Hole filling in a narrow conduction band inYBa2Cu3−xCoxO7−δ(x≤0.3)

Measurements of the temperature dependence of the absolute thermopower (S) on samples of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_{3\mathrm{-}\mathit{x}}$ ${\mathrm{Co}}_{\mathit{x}}$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ with 0≤x≤0.3 and δS has a maximum that depends on x, according to the relation ${\mathit{S}}_{\mathit{m}}$(x)≊(${\mathit{k}}_{\mathit{B}}$/‖e‖) 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 (ρ) 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 dρ/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.