THERMOELECTRIC-POWER OF HIGH-TC SUPERCONDUCTORS

Thermoelectric-power measurements are reported for four copper oxide superconductor systems: La2-ySryCuO4, Nd1+yBa2-yCu3O6+x, Y0.6Ca0.4Ba1.6-yLa0. 4+yCu3O6+x, and Bi2Sr2-2yLa2yCuO6+x. The Seebeck coefficient ± is interpreted in terms of an electron-diffusion component ±e that varies from the small-polaron limit for the antiferromagnetic compositions to the metallic limit for the normal-metal compositions. It is argued that the width of the conduction band increases exponentially with oxidation of the CuO2 sheets. At larger hole doping, the correlation splitting in the superconductor compositions becomes small enough to require the introduction of a two-band model for ±e. In those systems in which holes are trapped from the CuO2 sheets into the inactive intergrowth layers, the temperature dependence of ± develops a characteristic maximum at a temperature Tm. From a T-1 dependence for T>Tm, a trapping energy is extracted; this energy increases linearly with the formal charges of the intergrowth layers. The fall in ± with decreasing temperature for T<Tm is interpreted to reflect the consequences of a freezing out of the displacements of the c-axis oxygen that are associated with charge transfer between the active and inactive superconductor layers. © 1990 The American Physical Society.