Strong correlations produce the Curie-Weiss phase of Na_xCoO₂

Abstract

Within the t-J model we study several experimentally accessible properties of the 2D-triangular lattice system Na_xCoO₂, using a numerically exact canonical ensemble study of 12 to 18 site triangular toroidal clusters as well as the icosahedron. Focusing on the doping regime of x~0.7, we study the temperature dependent specific heat, magnetic susceptibility, and the dynamic Hall coefficient R_H(T,ω) as well as the magnetic field dependent thermopower. We find a crossover between two phases near x~0.75 in susceptibility and field suppression of the thermopower arising from strong correlations. An interesting connection is found between the temperature dependence of the diamagnetic susceptibility and the Hall coefficient. We predict a large thermopower enhancement, arising from transport corrections to the Heikes-Mott formula, in a model situation where the sign of hopping is reversed from that applicable to Na_xCoO₂.