Charge-density waves in one-dimensional Hubbard superlattices

We study the formation of charge density waves (CDW's) in one-dimensional Hubbard superlattices, modeled by a repeated pattern of repulsive (U>0) and free (U=0) sites. By means of Lanczos diagonalizations for the ground state, we calculate the charge structure factor. Our results show that while the superlattice structure affects the modulation of the charge density waves, the periodicity can still be predicted through an effective density. We also show that, for a fixed repulsive layer thickness, the periodicity of the CDW is an oscillatory function of the free layer thickness.Comment: 4 pages, 4 figure

Limits of the energy-momentum tensor in general relativity

A limiting diagram for the Segre classification of the energy-momentum tensor is obtained and discussed in connection with a Penrose specialization diagram for the Segre types. A generalization of the coordinate-free approach to limits of Paiva et al. to include non-vacuum space-times is made. Geroch's work on limits of space-times is also extended. The same argument also justifies part of the procedure for classification of a given spacetime using Cartan scalars.Comment: LaTeX, 21 page

Limits of space-times in five dimensions and their relation to the Segre Types

A limiting diagram for the Segre classification in 5-dimensional space-times is obtained, extending a recent work on limits of the energy-momentum tensor in general relativity. Some of Geroch's results on limits of space-times in general relativity are also extended to the context of five-dimensional Kaluza-Klein space-times.Comment: Late

s-wave Superconductivity Phase Diagram in the Inhomogeneous Two-Dimensional Attractive Hubbard Model

We study s-wave superconductivity in the two-dimensional square lattice attractive Hubbard Hamiltonian for various inhomogeneous patterns of interacting sites. Using the Bogoliubov-de Gennes (BdG) mean field approximation, we obtain the phase diagram for inhomogeneous patterns in which the on-site attractive interaction U_i between the electrons takes on two values, U_i=0 and -U/(1-f) (with f the concentration of non-interacting sites) as a function of average electron occupation per site n, and study the evolution of the phase diagram as f varies. In certain regions of the phase diagram, inhomogeneity results in a larger zero temperature average pairing amplitude (order parameter) and also a higher superconducting (SC) critical temperature T_c, relative to a uniform system with the same mean interaction strength (U_i=-U on all sites). These effects are observed for stripes, checkerboard, and even random patterns of the attractive centers, suggesting that the pattern of inhomogeneity is unimportant. The phase diagrams also include regions where superconductivity is obliterated due to the formation of various charge ordered phases. The enhancement of T_{c} due to inhomogeneity is robust as long as the electron doping per site n is less than twice the fraction of interacting sites [2(1-f)] regardless of the pattern. We also show that for certain inhomogeneous patterns, when n = 2(1-f), increasing temperature can work against the stability of existing charge ordered phases for large f and as a result, enhance T_{c}.Comment: 16 pages, 11 figure

Mimicking a robot: Facial EMG in response to emotional robotic facial expressions

Humans tend to anthropomorphize i.e., to attribute human-like characteristics (e.g. motivations, intentions, emotions) to non-humans. This suggests that we can interact with non-humans (televisions, computers, robots) in a similar way we interact with humans. Robots, in particular, have physical presence and can be programmed to display social interaction capabilities, i.e. to be social robots, amplifying those similarities. Past studies have shown that social robots in negative situations tend to elicit strong emotional responses and empathy in humans. However, it remains to be tested whether empathy can be felt towards a social robot, set in a situation of positive social interaction. We proposed that facial mimicry, one indicator of empathy, may occur towards a robot in a positive social context, i.e. while the robot is playing a board game with human opponents. Fifty-nine participants (46 females), aged 17 to 27 years (M=19.56, SD=2.11) were exposed to videos of a robotic head (EMYS, the EMotive headY System), previously programmed to display six emotional expressions (joy, surprise, anger, disgust, fear, sadness) and a neutral expression, while playing a board game. EMYS’s facial expressions were shown in two blocks: in the first, no social context was provided and sound was omitted; in the second, a positive social context was provided, which included sound of verbal interaction with humans. In each block, 14 videos were randomly presented. Facial electromyography (fEMG) activity, in response to EMYS’s facial expressions, was measured over the corrugator supercilii and zygomaticus major muscles. fEMG responses were calculated as difference from stimulus presentation to 1 sec baseline. Changes in fEMG reactivity, between conditions, were analyzed comparing fEMG responses to robotic emotional expressions with responses to robotic neutral expressions. In the positive social context condition, results revealed an overall reduction of corrugator supercilii reactivity for the majority of negative emotional expressions (except anger). There was also a significant reduction of the zygomaticus major activity to surprise, compared to neutral, in the positive social context. Overall, our results suggest the important role of the social context in our physiological responses to a robot, and more specifically a reduction of emotional negativity to non-threatening robotic facial expressions, displayed in a positive social context.info:eu-repo/semantics/publishedVersio

Modulation of charge-density waves by superlattice structures

We discuss the interplay between electronic correlations and an underlying superlattice structure in determining the period of charge density waves (CDW's), by considering a one-dimensional Hubbard model with a repeated (non-random) pattern of repulsive (U>0) and free (U=0) sites. Density matrix renormalization group diagonalization of finite systems (up to 120 sites) is used to calculate the charge-density correlation function and structure factor in the ground state. The modulation period can still be predicted through effective Fermi wavevectors, k_F*, and densities, and we have found that it is much more sensitive to electron (or hole) doping, both because of the narrow range of densities needed to go from q*=0 to \pi, but also due to sharp 2k_F*-4k_F* transitions; these features render CDW's more versatile for actual applications in heterostructures than in homogeneous systems.Comment: 4 pages, 5 figures, to appear in Phys Rev

Comparação entre folíolo central nos diferentes nós do feijoeiro.

O objetivo do trabalho foi observar se o índice foliar de diferentes nós do talo principal em vários estados de desenvolvimento guarda semelhança entre si

G_2 cosmological models separable in non-comoving coordinates

We study new separable orthogonally transitive abelian G_2 on S_2 models with two mutually orthogonal integrable Killing vector fields. For this purpose we consider separability of the metric functions in a coordinate system in which the velocity vector field of the perfect fluid does not take its canonical form, providing thereby solutions which are non-separable in comoving coordinates in general. Some interesting general features concerning this class of solutions are given. We provide a full classification for these models and present several families of explicit solutions with their properties.Comment: latex, 26 pages, accepted for publication in Class. Quantum Gra