6,326 research outputs found
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 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 . 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 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
Theory of the Fermi Arcs, the Pseudogap, and the Anisotropy in k-space of Cuprate Superconductors
The appearance of the Fermi arcs or gapless regions at the nodes of the Fermi
surface just above the critical temperature is described through
self-consistent calculations in an electronic disordered medium. We develop a
model for cuprate superconductors based on an array of Josephson junctions
formed by grains of inhomogeneous electronic density derived from a phase
separation transition. This approach provides physical insights to the most
important properties of these materials like the pseudogap phase as forming by
the onset of local (intragrain) superconducting amplitudes and the zero
resistivity critical temperature due to phase coherence activated by
Josephson coupling. The formation of the Fermi arcs and the dichotomy in
k-space follows from the direction dependence of the junctions tunneling
current on the d-wave symmetry on the planes. We show that this
semi-phenomenological approach reproduces also the main future of the cuprates
phase diagram.Comment: 5 pages 7 fig
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 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
Effective three-band structure in Fe-based superconductors
We present self-consistent calculations of the multi-gap structure measured
in some Fe-based superconductors. These materials are known to have structural
disorder in real space and a multi-gap structure due to the Fe-orbitals
contributing to a complex Fermi surface topology with hole and electron
pockets. Different experiments identify three s-wave like superconducting gaps
with a single critical temperature (). We investigate the temperature
dependence of these gaps by a multi-band Bogoliubov-deGennes theory at
different pockets in the presence of effective hybridizations between some
bands and an attractive temperature dependent intra-band interaction. We show
that this approach reproduces the three observed gaps and single in
different compounds of BaKFeAs, providing some insights
on the inter-band interactions
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