Density mismatch in thin diblock copolymer films

Thin films of diblock copolymer subject to gravitational field are simulated by means of a cell dynamical system model. The difference in density of the two sides of the molecule and the presence of the field causes the formation of lamellar patterns with orientation parallel to the confining walls even when they are neutral. The concentration profile of those films is analyzed in the weak segregation regime and a functional form for the profile is proposed.Comment: 9 pages and 8 figures. Needs EPSF macros. Submitted to PR

Brésil

Plus que la "frontière" pionnière de l'intérieur du pays, c'est la région du Centre-Sud, et notamment ses grandes villes, qui attire le plus grand nombre de migrants brésiliens, comme l'atteste l'analyse des recensements de 1985 et 1991. Le Centre-Sud, en effet, est la région la plus urbanisée, la plus industrialisée du pays; elle est en situation dominante dans les secteurs tertiaires, de l'agriculture et de l'élevage, et constitue ainsi le véritable moteur de l'organisation territoriale brésilienne. Ainsi, cette restructuration spatiale, placée sous le signe de l'urbanisation, s'opère aujourd'hui selon deux modèles. D'une part, des zones à la production agropastorale moderne, bien reliées aux circuits économiques et, notamment, au complexe agro-industriel. D'autre part, des régions, dotées d'une population nombreuse, tournées vers l'autoconsommation et, donc, peu intégrées dans les circuits commerciaux. (Résumé d'auteur

Microwave Electronics

Contains reports on three research projects.U.S Navy (Office of Naval Research) under Contract Nonr-1841(49)U.S. Air Force under Air Force Contract AF19(604)-5200Lincoln Laboratory, Purchase Order DDL-B22

Frequency Modulation

Contains reports on six research projects

Domain size effects in Barkhausen noise

The possible existence of self-organized criticality in Barkhausen noise is investigated theoretically through a single interface model, and experimentally from measurements in amorphous magnetostrictive ribbon Metglas 2605TCA under stress. Contrary to previous interpretations in the literature, both simulation and experiment indicate that the presence of a cutoff in the avalanche size distribution may be attributed to finite size effects.Comment: 5 pages, 3 figures, submitted so Physical Review

Physical Acoustics

Contains reports on eight research projects.Office of Naval Research (Contract Nonr-1841(42)United States Air Force, Air Force Cambridge Research Center, Air Research and Development Command (Contract AF19(604)-2051

The role of the alloy structure in the magnetic behavior of granular systems

The effect of grain size, easy magnetization axis and anisotropy constant distributions in the irreversible magnetic behavior of granular alloys is considered. A simulated granular alloy is used to provide a realistic grain structure for the Monte Carlo simulation of the ZFC-FC curves. The effect of annealing and external field is also studied. The simulation curves are in good agreement with the FC and ZFC magnetization curves measured on melt spun Cu-Co ribbons.Comment: 13 pages, 10 figures, submitted to PR

Demagnetization via Nucleation of the Nonequilibrium Metastable Phase in a Model of Disorder

We study both analytically and numerically metastability and nucleation in a two-dimensional nonequilibrium Ising ferromagnet. Canonical equilibrium is dynamically impeded by a weak random perturbation which models homogeneous disorder of undetermined source. We present a simple theoretical description, in perfect agreement with Monte Carlo simulations, assuming that the decay of the nonequilibrium metastable state is due, as in equilibrium, to the competition between the surface and the bulk. This suggests one to accept a nonequilibrium "free-energy" at a mesoscopic/cluster level, and it ensues a nonequilibrium "surface tension" with some peculiar low-T behavior. We illustrate the occurrence of intriguing nonequilibrium phenomena, including: (i) Noise-enhanced stabilization of nonequilibrium metastable states; (ii) reentrance of the limit of metastability under strong nonequilibrium conditions; and (iii) resonant propagation of domain walls. The cooperative behavior of our system may also be understood in terms of a Langevin equation with additive and multiplicative noises. We also studied metastability in the case of open boundaries as it may correspond to a magnetic nanoparticle. We then observe burst-like relaxation at low T, triggered by the additional surface randomness, with scale-free avalanches which closely resemble the type of relaxation reported for many complex systems. We show that this results from the superposition of many demagnetization events, each with a well- defined scale which is determined by the curvature of the domain wall at which it originates. This is an example of (apparent) scale invariance in a nonequilibrium setting which is not to be associated with any familiar kind of criticality.Comment: 26 pages, 22 figure

Modeling self-assembly of diblock copolymer-nanoparticle composites

A cell dynamics method for domain separation of diblock copolymers (DBCPs) interacting with nanoparticles (NPs) whose diffusion coefficients depend on chain configuration is proposed for self-assembly of DBCP/NP composites. Increasing NP concentration slows down domain separation, but matching NP diffusion lengths and lamellar size of DBCPs reduces this effect. The model also explains features of different nanocomposites, such as morphological transitions induced by NPs, the coexistence of lamellar and hexagonal patterns in a single sample and peaked NP density profiles across the parallel domains.Comment: 21 pages, 7 figures include