Anisotropy and magnetism of high temperature oxides superconductors

Abstract

Phonon or electron mediated weak BCS attraction is enough to have high critical temperature if a van Hove anomaly is at work. This could apply to electron doped compounds and also to compounds with CuO$_2$ planes overdoped in holes, where $T_c$ decreases with increasing doping. If phonons dominate, it should lead to an anisotropic but mainly $s$ superconductive gap, as observed recently in overdoped LaSrCuO, and probably also in electron doped compounds. If electrons dominate, a $d$ gap should develop as observed in a number of cases. In the underdoped range, the observed decrease of $T_c$ with hole doping can be related in all cases to the development of antiferromagnetic fluctuations which produces a magnetic pseudogap, thus lowering the density of states at the Fermi level. The observed mainly $d$ superconductive gap then can be due to a prevalent superconductive coupling through antiferromagnetic fluctuations; it could also possibly be attributed to the same phonon coupling as in the overdoped range, now acting on Bloch functions scattered in the magnetic pseudogap. More systematic studies of superconductive gap anisotropy and of magnetic fluctuations would be in order.