Thermoelectric effects in a strongly correlated model for Na_xCoO₂

Abstract

Thermal response functions of strongly correlated electron systems are of appreciable interest to the larger scientific community both theoretically and technologically. Here, we focus on the infinitely correlated t-J model on a geometrically frustrated two-dimensional triangular lattice. Using exact diagonalization on a finite sized system, we calculate the dynamical thermal response functions in order to determine the thermopower, Lorenz number, and dimensionless figure of merit. The dynamical thermal response functions are compared to the infinite frequency limit and shown to be very weak functions of frequency, hence, establishing the validity of the high frequency formalism recently proposed by Shastry [Phys. Rev. B 73, 085117 (2006)] for the thermopower, Lorenz number, and the dimensionless figure of merit. Further, the thermopower is demonstrated to have a low to intermediate temperature enhancement when the sign of the hopping parameter t is switched from positive to negative for the geometrically frustrated lattice [A. P. Ramirez, in More Is Different, edited by N. P. Org and R. N. Bhatt (Princeton University Press, New Jersey, 2001), p. 255] considered.