167 research outputs found
Wetting and Growth Behaviors in Adsorbed Systems with Long-Range Forces
The growth and possible wetting behaviors of an adsorbed film are studied employing a solid-on-solid model in the presence of a hard wall and external potential V(h) which is of long range. The model is analyzed with the use of position-space renormalization-group methods within the Migdal approximation. The existence of wetting transitions and their nature depends on the asymptotic behavior of V(h) at large distances. We find that critical wetting cannot take place in this model. From what is known of V(h), we conclude that wetting can be observed only along the gas-liquid phase boundary; however, first-order transitions between thin and thick films, which may be experimentally difficult to distinguish from wetting, can be observed along any phase boundary. The nature of the global phase diagram depends on the form of V(h) and several general behaviors are presented. In particular, in the layering subregime we find that the limit of layering critical points is indeed the bulk roughening temperature as had been suggested by de Oliveira and Griffiths. The scaling of these layering critical points is given explicitly
Analytical Investigation of Innovation Dynamics Considering Stochasticity in the Evaluation of Fitness
We investigate a selection-mutation model for the dynamics of technological
innovation,a special case of reaction-diffusion equations. Although mutations
are assumed to increase the variety of technologies, not their average success
("fitness"), they are an essential prerequisite for innovation. Together with a
selection of above-average technologies due to imitation behavior, they are the
"driving force" for the continuous increase in fitness. We will give analytical
solutions for the probability distribution of technologies for special cases
and in the limit of large times.
The selection dynamics is modelled by a "proportional imitation" of better
technologies. However, the assessment of a technology's fitness may be
imperfect and, therefore, vary stochastically. We will derive conditions, under
which wrong assessment of fitness can accelerate the innovation dynamics, as it
has been found in some surprising numerical investigations.Comment: For related work see http://www.helbing.or
Osmotic pressure of matter and vacuum energy
The walls of the box which contains matter represent a membrane that allows
the relativistic quantum vacuum to pass but not matter. That is why the
pressure of matter in the box may be considered as the analog of the osmotic
pressure. However, we demonstrate that the osmotic pressure of matter is
modified due to interaction of matter with vacuum. This interaction induces the
nonzero negative vacuum pressure inside the box, as a result the measured
osmotic pressure becomes smaller than the matter pressure. As distinct from the
Casimir effect, this induced vacuum pressure is the bulk effect and does not
depend on the size of the box. This effect dominates in the thermodynamic limit
of the infinite volume of the box. Analog of this effect has been observed in
the dilute solution of 3He in liquid 4He, where the superfluid 4He plays the
role of the non-relativistic quantum vacuum, and 3He atoms play the role of
matter.Comment: 5 pages, 1 figure, JETP Lett. style, version accepted in JETP Letter
Universal Long-time Behavior of Nuclear Spin Decays in a Solid
Magnetic resonance studies of nuclear spins in solids are exceptionally well
suited to probe the limits of statistical physics. We report experimental
results indicating that isolated macroscopic systems of interacting nuclear
spins possess the following fundamental property: spin decays that start from
different initial configurations quickly evolve towards the same long-time
behavior. This long-time behavior is characterized by the shortest ballistic
microscopic timescale of the system and therefore falls outside of the validity
range for conventional approximations of statistical physics. We find that the
nuclear free induction decay and different solid echoes in hyperpolarized solid
xenon all exhibit sinusoidally modulated exponential long-time behavior
characterized by identical time constants. This universality was previously
predicted on the basis of analogy with resonances in classical chaotic systems.Comment: 4 pages main paper + 3 pages supporting material, 3 figure
Bose-Einstein Condensation at a Helium Surface
Path Integral Monte Carlo was used to calculate the Bose-Einstein condensate
fraction at the surface of a helium film at , as a function of
density. Moving from the center of the slab to the surface, the condensate
fraction was found to initially increase with decreasing density to a maximum
value of 0.9 before decreasing. Long wavelength density correlations were
observed in the static structure factor at the surface of the slab. Finally, a
surface dispersion relation was calculated from imaginary-time density-density
correlations.Comment: 8 pages, 5 figure
Bunching Transitions on Vicinal Surfaces and Quantum N-mers
We study vicinal crystal surfaces with the terrace-step-kink model on a
discrete lattice. Including both a short-ranged attractive interaction and a
long-ranged repulsive interaction arising from elastic forces, we discover a
series of phases in which steps coalesce into bunches of n steps each. The
value of n varies with temperature and the ratio of short to long range
interaction strengths. We propose that the bunch phases have been observed in
very recent experiments on Si surfaces. Within the context of a mapping of the
model to a system of bosons on a 1D lattice, the bunch phases appear as quantum
n-mers.Comment: 5 pages, RevTex; to appear in Phys. Rev. Let
Adsorption in non interconnected pores open at one or at both ends: A reconsideration of the origin of the hysteresis phenomenon
We report on an experimental study of adsorption isotherme of nitrogen onto
porous silicon with non interconnected pores open at one or at both ends in
order to check for the first time the old (1938) but always current idea based
on Cohan's description which suggests that the adsorption of gaz should occur
reversibly in the first case and irreversibly in the second one. Hysteresis
loops, the shape of which is usually associated to interconnections in porous
media, are observed whether the pores are open at one or at both ends in
contradiction with Cohan's model.Comment: 5 pages, 4 EPS figure
Surface Region of Superfluid Helium as an Inhomogeneous Bose-Condensed Gas
We present arguments that the low density surface region of self-bounded
superfluid He systems is an inhomogeneous dilute Bose gas, with almost all
of the atoms occupying the same single-particle state at . Numerical
evidence for this complete Bose-Einstein condensation was first given by the
many-body variational calculations of He droplets by Lewart, Pandharipande
and Pieper in 1988. We show that the low density surface region can be treated
rigorously using a generalized Gross-Pitaevskii equation for the Bose order
parameter.Comment: 4 pages, 1 Postscript figur
To wet or not to wet: that is the question
Wetting transitions have been predicted and observed to occur for various
combinations of fluids and surfaces. This paper describes the origin of such
transitions, for liquid films on solid surfaces, in terms of the gas-surface
interaction potentials V(r), which depend on the specific adsorption system.
The transitions of light inert gases and H2 molecules on alkali metal surfaces
have been explored extensively and are relatively well understood in terms of
the least attractive adsorption interactions in nature. Much less thoroughly
investigated are wetting transitions of Hg, water, heavy inert gases and other
molecular films. The basic idea is that nonwetting occurs, for energetic
reasons, if the adsorption potential's well-depth D is smaller than, or
comparable to, the well-depth of the adsorbate-adsorbate mutual interaction. At
the wetting temperature, Tw, the transition to wetting occurs, for entropic
reasons, when the liquid's surface tension is sufficiently small that the free
energy cost in forming a thick film is sufficiently compensated by the fluid-
surface interaction energy. Guidelines useful for exploring wetting transitions
of other systems are analyzed, in terms of generic criteria involving the
"simple model", which yields results in terms of gas-surface interaction
parameters and thermodynamic properties of the bulk adsorbate.Comment: Article accepted for publication in J. Low Temp. Phy
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