Issues in Solid-State Physics

In the first sections, we bring into the present context some of our past contributions on the influence of quantum correlations on the formation of tightly bound solids. We discuss the effects of the overlap between neighbor orbitals in diverse situations of interest—involving both bulk and surface states—and call the reader’s attention to an exact tight-binding calculation which allows gauging the errors introduced by the underlying hypotheses of the usual tight-binding approximation. We round up this part by reviewing a quantum Monte Carlo method specific for strongly correlated fermion systems. In the last section, we explore some non-equilibrium routes to (not necessarily tightly bound) solid state: we discuss spatiotemporal pattern formation in arrays of FitzHugh-Nagumo (FHN) neurons, akin to resonant crystal structures

Energy harvesting from noise

Se realiza una serie de experimentos tendientes a contribuir al diseño y prototipo de una nueva clase de generadores para los dispositivos electrónicos de bajo consumo (telefonía móvil, sensores remotos, etc), capaces de extraer la energía del ambiente cuando y donde se necesita. En el primer experimento se reprodujo una propuesta de la literatura, excitando al sistema con ruidos sintetizados digital y analógicamente para que tengan distintas características espectrales y estadísticas. Los resultados preliminares indican un incremento en la potencia generada cuando la estadística es de tipo Lévy. En el segundo (en fase de prueba) abordamos un diseño alternativo original, basado en inducción electromagnética.A series of experiments is performed to help design and prototype a new class of generators for low-power electronic devices (mobile phones, remote sensing, etc.) capable of extracting energy from the environment when and where needed. In the first experiment a proposal in the literature was repeated, exciting the system with digitally-and analogically-synthesized sounds to have different spectral and statistical characteristics. Preliminary results indicate an increase in generated power when the statistics is of Lévy type. In the second (in test phase), an original alternative design based on electromagnetic induction is approachedFil: Peña Rosselló, J. I.. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales (UNMdP-FCEyN). Buenos Aires. ArgentinaFil: Murias, G. F.. Universidad Nacional de Mar del Plata. Facultad de Ingeniería (UNMdP-FI). Buenos Aires. ArgentinaFil: Deza, Roberto Raúl. Universidad Nacional de Mar del Plata - CONICET. Instituto de Investigaciones Físicas de Mar del Plata (IFIMAR). Buenos Aires. Argentin

Nonequilibrium phase transitions induced by multiplicative noise: effects of self-correlation

A recently introduced lattice model, describing an extended system which exhibits a reentrant (symmetry-breaking, second-order) noise-induced nonequilibrium phase transition, is studied under the assumption that the multiplicative noise leading to the transition is colored. Within an effective Markovian approximation and a mean-field scheme it is found that when the self-correlation time of the noise is different from zero, the transition is also reentrant with respect to the spatial coupling D. In other words, at variance with what one expects for equilibrium phase transitions, a large enough value of D favors disorder. Moreover, except for a small region in the parameter subspace determined by the noise intensity and D, an increase in the self-correlation time usually preventsthe formation of an ordered state. These effects are supported by numerical simulations.Comment: 15 pages. 9 figures. To appear in Phys.Rev.

Nonequilibrium phase transitions induced by multiplicative noise: Effects of self-correlation

A recently introduced lattice model, describing an extended system which exhibits a reentrant (symmetry-breaking, second-order) noise-induced nonequilibrium phase transition, is studied under the assumption that the multiplicative noise leading to the transition is colored. Within an effective Markovian approximation and a mean-field scheme it is found that when the self-correlation time τ of the noise is different from zero, the transition is also reentrant with respect to the spatial coupling D. In other words, at variance with what one expects for equilibrium phase transitions, a large enough value of D favors disorder. Moreover, except for a small region in the parameter subspace determined by the noise intensity σ and D, an increase in τ usually prevents the formation of an ordered state. These effects are supported by numerical simulations.R. T. acknowledges financial support from DGICyT, project numbers PB94-1167 and PB97- 0141-C02-01. H. S. W, R. R. D. and S. E. M. acknowledge financial support from CONICET, project number PIP-4953/96, and from ANPCyT, project number .Peer Reviewe

