Thermoelectric power of high-Tcsuperconductors

Abstract

Thermoelectric-power measurements are reported for four copper oxide superconductor systems: ${\mathrm{La}}_{2\mathrm{-}\mathit{y}}$ ${\mathrm{Sr}}_{\mathit{y}}$ ${\mathrm{CuO}}_4$, ${\mathrm{Nd}}_{1+\mathit{y}}$ ${\mathrm{Ba}}_{2\mathrm{-}\mathit{y}}$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$, ${\mathrm{Y}}_{0.6}$ ${\mathrm{Ca}}_{0.4}$ ${\mathrm{Ba}}_{1.6\mathrm{-}\mathit{y}}$ ${\mathrm{La}}_{0.4+\mathit{y}}$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$, and ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_{2\mathrm{-}2\mathit{y}}$ ${\mathrm{La}}_{2\mathit{y}}$ ${\mathrm{CuO}}_{6+\mathit{x}}$. The Seebeck coefficient α is interpreted in terms of an electron-diffusion component ${\mathrm{\alpha }}^{\mathit{e}}$ 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 ${\mathrm{CuO}}_2$ 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 ${\mathrm{\alpha }}^{\mathit{e}}$. In those systems in which holes are trapped from the ${\mathrm{CuO}}_2$ sheets into the ‘‘inactive’’ intergrowth layers, the temperature dependence of α develops a characteristic maximum at a temperature ${\mathit{T}}_{\mathit{m}}$. From a ${\mathit{T}}^{\mathrm{-}1}$ dependence for T>${\mathit{T}}_{\mathit{m}}$, a trapping energy is extracted; this energy increases linearly with the formal charges of the intergrowth layers. The fall in α with decreasing temperature for T<${\mathit{T}}_{\mathit{m}}$ is interpreted to reflect the consequences of a freezing out of the displacements of the...