2,901 research outputs found

### An introduction to the Ginzburg-Landau theory of phase transitions and nonequilibrium patterns

This paper presents an introduction to phase transitions and critical
phenomena on the one hand, and nonequilibrium patterns on the other, using the
Ginzburg-Landau theory as a unified language. In the first part, mean-field
theory is presented, for both statics and dynamics, and its validity tested
self-consistently. As is well known, the mean-field approximation breaks down
below four spatial dimensions, where it can be replaced by a scaling
phenomenology. The Ginzburg-Landau formalism can then be used to justify the
phenomenological theory using the renormalization group, which elucidates the
physical and mathematical mechanism for universality. In the second part of the
paper it is shown how near pattern forming linear instabilities of dynamical
systems, a formally similar Ginzburg-Landau theory can be derived for
nonequilibrium macroscopic phenomena. The real and complex Ginzburg-Landau
equations thus obtained yield nontrivial solutions of the original dynamical
system, valid near the linear instability. Examples of such solutions are plane
waves, defects such as dislocations or spirals, and states of temporal or
spatiotemporal (extensive) chaos

### Compatible Quantum Theory

Formulations of quantum mechanics can be characterized as realistic,
operationalist, or a combination of the two. In this paper a realistic theory
is defined as describing a closed system entirely by means of entities and
concepts pertaining to the system. An operationalist theory, on the other hand,
requires in addition entities external to the system. A realistic formulation
comprises an ontology, the set of (mathematical) entities that describe the
system, and assertions, the set of correct statements (predictions) the theory
makes about the objects in the ontology. Classical mechanics is the prime
example of a realistic physical theory. The present realistic formulation of
the histories approach originally introduced by Griffiths, which we call
'Compatible Quantum Theory (CQT)', consists of a 'microscopic' part (MIQM),
which applies to a closed quantum system of any size, and a 'macroscopic' part
(MAQM), which requires the participation of a large (ideally, an infinite)
system. The first (MIQM) can be fully formulated based solely on the assumption
of a Hilbert space ontology and the noncontextuality of probability values,
relying in an essential way on Gleason's theorem and on an application to
dynamics due in large part to Nistico. The microscopic theory does not,
however, possess a unique corpus of assertions, but rather a multiplicity of
contextual truths ('c-truths'), each one associated with a different framework.
This circumstance leads us to consider the microscopic theory to be physically
indeterminate and therefore incomplete, though logically coherent. The
completion of the theory requires a macroscopic mechanism for selecting a
physical framework, which is part of the macroscopic theory (MAQM). Detailed
definitions and proofs are presented in the appendice

### Pattern formation outside of equilibrium

A comprehensive review of spatiotemporal pattern formation in systems driven away from equilibrium is presented, with emphasis on comparisons between theory and quantitative experiments. Examples include patterns in hydrodynamic systems such as thermal convection in pure fluids and binary mixtures, Taylor-Couette flow, parametric-wave instabilities, as well as patterns in solidification fronts, nonlinear optics, oscillatory chemical reactions and excitable biological media. The theoretical starting point is usually a set of deterministic equations of motion, typically in the form of nonlinear partial differential equations. These are sometimes supplemented by stochastic terms representing thermal or instrumental noise, but for macroscopic systems and carefully designed experiments the stochastic forces are often negligible. An aim of theory is to describe solutions of the deterministic equations that are likely to be reached starting from typical initial conditions and to persist at long times. A unified description is developed, based on the linear instabilities of a homogeneous state, which leads naturally to a classification of patterns in terms of the characteristic wave vector q0 and frequency Ï‰0 of the instability. Type Is systems (Ï‰0=0, q0â‰ 0) are stationary in time and periodic in space; type IIIo systems (Ï‰0â‰ 0, q0=0) are periodic in time and uniform in space; and type Io systems (Ï‰0â‰ 0, q0â‰ 0) are periodic in both space and time. Near a continuous (or supercritical) instability, the dynamics may be accurately described via "amplitude equations," whose form is universal for each type of instability. The specifics of each system enter only through the nonuniversal coefficients. Far from the instability threshold a different universal description known as the "phase equation" may be derived, but it is restricted to slow distortions of an ideal pattern. For many systems appropriate starting equations are either not known or too complicated to analyze conveniently. It is thus useful to introduce phenomenological order-parameter models, which lead to the correct amplitude equations near threshold, and which may be solved analytically or numerically in the nonlinear regime away from the instability. The above theoretical methods are useful in analyzing "real pattern effects" such as the influence of external boundaries, or the formation and dynamics of defects in ideal structures. An important element in nonequilibrium systems is the appearance of deterministic chaos. A greal deal is known about systems with a small number of degrees of freedom displaying "temporal chaos," where the structure of the phase space can be analyzed in detail. For spatially extended systems with many degrees of freedom, on the other hand, one is dealing with spatiotemporal chaos and appropriate methods of analysis need to be developed. In addition to the general features of nonequilibrium pattern formation discussed above, detailed reviews of theoretical and experimental work on many specific systems are presented. These include Rayleigh-BÃ©nard convection in a pure fluid, convection in binary-fluid mixtures, electrohydrodynamic convection in nematic liquid crystals, Taylor-Couette flow between rotating cylinders, parametric surface waves, patterns in certain open flow systems, oscillatory chemical reactions, static and dynamic patterns in biological media, crystallization fronts, and patterns in nonlinear optics. A concluding section summarizes what has and has not been accomplished, and attempts to assess the prospects for the future

### Modeling of droplet breakup in a microfluidic T--shaped junction with a phase--field model

A phase--field method is applied to the modeling of flow and breakup of
droplets in a T--shaped junction in the hydrodynamic regime where capillary and
viscous stresses dominate over inertial forces, which is characteristic of
microfluidic devices. The transport equations are solved numerically in the
three--dimensional geometry, and the dependence of the droplet breakup on the
flow rates, surface tension and viscosities of the two components is
investigated in detail. The model reproduces quite accurately the phase diagram
observed in experiments performed with immiscible fluids. The critical
capillary number for droplet breakup depends on the viscosity contrast, with a
trend which is analogous to that observed for free isolated droplets in
hyperbolic flow

### Analysis of the Scanning Tunneling Microscopy Images of the Charge Density Wave Phase in Quasi-one-dimensional Rb0.3MoO3

The experimental STM images for the CDW phase of the blue bronze RbMoO3 have
been successfully explained on the basis of first-principles DFT calculations.
Although the density of states near the Fermi level strongly concentrates in
two of the three types of Mo atoms Mo-II and Mo-III, the STM measurement mostly
probes the contribution of the uppermost O atoms of the surface, associated
with the Mo-IO6 octahedra. In addition, it is found that the surface
concentration of Rb atoms plays a key role in determining the surface nesting
vector and hence the periodicity of the CDW modulation. Significant
experimental inhomogeneities of the b* surface component of the wavevector of
the modulation, probed by STM, are reported. The calculated changes in the
surface nesting vector are consistent with the observed experimental
inhomogeneities.Comment: 4 pages 5 Figure

### Lorentz Covariance and the Dimensional Crossover of 2d-Antiferromagnets

We derive a lattice $\beta$-function for the 2d-Antiferromagnetic Heisenberg
model, which allows the lattice interaction couplings of the nonperturbative
Quantum Monte Carlo vacuum to be related directly to the zero-temperature fixed
points of the nonlinear sigma model in the presence of strong interplanar and
spin anisotropies. In addition to the usual renormalization of the gapful
disordered state in the vicinity of the quantum critical point, we show that
this leads to a chiral doubling of the spectra of excited states

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