Systematics in the thermopower behaviour of several series of bismuth and thallium cuprate superconductors: an interpretation of the temperature variation and the sign of the thermopower

Abstract

The thermopower, α, of many members of the Bi₂Ca_1−xY_xSr₂Cu₂O_8+δ, Bi_2−yPb_yCaSr₂Cu₂O_8+δ, Tl_yCa_1−xY_xBa₂Cu₂O_6+δ, TlCa_1−xLn_xSr₂Cu₂O_6+δ and TlSr_2−xLa_xCuO_5+δ systems is positive down to T_c, but is negative in some of them. The slope of the α-T plots above T_c is generally negative independent of the sign of α. The value of α extrapolated to 0 K, α(0), from the high-temperature 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, Tl_1−xPb_xCaSr₂Cu₂O_6+δ containing 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-T_c electron-superconductor. The thermopower of cuprates has been discussed in some detail in terms of entropic and quasiparticle contributions.