Strong Correlations Produce the Curie-Weiss Phase of Na$_{x}$CoO$_2$

TL;DR

This study uses the t-J model to analyze strongly correlated electronic properties of Na$_x$CoO$_2$, revealing phase crossover behavior and predicting thermopower enhancement due to transport effects.

Contribution

It provides the first detailed numerical analysis of various experimentally relevant properties of Na$_x$CoO$_2$ within the t-J model, highlighting phase crossover and thermopower enhancement.

Findings

Identifies a crossover near x~0.75 in susceptibility and thermopower.

Finds a connection between diamagnetic susceptibility and Hall coefficient.

Predicts large thermopower enhancement with reversed hopping sign.

Abstract

Within the t-J model we study several experimentally accessible properties of the 2D-triangular lattice system Na$_x$CoO$_2$, 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 $x\sim0.7$, we study the temperature dependent chemical potential, specific heat, magnetic susceptibility and the dynamic Hall coefficient $R_H(T,\omega)$ as well as the magnetic field dependent thermopower. We find a crossover between two phases near $x \sim 0.75$ 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 {\em transport corrections} to the Heikes-Mott formula, in a model situation where the sign of hopping is reversed from that applicable to Na$_x$CoO$_2$.