Contribución de cadenas pendientes al módulo viscoelástico de gomas

Se estudia la contribución de una cadena pendiente al espectro de relajación de una red perfecta y monofuncional, variando la relación de dicha cadena y la de los tramos de la red, y la funcionalidad de ésta. El espectro de relajación se calcula como en la ref. 1 a partir de los autovalores de la matriz de conectividad de la red, esta vez acoplada con la cadena. En contraste con¹, que muestra la contribución a dicho espectro de una "cáscara" suficientemente alejada del borde en el límite de red infinita, en nuestro trabajo encontramos el espectro completo (para una red cuyo tamaño está limitado por la capacidad de la VAX 780) que es cualitativamente diferente del mostrado en¹ y permite cálculos del módulo viscoelástico complejo susceptibles de comparación con datos experimentales. En particular econtramos que los tiempos de relajación mas largos están asociados a la presencia de la cadena pendiente y aumentan con la longitud de ésta, siguiendo una ley de escala tipo potencia. El exponente obtenido dista mucho del medido que es aproximadamente de 3,4) indicando la necesidad de introducir en el cálculo la contribución de los entrelazamientos entre cadenasFil: Carignano, Marcelo Andrés. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física de La Plata (UNLP-FCE-IFLP). Buenos Aires. ArgentinaFil: Deza, Roberto Raúl. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Departamento de Física (UNMdP-FCEyN). Buenos Aires. Argentina

Nonequilibrium phase transitions induced by multiplicative noise: Effects of self-correlation,” Phys

A recently introduced lattice model, describing an extended system which exhibits a reentrant �symmetrybreaking, second-order � noise-induced nonequilibrium phase transition, is studied under the assumption that the multiplicative noise leading to the transition is colored. Within an effective Markovian approximation and a mean-field scheme it is found that when the self-correlation time � of the noise is different from zero, the transition is also reentrant with respect to the spatial coupling D. In other words, at variance with what one expects for equilibrium phase transitions, a large enough value of D favors disorder. Moreover, except for a small region in the parameter subspace determined by the noise intensity � and D, an increase in � usually prevents the formation of an ordered state. These effects are supported by numerical simulations. PACS number�s�: 05.40.�a, 47.20.Ky, 47.20.Hw I

Influencia de las condiciones de contorno de albedo sobre la formación de estructuras estacionarias en un modelo de conducción nerviosa

En la versión lineal por partes del sistema activador-inhibidor conocido como modelo de FitzHugh-Nagumo se estudian, para un sistema restringido a un intervalo unidimensional finito, los efectos de las condiciones de contorno parcialmente reflectivas sobre la formación y estabilidad de estructuras estacionarias. Suponemos que los campos de activación e inhibición están sujetos a diferentes condiciones de contorno (pero simétricas para cada uno de los campos por separado)In the piccewise-linear version of the activator-inhibitor system known as FitzHugh Nagumo model we study - for a system restricted to a finite one dimensional interval the effects on stationary pattern formation of imposing patially rellecting or albedo boundary conditions. The activator and inhibitor fields are assumed to "see" different albedo conditions. A variety of transitions in behavior can be observed as the plane of the corresponding albedo parameters is swept. The stability of the different patterns that arise is under studyFil: Wio, Horacio Sergio. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche. División Teoría (CNEA-CAB). Río Negro. ArgentinaFil: Izús, Gonzalo Gregorio. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Departamento de Física (UNMdP-FCEyN). Buenos Aires. ArgentinaFil: Ramírez, O. S.. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Departamento de Física (UNMdP-FCEyN). Buenos Aires. ArgentinaFil: Deza, Roberto Raúl. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Departamento de Física (UNMdP-FCEyN). Buenos Aires. ArgentinaFil: Borzi, C. H.. Comisión Nacional de Energía Atómica (CNEA). CABA. Argentin