Preasymptotic multiscaling in the phase-ordering dynamics of the kinetic Ising model

The evolution of the structure factor is studied during the phase-ordering dynamics of the kinetic Ising model with conserved order parameter. A preasymptotic multiscaling regime is found as in the solution of the Cahn-Hilliard-Cook equation, revealing that the late stage of phase-ordering is always approached through a crossover from multiscaling to standard scaling, independently from the nature of the microscopic dynamics.Comment: 11 pages, 3 figures, to be published in Europhys. Let

Universality of citation distributions: towards an objective measure of scientific impact

We study the distributions of citations received by a single publication within several disciplines, spanning broad areas of science. We show that the probability that an article is cited $c$ times has large variations between different disciplines, but all distributions are rescaled on a universal curve when the relative indicator $c_f=c/c_0$ is considered, where $c_0$ is the average number of citations per article for the discipline. In addition we show that the same universal behavior occurs when citation distributions of articles published in the same field, but in different years, are compared. These findings provide a strong validation of $c_f$ as an unbiased indicator for citation performance across disciplines and years. Based on this indicator, we introduce a generalization of the h-index suitable for comparing scientists working in different fields.Comment: 7 pages, 5 figures. accepted for publication in Proc. Natl Acad. Sci. US

Critical Exponents of the KPZ Equation via Multi-Surface Coding Numerical Simulations

We study the KPZ equation (in D = 2, 3 and 4 spatial dimensions) by using a RSOS discretization of the surface. We measure the critical exponents very precisely, and we show that the rational guess is not appropriate, and that 4D is not the upper critical dimension. We are also able to determine very precisely the exponent of the sub-leading scaling corrections, that turns out to be close to 1 in all cases. We introduce and use a {\em multi-surface coding} technique, that allow a gain of order 30 over usual numerical simulations.Comment: 10 pages, 8 eps figures (2 figures added). Published versio

Sequential multi-photon strategy for semiconductor-based terahertz detectors

A semiconductor-based terahertz-detector strategy, exploiting a bound-to-bound-to-continuum architecture, is presented and investigated. In particular, a ladder of equidistant energy levels is employed, whose step is tuned to the desired detection frequency and allows for sequential multi-photon absorption. Our theoretical analysis demonstrates that the proposed multi-subband scheme could represent a promising alternative to conventional quantum-well infrared photodetectors in the terahertz spectral region.Comment: Submitted to Journal of Applied Physic

Heterogeneous pair approximation for voter models on networks

For models whose evolution takes place on a network it is often necessary to augment the mean-field approach by considering explicitly the degree dependence of average quantities (heterogeneous mean-field). Here we introduce the degree dependence in the pair approximation (heterogeneous pair approximation) for analyzing voter models on uncorrelated networks. This approach gives an essentially exact description of the dynamics, correcting some inaccurate results of previous approaches. The heterogeneous pair approximation introduced here can be applied in full generality to many other processes on complex networks.Comment: 6 pages, 6 figures, published versio

The non-linear q-voter model

We introduce a non-linear variant of the voter model, the q-voter model, in which q neighbors (with possible repetition) are consulted for a voter to change opinion. If the q neighbors agree, the voter takes their opinion; if they do not have an unanimous opinion, still a voter can flip its state with probability $\epsilon$. We solve the model on a fully connected network (i.e. in mean-field) and compute the exit probability as well as the average time to reach consensus. We analyze the results in the perspective of a recently proposed Langevin equation aimed at describing generic phase transitions in systems with two ($Z_2$ symmetric) absorbing states. We find that in mean-field the q-voter model exhibits a disordered phase for high $\epsilon$ and an ordered one for low $\epsilon$ with three possible ways to go from one to the other: (i) a unique (generalized voter-like) transition, (ii) a series of two consecutive Ising-like and directed percolation transition, and (iii) a series of two transitions, including an intermediate regime in which the final state depends on initial conditions. This third (so far unexplored) scenario, in which a new type of ordering dynamics emerges, is rationalized and found to be specific of mean-field, i.e. fluctuations are explicitly shown to wash it out in spatially extended systems.Comment: 9 pages, 7 figure

Terahertz detection schemes based on sequential multi-photon absorption

We present modeling and simulation of prototypical multi bound state quantum well infrared photodetectors and show that such a detection design may overcome the problems arising when the operation frequency is pushed down into the far infrared spectral region. In particular, after a simplified analysis on a parabolic-potential design, we propose a fully three-dimensional model based on a finite difference solution of the Boltzmann transport equation for realistic potential profiles. The performances of the proposed simulated devices are encouraging and support the idea that such design strategy may face the well-known dark-current problem.Comment: 3 pages, 2 figures; submitted to Applied Physics Letter