Ferromagnetism, paramagnetism and a Curie-Weiss metal in an electron doped Hubbard model on a triangular lattice

TL;DR

This study uses dynamical mean-field theory to explore the magnetic and electronic phases of a doped Hubbard model on a triangular lattice, revealing ferromagnetism, Curie-Weiss behavior, and charge ordering phenomena relevant to NaxCoO2.

Contribution

It demonstrates how the sign of hopping amplitude influences magnetic properties and introduces a model explaining the Curie-Weiss metal phase in NaxCoO2.

Findings

Ferromagnetism and Curie-Weiss susceptibility for t>0

Nagaoka ferromagnetism consistent with experiments

Charge ordering near Mott insulating phase

Abstract

Motivated by the unconventional properties and rich phase diagram of NaxCoO2 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, ferromagnetism and a Curie-Weiss magnetic susceptibility are found in a broad electron doping range. Our observation of Nagaoka ferromagnetism is consistent with the A-type antiferromagnetism (i.e. ferromagnetic layers stacked antiferromagnetically) observed in neutron scattering experiments on NaxCoO2. We propose that `Curie-Weiss metal' phase observed in NaxCoO2 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. We propose a model which contains the charge ordering phenomena observed in the system which, we propose, drives the system close to the Mott insulating phase even at large dopings.