Certain novel aspects of thallium and bismuth cuprate superconductors

Novel superconducting thallium cuprates of the type TlCa<SUB>1−X</SUB>Ln<SUB>X</SUB>Sr<SUB>2</SUB>Cu<SUB>2</SUB>O<SUB>6+δ</SUB> (Ln=Y or rare earth), TlSr<SUB>n+1−x</SUB>Ln<SUB>x</SUB>Cu<SUB>n</SUB>O<SUB>Y</SUB> and Tl<SUB>1−x</SUB>Pb<SUB>X</SUB>Sr<SUB>n+1</SUB>Cu<SUB>n</SUB>0<SUB>8+δ</SUB> are described. These cuprates as well as Bi<SUB>2</SUB>Ca<SUB>1−x</SUB>Ln<SUB>x</SUB>Sr<SUB>2</SUB>Cu<SUB>2</SUB>O<SUB>8+δ</SUB> and TlCa<SUB>1−x</SUB>Y<SUB>x</SUB>Ba<SUB>2</SUB>Cu<SUB>2</SUB>O<SUB>6+δ</SUB> show maximum T around a specific composition or oxygen content. They also show interesting changes in the sign and magnitude of the thermopower with the composition. Specially noteworthy is the negative slope of the thermopower-temperature plots. The thermopower behaviour in these two-band systems can be understood in terms of entropie and quasiparticle contributions. It appears that Tl<SUB>1−x</SUB>Pb<SUB>x</SUB>CaSr<SUB>2</SUB>Cu<SUB>2</SUB>O<SUB>6+δ</SUB> is a genuine high T electron superconductor