Ferromagnetism, paramagnetism and a Curie-Weiss metal in NaxCoO2

Motivated by the unconventional properties and rich phase diagram of Na$_x$CoO$_2$ we consider the electronic and magnetic properties of a two-dimensional Hubbard model on an isotropic triangular lattice doped with electrons away from half-filling. Dynamical mean-field theory (DMFT) calculations predict that for negative inter-site hopping amplitudes ($t<0$) and an on-site Coulomb repulsion, $U$, comparable to the bandwidth, the system displays properties typical of a weakly correlated metal. In contrast, for $t>0$ a large enhancement of the effective mass, itinerant ferromagnetism and a metallic phase with a Curie-Weiss magnetic susceptibility are found in a broad electron doping range. The transport and magnetic properties measured in Na$_x$CoO$_2$ are consistent with DMFT predictions of a metal close to the Mott insulator and we discuss the role of Na ordering in driving the system towards the Mott transition. We propose that the Curie-Weiss metal phase observed in Na$_x$CoO$_2$ is a consequence of the crossover from ``bad metal'' with incoherent quasiparticles at temperatures T$>$T$^*$ and Fermi liquid behavior with enhanced parameters below T$^*$, where T$^*$ is a low energy coherence scale induced by strong local Coulomb electron correlations. Our analysis shows that the one band Hubbard model on a triangular lattice is not enough to describe the unusual properties of Na$_x$CoO$_2$.