Interlayer coupling and the thermopower of cuprate superconductors

Abstract

A review of the extant literature on the thermopower (TEP) of the cuprate superconductors reveals a transition from polaronic to homogeneous itinerant electron behavior with increasing hole concentration in the CuO2 sheets. Superconductivity appears at the compositions of crossover from one regime to the other, and the normal state of the superconductors retains a heterogeneous electronic structure of mobile hole-rich and hole-poor domains, which introduces both a statistical and a transport component to the Seebeck coefficient. A primary manifestation of the transport component is an enhancement term δα(T) having a maximum at a Tmax=100–140 K, which is characteristic of a strong coupling of the charge carriers to optical rather than acoustic phonons. Comparison of the TEP for La1.85Sr0.15CuO4 vs La0.85Sr1.15GaCuO5 and La1.8Sr0.2CaCu2O6+δ vs Bi2Sr2CaCu2O8+δ indicates that an increase in the c-axis coupling between neighboring CuO2 sheets increases the size of the polaron or hole-rich domains in the underdoped compositions, thereby lowering the room-temperature value of the Seebeck coefficient α(300 K) and increasing the magnitude of δα(T). These observations implicate a strong elastic component, enhanced by electron-lattice interactions, in the c-axis coupling as well as in the formation of larger nonadiabatic polarons and their interactions within the CuO2 planes. They also signal that caution must be exercised in any application of a ‘‘universal plot’’ of α(300 K) versus hole concentration p per Cu atom in a CuO2 plane to obtain a value of p from TEP data. © 1996 The American Physical Society.

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