Abstract
It is suggested, following many other investigators, that, both in the new high temperature superconductors and the cubic materials such as Ba1-xKxBiO3 (Tc=34 K), the phenomenon is caused by real space bosons, the phonons playing no major role. In the latter, Bi3+ moving through heavily screened Bi5+ are the bosons; Tc is the temperature at which the boson gas becomes non-degenerate, so bosons carry the current above Tc. In the quasi-two dimensional copper oxide materials it is maintained, following previous papers, that spin bipolarons are formed, but there is now strong evidence that only a fraction of the carriers (oxygen 2p holes hybridised with Cu3d8) form bosons, and that both above and below Tc a condensed gas of fermions exists. The fermions are heavy; a new description of the role of the copper moments is given. The fermions appear not to be responsible for the conductivity above Tc, being probably localized by disorder, so here again the current is carried by bosons.