3,867 research outputs found
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
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