Multi-Legged Robot Control Using GA-Based Q-Learning Method With Neighboring Crossover
Tadahiko Murata, Masatoshi Yamaguchi · 2008
According to the type of charge carriers, superconductors can be classified in two types: ntype superconductors, when the charge carriers are Cooper pairs of electrons and p-type superconductors, when the charge carriers are Cooper pairs of holes.We know that BCS theory (Bardeen et al., 1957) explains the microscopic mechanisms of superconductivity in metals.These materials are clearly n-type superconductors.According to BCS theory, electrons in a metallic superconductor are paired by exchanging phonons.Microscopic mechanisms in some types of non-metallic superconductors, like MgB 2 (Nagamatsu et al., 2001), probably may be explained by BCS theory.However, according to many researchers (De Jongh, 1988;Emin, 1991;Hirsch, 1991;Ranninger, 1994), BCS theory is not appropriate to be applied to explain the mechanisms of superconductivity in oxide superconductors.Nevertheless, other models relying on a BCS-like picture replace the phonons by another bosons, such as: plasmons, excitons and magnons, as the mediators causing the attractive interaction between a pair of electrons and many authors claim that superconductivity in the oxide superconductors can be explained by the conventional BCS theory or BCS-like theories (Canright & Vignale, 1989;Prelovsek, 1988;Tachiki & Takahashi, 1988;Takada, 1993).In this chapter we discuss this controversy.That is, we discus the microscopic mechanisms to explain the condensation of the superconductor state of oxide superconductors.This discussion may be useful to study all types of oxide superconductors, that is, oxide superconductors containing copper, as well as oxide superconductors that do not contain copper.However, the main objective of this chapter is to discuss the role of double valence fluctuations in p-type oxide superconductors.In a previous work (Luiz, 2008) we have suggested a simple phenomenological model useful to calculate the optimal doping of p-type high-T c oxide superconductors.In this chapter we study possible microscopic mechanisms in high-T c superconductors in order to give theoretical support for that simple model.