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

The thermopower, $\alpha$, of many members of the $Bi_2Ca_{1-x}Y_xSr_2Cu_2O_{8+\delta}$, $Bi_{2-y}Pb_yCaSr_2Cu_2O_{8+\delta}$, $Tl_yCa_{1-x}Y_xBa_2Cu_2O_{6+\delta}$ $TlCa_{1-x}Ln_xSr_2Cu_2O_{6+\delta}$ and $TlSr_{2-x}La_xCuO_{5+\delta}$ systems is positive down to $T_c$, but is negative in some of them. The slope of the $\alpha-T$ plots above $T_c$ is generally negative independent of the sign of $\alpha$. The value of $\alpha$ extrapolated to 0 K, $\alpha(0)$, from the high-tem perature regime is finite and generally, positive. Compared to the behaviour of $YBa_2Cu_3O_7$, 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 $\alpha$ and the $\alpha-T$ slope are negative. Of the different systems examined here, $Tl_{1-x}Pb_xCaSr_2Cu_2O_{6+\delta}$ containing $Pb$ in the 4+state, deserves special mention since it shows negative $\alpha$ 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