Thermoelectric power of YBa2Cu3O7−δ: Phonon drag and multiband conduction

Abstract

We report measurements of the a-b-plane thermopower (S) on crystalline ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (δ≤0.16) for temperatures below 310 K. Much sharper features in the T dependence of S are observed than have been previously reported. Both the temperature and oxygen-doping dependences of S, which have caused confusion, are consistently accounted for by conventional metallic theory for the first time. Strong phonon drag and two carrier species are indicated, with holes on the planes and electrons on the chains. An anomalous, positive component to S, observed for T<160 K, is attributed to a freeze-out of carrier-phonon umklapp processes involving holes in the ${\mathrm{CuO}}_2$ planes and optical-mode phonons.