Anomalous high-temperature Hall effect on the triangular lattice in Na_xCoO₂

The Hall coefficient R_H of Na_xCoO_2 (x = 0.68) behaves anomalously at high temperatures. From 200 to 500 K, R_H increases linearly with T to 8 times the expected Drude value, with no sign of saturation. Together with the thermopower Q, the behavior of R_H provides firm evidence for strong correlation. We discuss the effect of hopping on a triangular lattice and compare R_H with a recent prediction by Kumar and Shastry.