Thermoelectric power in single-layer copper oxides.

Abstract

The temperature variation of the thermoelectric power has been measured and used as a diagnostic tool to determine the character of the charge carriers in the normal state of the superconductive copper oxides and in the overdoped compositions. We used copper oxides with single ${\mathrm{CuO}}_{2}$ sheets separated by nonsuperconductive layers that are not charge reservoirs in order to simplify interpretation of the data. Both the superconductive and the overdoped samples are characterized by a ``hump'' in the variation of the Seebeck coefficient \ensuremath{\alpha} with temperature T that is superposed on a weakly temperature-dependent background term ${\mathrm{\ensuremath{\alpha}}}_{0}$ that decreases systematically with increased doping in the superconductive phase to a small, doping-independent value in the overdoped samples. Perturbation of the periodic potential in the ${\mathrm{CuO}}_{2}$ sheets and increasing the separation between the sheets suppresses both the hump in \ensuremath{\alpha} versus T and superconductivity. The maximum in the hump occurs at ${\mathit{T}}_{\mathrm{max}}$\ensuremath{\approxeq}140 K, which requires a characteristic energy for any enhancement mechanism that is higher than can be provided by acoustic phonons. It is difficult for theoretical models of the normal state that are based only on electron-electron and/or conventional electron-phonon interactions to explain these data. However, we can interpret the hump with a mass-enhancement phenomenon that utilizes vibronic coupling of electrons to optical-mode lattice vibrations. From a first-principles analysis of the thermopower data, we distinguish two closely related types of the itinerant-particle state: one is applicable to the normal state of the copper-oxide superconductor compositions in the temperature range ${\mathit{T}}_{\mathit{c}}$T300 K and the other to the overdoped phase.