Theory of charge fluctuations and domain relocation times in semiconductor superlattices

Shot noise affects differently the nonlinear electron transport in semiconductor superlattices depending on the strength of the coupling among the superlattice quantum wells. Strongly coupled superlattices can be described by a miniband Boltzmann-Langevin equation from which a stochastic drift-diffusion equation is derived by means of a consistent Chapman-Enskog method. Similarly, shot noise in weakly coupled, highly doped semiconductor superlattices is described by a stochastic discrete drift-diffusion model. The current-voltage characteristics of the corresponding deterministic model consist of a number of stable branches corresponding to electric field profiles displaying two domains separated by a domain wall. If the initial state corresponds to a voltage on the middle of a stable branch and is suddenly switched to a final voltage corresponding to the next branch, the domains relocate after a certain delay time, called relocation time. The possible scalings of this mean relocation time are discussed using bifurcation theory and the classical results for escape of a Brownian particle from a potential well.Comment: 14 pages, 2 figure

Energy prices, production

This paper investigates economic incentives influencing the adoption of energy saving technology by industry, namely, CHP in UK and Dutch manufacturing sectors. The empirical analysis is based on a cross sectional time series econometric model, and examines how industrial output and historical increases in the price of electricity relative to gas prices, spark the diffusion of CHP. Production and price elasticities are estimated across heterogeneous industrial groups. Using data for 13 manufacturing sectors the model shows that fuel cost savings and industry output impact significantly on CHP uptake. Model outcomes are found to differ depending on the period of estimation and the estimation period is key in determining the impact of gas price and purchased power prices on adoption of CHP

Asymptotics of the trap-dominated Gunn effect in p-type Ge

We present an asymptotic analysis of the Gunn effect in a drift-diffusion model---including electric-field-dependent generation-recombination processes---for long samples of strongly compensated p-type Ge at low temperature and under dc voltage bias. During each Gunn oscillation, there are different stages corresponding to the generation, motion and annihilation of solitary waves. Each stage may be described by one evolution equation for only one degree of freedom (the current density), except for the generation of each new wave. The wave generation is a faster process that may be described by solving a semiinfinite canonical problem. As a result of our study we have found that (depending on the boundary condition) one or several solitary waves may be shed during each period of the oscillation. Examples of numerical simulations validating our analysis are included.Comment: Revtex, 25 pag., 5 fig., to appear Physica

Time-periodic phases in populations of nonlinearly coupled oscillators with bimodal frequency distributions

The mean field Kuramoto model describing the synchronization of a population of phase oscillators with a bimodal frequency distribution is analyzed (by the method of multiple scales) near regions in its phase diagram corresponding to synchronization to phases with a time periodic order parameter. The richest behavior is found near the tricritical point were the incoherent, stationarily synchronized, ``traveling wave'' and ``standing wave'' phases coexist. The behavior near the tricritical point can be extrapolated to the rest of the phase diagram. Direct Brownian simulation of the model confirms our findings.Comment: Revtex,16 pag.,10 fig., submitted to Physica

Asymptotic description of transients and synchronized states of globally coupled oscillators

A two-time scale asymptotic method has been introduced to analyze the multimodal mean-field Kuramoto-Sakaguchi model of oscillator synchronization in the high-frequency limit. The method allows to uncouple the probability density in different components corresponding to the different peaks of the oscillator frequency distribution. Each component evolves toward a stationary state in a comoving frame and the overall order parameter can be reconstructed by combining them. Synchronized phases are a combination of traveling waves and incoherent solutions depending on parameter values. Our results agree very well with direct numerical simulations of the nonlinear Fokker-Planck equation for the probability density. Numerical results have been obtained by finite differences and a spectral method in the particular case of bimodal (symmetric and asymmetric) frequency distribution with or without external field. We also recover in a very easy and intuitive way the only other known analytical results: those corresponding to reflection-symmetric bimodal frequency distributions near bifurcation points.Comment: Revtex,12 pag.,9 fig.;submitted to Physica

Microscopic Model for Sequential Tunneling in Semiconductor Multiple Quantum Wells

We propose a selfconsistent microscopic model of vertical sequential tunneling through a multi-quantum well.The model includes a detailed description of the contacts,uses the Transfer Hamiltonian for expressions of the current and it treats the Coulomb interaction within a mean field approximation. We analyze the current density through a double well and a superlattice and study the formation of electric field domains and multistability coming from the Coulomb interaction. Phase diagrams of parameter regions (bias, doping in the heterostructure and in the contacts,etc) where the different solutions exist are given.Comment: 4 pages, 8 Postscript Figure

On the Photorefractive Gunn Effect

We present and numerically solve a model of the photorefractive Gunn effect. We find that high field domains can be triggered by phase-locked interference fringes, as it has been recently predicted on the basis of linear stability considerations. Since the Gunn effect is intrinsically nonlinear, we find that such considerations give at best order-of-magnitude estimations of the parameters critical to the photorefractive Gunn effect. The response of the system is much more complex including multiple wave shedding from the injecting contact, wave suppression and chaos with spatial structure.Comment: Revtex, 8 pag., 4 fig. (jpg), submit to Physical Review

The influence of social value and selfcongruity on interpersonal connections in virtual social networks by Gen-Y tourists

This research focuses on the relationship of self-congruity and perceived social value with the interpersonal connections established by Generation Y tourists in virtual social networks. A quantitative study was performed using a sample of young travelers from Spain. The methodologies of Confirmatory Factor Analysis (CFA) and Structural Equation Models (SEM) were used to analyze the results. The findings of the research show that self-congruity influences the perceived social value; the perceived social value leads to satisfaction and the creation of interpersonal connections in virtual social networks; and the interpersonal connections in virtual social networks influence the use of these tools by Generation Y travelers