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

The Schwarzschild-de Sitter solution in five-dimensional general relativity briefly revisited

We briefly revisit the Schwarzschild-de Sitter solution in the context of five-dimensional general relativity. We obtain a class of five-dimensional solutions of Einstein vacuum field equations into which the four-dimensional Schwarzschild-de Sitter space can be locally and isometrically embedded. We show that this class of solutions is well-behaved in the limit of lambda approaching zero. Applying the same procedure to the de Sitter cosmological model in five dimensions we obtain a class of embedding spaces which are similarly well-behaved in this limit. These examples demonstrate that the presence of a non-zero cosmological constant does not in general impose a rigid relation between the (3+1) and (4+1)-dimensional spacetimes, with degenerate limiting behaviour.Comment: 7 page

A layering model for superconductivity in the borocarbides

We propose a superlattice model to describe superconductivity in layered materials, such as the borocarbide families with the chemical formul\ae\ $RT_2$B$_2$C and $RT$BC, with $R$ being (essentially) a rare earth, and $T$ a transition metal. We assume a single band in which electrons feel a local attractive interaction (negative Hubbard-$U$) on sites representing the $T$B layers, while U=0 on sites representing the $R$C layers; the multi-band structure is taken into account minimally through a band offset $\epsilon$. The one-dimensional model is studied numerically through the calculation of the charge gap, the Drude weight, and of the pairing correlation function. A comparison with the available information on the nature of the electronic ground state (metallic or superconducting) indicates that the model provides a systematic parametrization of the whole borocarbide family.Comment: 4 figure

Taub's plane-symmetric vacuum spacetime revisited

The gravitational properties of the {\em only} static plane-symmetric vacuum solution of Einstein's field equations without cosmological term (Taub's solution, for brevity) are presented: some already known properties (geodesics, weak field limit and pertainment to the Schwarzschild family of spacetimes) are reviewed in a physically much more transparent way, as well as new results about its asymptotic structure, possible matchings and nature of the source are furnished. The main results point to the fact that the solution must be interpreted as representing the exterior gravitational field due to a {\em negative} mass distribution, confirming previous statements to that effect in the literature. Some analogies to Kasner's spatially homogeneous cosmological model are also referred to.Comment: plain LaTex, four Postscript figure

Virtual Meson Cloud of the Nucleon and Intrinsic Strangeness and Charm

We have applied the Meson Cloud Model (MCM) to calculate the charm and strange antiquark distribution in the nucleon. The resulting distribution, in the case of charm, is very similar to the intrinsic charm momentum distribution in the nucleon. This seems to corroborate the hypothesis that the intrinsic charm is in the cloud and, at the same time, explains why other calculations with the MCM involving strange quark distributions fail in reproducing the low x region data. From the intrinsic strange distribution in the nucleon we have extracted the strangeness radius of the nucleon, which is in agreement with other meson cloud calculations.Comment: 9 pages RevTex, 4 figure

Identificação e purificação de um vírus da poliedrose nuclear da lagarta Spodoptera frugiperda (J.E. Smith, 1797) (Lepidoptera Noctuidae).

A lagarta-do-cartucho e considerado uma das principais pragas da cultura do milho e seu controle tem sido feito basicamente pelo uso de inseticidas quimicos. O CNPMS/EMBRAPA considerando as possibilidades de um controle biologico desta praga, tem procurado um virus de granulose. Recentemente isolou-se , a partir de uma lagarta do campo, um virus de poliedrose nuclear (VPN), que tem sido mantido em larvas em condicoes de laboratorio, causando nestas sintomas identicos ao observado no campo. Poliedrose puderam ser purificados a partir de extratos de lagarta infectada atraves de centrifugacao diferencial em um gradiente de densidade de sacarose. Teste de infectividade mostraram que larvas de S. frugiperda, criadas artificialmente, de 3, 6 e 7 dias de idade,alimentadas com suspensoes poliedros, em concentracoes de 2 x 106 ml, tiveram alta taxa de mortalidade (praticamente 100%), exceto as de 7 dias, na menor dose cuja mortandade foi da ordem de 70%. Exames de extratos das lagartas infectadas mostraram a presenca de grande numero de poliedros, mas a identificacao adequada de que se tratava de um virus de poliedrose nuclear, do tipo "multi-enveloped", (MECPN) so pode ser feita atraves de exames de seccoes ultra finas ao microscopio eletronico: nucleos de ectoderma, tecido adiposo e traqueocitos continham grande numero de poliedros e, ocluidas nestes, particulas baciliformes, em geral varias delas envolvidas por um envoltorio comum.Suplemento

"Enfezamento pálido" e "enfezamento vermelho" na cultura do milho no Brasil Central.

Foi constatada alta incidencia de plantas com sintomas de "Enfezamento Palido" e de "Enfezamento vermelho" em lavouras de milho no Brasil Central. Na mesma area observou-se a presenca de cigarrinha Dalbulus maidis em alta densidade. A presenca de espiroplasma foi confirmada apenas em plantas com sintomas de "Enfezamento Palido", atraves de teste DAS-ELISA. Os resultados da analise por PCR indicaram a presenca de fitoplasma em algumas amostras de plantas com sintomas do "enfezamento Vermelho"