Hole filling in a narrow conduction band in YBa2Cu3-xCoxO7- delta (x

Abstract

Measurements of the temperature dependence of the absolute thermopower (S) on samples of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ with 0\ensuremath{\le}x\ensuremath{\le}0.3 and \ensuremath{\delta}0.05 show that, between 100--200 K, S has a maximum that depends on x, according to the relation ${\mathit{S}}_{\mathit{m}}$(x)\ensuremath{\approxeq}(${\mathit{k}}_{\mathit{B}}$/\ensuremath{\Vert}e\ensuremath{\Vert}) ln[(1+x)/(1-x)]. This is the expected saturation value of S, for noninteracting carriers in a narrow conduction band with x doped electrons above half-filling. The temperature dependence of the resistivity (\ensuremath{\rho}) exhibits a crossover from wide-band to narrow-band behavior, in a conductor with a mean free path limited by impurities. The drop of S at higher temperatures, accompanied by a drop in d\ensuremath{\rho}/dT, can be attributed to excitation of electrons from the conduction band to higher-energy levels associated with the dopant, or to the contribution of activated hopping to the transport coefficients.