimal or no intervention controls.
Interventions that target non-drinking youth are known to delay the onset of drinking behaviours. Web based alcohol interventions for adolescents also demonstrate significantly greater reductions in consumption and harm among ‘high-risk’ drinkers; however changes in risk status at follow-up for non-drinkers or low-risk
drinkers have not been assessed in controlled trials of brief alcohol interventions
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