In-plane thermoelectric power of untwinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We present the first measurements of the temperature and oxygen-doping dependences of thermoelectric power in the ab plane of untwinned ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (δ≊0.10) for T≤330 K. Employing a two-band model and measured electrical resistivities, we derive the chain and plane components of the thermopower, ${\mathit{S}}_{\mathrm{ch}}$ and ${\mathit{S}}_{\mathrm{pl}}$. The behavior of ${\mathit{S}}_{\mathrm{ch}}$ indicates that some of the chain conduction occurs via carriers in a narrow band. ${\mathit{S}}_{\mathrm{pl}}$(T) is similar to the in-plane thermopower of the other cuprates, suggesting a ‘‘universal’’ behavior for the ${\mathrm{CuO}}_2$ planes. The results are discussed with reference to the known Fermi surface.