The extended Hubbard model applied to phase diagram and the pressure effects in \Bi superconductors

We use the two dimensional extended Hubbard Hamiltonian with the position of the attractive potential as a variable parameter with a BCS type approach to study the interplay between the superconductor transition temperature $T_c$ and hole content for high temperature superconductors. This novel method gives some insight on the range and intensity of the Cooper pair interaction and why different compounds have different values for their measured coherence lengths and it describes well the experimental results of the superconducting phase diagram $T_c \times n$. The calculations may also be used to study the effect of the applied pressure with the assumption that it increases the attractive potential which is accompanied by an increase in the superconductor gap. In this way we obtain a microscopic interpretation for the intrinsic term and a general expansion for $T_c$ in terms of the pressure which reproduces well the experimental measurements on the \Bi superconductors.Comment: 11 pags in RevTex, 5 fi2s. in eps, accepted in Braz. J. of Physic

Casimir Densities for a Massive Fermionic Quantum Field in a Global Monopole Background with Spherical Boundary

We investigate the vacuum expectation value of the energy-momentum tensor associated with a massive fermionic field obeying the MIT bag boundary condition on a spherical shell in the global monopole spacetime. The asymptotic behavior of the vacuum densities is investigated near the sphere center and surface, and at large distances from the sphere. In the limit of strong gravitational field corresponding to small values of the parameter describing the solid angle deficit in global monopole geometry, the sphere-induced expectation values are exponentially suppressed.Comment: 8 pages, 4 figures, 6th Alexander Friedmann International Seminar on Gravitation and Cosmolog

Vacuum polarization by topological defects in de Sitter spacetime

In this paper we investigate the vacuum polarization effects associated with a massive quantum scalar field in de Sitter spacetime in the presence of gravitational topological defects. Specifically we calculate the vacuum expectation value of the field square, $$. Because this investigation has been developed in a pure de Sitter space, here we are mainly interested on the effects induced by the presence of the defects.Comment: Talk presented at the 1st. Mediterranean Conference on Classical and Quantum Gravity (MCCQG

Electronic Phase Separation Transition as the Origin of the Superconductivity and the Pseudogap Phase of Cuprates

We propose a new phase of matter, an electronic phase separation transition that starts near the upper pseudogap and segregates the holes into high and low density domains. The Cahn-Hilliard approach is used to follow quantitatively this second order transition. The resulting grain boundary potential confines the charge in domains and favors the development of intragrain superconducting amplitudes. The zero resistivity transition arises only when the intergrain Josephson coupling $E_J$ is of the order of the thermal energy and phase locking among the superconducting grains takes place. We show that this approach explains the pseudogap and superconducting phases in a natural way and reproduces some recent scanning tunneling microscopy dataComment: 4 pages and 5 eps fig