Abstract
Motivated by the unconventional properties and rich phase diagram of 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 intersite hopping amplitudes and an on-site Coulomb repulsion, , comparable to the bandwidth, the system displays properties typical of a weakly correlated metal. In contrast, for 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 different behavior encountered is a consequence of the larger noninteracting density of states (DOS) at the Fermi level for than for , which effectively enhances the mass and the scattering amplitude of the quasiparticles. The shape of the DOS is crucial for the occurrence of ferromagnetism as for the energy cost of polarizing the system is much smaller than for . Our observation of Nagaoka ferromagnetism is consistent with the A-type antiferromagnetism (i.e., ferromagnetic layers stacked antiferromagnetically) observed in neutron scattering experiments on . The transport and magnetic properties measured in 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 is a consequence of the crossover from a “bad metal” with incoherent quasiparticles at temperatures and Fermi liquid behavior with enhanced parameters below , where is a low energy coherence scale induced by strong local Coulomb electron correlations. Our analysis also shows that the one band Hubbard model on a triangular lattice is not enough to describe the unusual properties of and is used to identify the simplest relevant model that captures the essential physics in . We propose a model which allows for the Na ordering phenomena observed in the system which, we propose, drives the system close to the Mott insulating phase even at large dopings.
8 More- Received 4 January 2006
DOI:https://doi.org/10.1103/PhysRevB.73.235107
©2006 American Physical Society