EXPERIMENTAL PROGRESS IN UNDERSTANDING HIGH-TEMPERATURE SUPERCONDUCTIVITY

Abstract

In this paper first the structure of the newest layered compounds is presented, which do not contain rare-earth ions, instead either Bi- or Tl-oxide layers are sandwiched between the copper-oxide layers. The latter consist of either corner-connected octahedra, pyramids or planar squares: the transition temperature T/sub c/ increases with the density of copper-oxide layers per unit cell and is accounted for by single-electron interlayer tunneling. The Bi- or Tl-oxide layers can either be double or single, with the former having the higher T/sub c/. Of considerable scientific interest is the discovery of a cubic perovskite with a high T/sub c/ of 29.6 {Kappa} which contains no copper. Ba/sub 1-x/K/sub x/BiO/sub 3/. Owing to its doping with K/sup +/ for Ba/sup 2+/, holes must again be present. If they are on the oxygens, this would be a common feature with the cuprates. This paper is aimed at determining the superconducting gap {Delta} and its anisotropy. Some measurements of infrared reflectivity on single crystals and nuclear magnetic resonance in powders indicate that the ratio is 2{Delta}/{kappa}T/sub c/ {ge} 7. Infrared data supporting this ratio in BaY/sub 2/Cu/sub 3/O/sub 7/ have now been extended. Others however yield values near the BCS weak couplingmore » limit of 3.5. Determination of this ratio is important, because it indicates whether the coupling is weak or strong. A Kramer-Kronig analysis of IR data in the normal-conducting regime supports intermediately strong coupling, but weaker than in many fermion systems. It is also pointed out that the tunneling experiments yield gaps which are somewhat too low because {Delta} is depressed at the surface owing to the intrinsic short coherence lengths.« less