SYSTEMATICS IN THE THERMOPOWER BEHAVIOR OF SEVERAL SERIES OF BISMUTH AND THALLIUM CUPRATE SUPERCONDUCTORS - AN INTERPRETATION OF THE TEMPERATURE-VARIATION AND THE SIGN OF THE THERMOPOWER

The thermopower, α, of many members of the Bi2Ca1-xYxSr2Cu2O8+δ, Bi2-yPbyCaSr2Cu2O8 +δ, TlyCa1-xYxBa2Cu2O6+δ, TlCa1-xLnxSr2Cu2O6+δ and TlSr2-xLaxCuO5+δ systems is positive down to Tc, but is negat ive in some of them. The slope of the α-T plots above Tc is generally negative independent of the sign of α. The value of α extrapolated to 0 K, α(0), from the high-tem perature regime is finite and generally, positive. Compared to the behaviour of YBa2Cu3O7, the data on the Bi and Tl cuprate systems examined here seem to suggest that electrons may be involved in the conduction process as well, at least in systems where both the α and the α-T slope are negative. Of the different systems examined here, Tl1-xPbxCaSr2Cu2O6+δ contai ning Pb in the 4+state, deserves special mention since it shows negative α over the entire temperature range in addition to a negative slope suggesting that this is likely to be a genuine high-Tc electron-superconductor. The thermopower of cuprates has been discussed in some detail in terms of entropic and quasiparticle contributions. © 1990.