Screening effects in superconductors

The partition function of the Hubbard model with local attraction and long range Coulomb repulsion between electrons is written as a functional integral with an action $A$ involving a pairing field $\Delta$ and a local potential $V$. After integration over $V$ and over fluctuations in $|\Delta|^{2}$, the final form of $A$ involves a Josephson coupling between the local phases of $\Delta$ and a "kinetic energy" term, representing the screened Coulomb interaction between charge fluctuations. The competition between Josephson coupling and charging energy allows to understand the relation between $T_{C}$ and composition in high-$T_{C}$ materials, in particular superlattices, alloys and bulk systems of low doping.Comment: 4 pages, revtex, no figures, submitted to Physica B (Proceedings of SCES '96 International Conference, held in Zurich from 19th to 21st of August

Fully Frustrated Cold Atoms

Fully frustrated Josephson Junction arrays (FF-JJA's) exhibit a subtle compound phase transition in which an Ising transition associated with discrete broken translational symmetry and a Berezinskii-Kosterlitz-Thouless (BKT) transition associated with quasi-long-range phase coherence occur nearly simultaneously. In this Letter we discuss a cold atom realization of the FF-JJA system. We demonstrate that both orders can be studied by standard momentum-distribution-function measurements and present numerical results, based on a successful self-consistent spin-wave approximation, that illustrate the expected behavior of observables.Comment: 5 pages, 3 figures, submitte

Utilização de aspectos contábeis voltados para análises de investimentos

Neste trabalho acadêmico, a seguinte questão é investigada: Qual é a importância e como são realizadas as análises de investimentos das empresas? O presente estudo pretende como objetivo geral compreender sobre as demonstrações contábeis e a importância da análise contábil das mesmas. Como objetivos específicos buscou avaliar sobre a contabilidade e as normas da mesma, analisar sobre balanço patrimonial e mais o que o mesmo envolve nas análises contábeis, estudar sobre os índices econômicos e sua importância para análise contábil e por fim, compreender sobre a análise econômico-financeira das demonstrações contábeis. Este estudo classifica-se como uma pesquisa bibliográfica que proporcionará a fundamentação teórica do tema em questão, apresentando visões de diversos autores

Direct angle resolved photoemission spectroscopy and superconductivity of strained high- T c films

Since 1997 we systematically perform direct angle resolved photoemission spectroscopy (ARPES) on in-situ grown thin (<30 nm) cuprate films. Specifically, we probe low-energy electronic structure and properties of high-T c superconductors (HTSC) under different degrees of epitaxial (compressive vs. tensile) strain. In overdoped and underdoped in-plane compressed (the strain is induced by the choice of substrate) ≈15 nm thin La2 − x Sr x CuO4 (LSCO) films we almost double T c to 40 K, from 20 K and 24 K, respectively. Yet the Fermi surface (FS) remains essentially two-dimensional. In contrast, ARPES data under tensile strain exhibit the dispersion that is three-dimensional, yet T c drastically decreases. It seems that the in-plane compressive strain tends to push the apical oxygen far away from the CuO2 plane, enhances the two-dimensional character of the dispersion and increases T c, while the tensile strain acts in the opposite direction and the resulting dispersion is three-dimensional. We have established the shape of the FS for both cases, and all our data are consistent with other ongoing studies, like EXAFS. As the actual lattice of cuprates is like a ‘Napoleon-cake', i.e. rigid CuO2 planes alternating with softer ‘reservoir', that distort differently under strain, our data rule out all oversimplified two-dimensional (rigid lattice) mean field models. The work is still in progress on optimized La-doped Bi-2201 films with enhanced T

Formalización de Relaciones: Un Compás de Espera en el Acontecer Bilateral

Historicamente han existido diversos puntos de contacto entre Cuba y el continente asiatico. Nuestras relaciones tienen mas de 40 anos de existencia y Cuba siempre se ha esforzado por desarrollar vinculos y relaciones estables y prosperas sobre la base de los principios de respeto a la autodeterminacion, soberania e independencia de nuestros pueblos y actualmente mantiene relaciones diplomaticas con mas de 25 paises de la region. En el plano politico, las relaciones bilaterales han estado primadas por el intercambio con los paises socialistas y con los paises miembros del Movimiento No Alineados..

La modelación descriptiva en el ejemplo de los yacimientos lateríticos de Cuba Oriental

El trabajo aporta un instrumento de generalización metodológico inexistente en nuestro país, de aplicación y utilidad práctica para la sistematización de la información sobre los yacimientos minerales de la República de Cuba y la confección de sus modelos descriptivos, de probada eficiencia en la exploración y evaluación de los recursos minerales en otros países de elevado nivel de desarrollo en la rama de Geología Económica vinculada a los yacimientos minerales como son Estados Unidos, Canadá y Australia

Experimental Electronic Structure and Interband Nesting in BaVS_3

The correlated 3d sulphide BaVS_3 is a most interesting compound because of the apparent coexistence of one-dimensional and three-dimensional properties. Our experiments explain this puzzle and shed new light on its electronic structure. High-resolution angle-resolved photoemission measurements in a 4eV wide range below the Fermi level explored the coexistence of weakly correlated a_1g wide-band and strongly correlated e_g narrow-band d-electrons that is responsible for the complicated behavior of this material. The most relevant result is the evidence for a_1g--e_g inter-band nesting condition.Comment: 4 pages, 3 figure

On Phase Transition and Vortex Stability in the Generalized XY Models

We study a recent generalization proposed for the XY model in two and three dimensions. Using both, the continuum limit and discrete lattice, we obtained the vortex configuration and shown that out-of-plane vortex solutions are deeply jeopardized whenever the parameter of generalization, $L$, is increased. The critical temperature for such models is calculated using the self consistent harmonic approximation. In both, two- and three-dimensional cases, such a temperature decreases with raising $L$. Our results are also compared with other approximated methods available in the literature.Comment: 7 pages, 1 figur