Properties of high TC superconductors

Abstract Gaining a basic description of the high Tc superconductors is a foremost challenge in physics today. Some insights are being obtained by studying a variety of properties in many laboratories. The most important property is the occurrence of superconductivity itself at such high temperatures, and how it depends upon chemical and structural parameters. There have been all sorts of pairing mechanisms and structural features singled out as the cause of the high Tc's. During the past few months, with the discovery of superconductivity above 100K in a series of Bi(1) and Tl(2) containing copper-oxide-layered structures, it has been possible to settle some issues: The new series may be written, following the notation of Torardi et al(3) as: (AIIIO)2 A2II Can−1 Cun O2+2n, where AIII is Bi or Tl, AII is Ba or Sr and n is the number of consecutive CuO sheets which are separated only by sheets of Ca ions. The fact that the superconductivity occurs in systems with no linear CuO chains eliminates a significant number of theoretical models in which the superconductivity was attributed to linear CuO chains.(4) On the other hand, up to now, all the superconductors with Tc's above LN2 temperatures are known to have the 2-dimensional buckled sheets of CuO. Therefore, most theories, and from now on unless experiment produces a counter-example, all reasonable theories will start with 2d sheets of CuO. What the new data demonstrate is that n Cu O sheets are better than one so that coupling between the sheets enhances the Tc to a certain extent. Other features of the new systems are also helpful in eliminating false trails. For example the ubiquitous twinning of the planes which occurs on fine scale even in single crystals of YBa2Cu3O7 does not occur in the Bi and Tl-containing series. This eliminates another class of theory in which the twin-planes were supposed to play a crucial role. The twin-planes have also been considered as essential ingredients for pinning, however, large critical currents have already been observed in the Tl and Bi compounds. Up to the present most investigations, have been carried out on La Sr Cu O4 and (RE) Ba2 Cu3 O7. RE can be any rare earth (except Ce, Pr, and Tb) without effecting Tc. The investigations are hampered by the difficulty of obtaining perfect enough samples. Even though the preparation has improved substantially in the past few months so that many investigations have been made on single crystals and/or well-oriented thin films with sharp superconducting transitions, as measured by resistivity, it is difficult to establish whether the properties being observed are intrinsic or are due to subtle effects of defects. Short coherence distances in the superconducting state suggest that small scale defects can be more important than in conventional superconductors. The extreme sensitivity to oxygen ordering and vacancies even for the normal state properties is in strong contrast with robust superconducting properties of three dimensional conventional superconductors such as NbN. It may be that the materials science of defect oxides will emerge from its dark ages much as that of germanium and silicon did in the early transistor days and as new techniques--the equivalent of zone-refining--are discovered. For the present, the field is a challenge to the combined efforts of materials scientists, experimentalists and theorists.