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

Abstract

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 ͑DMFT͒ calculations predict that for negative intersite hopping amplitudes ͑t Ͻ 0͒ 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 Ͼ 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 different behavior encountered is a consequence of the larger noninteracting density of states ͑DOS͒ at the Fermi level for t Ͼ 0 than for t Ͻ 0, 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 t Ͼ 0 the energy cost of polarizing the system is much smaller than for t Ͻ 0. Our observation of Nagaoka ferromagnetism is consistent with the A-type antiferromagnetism ͑i.e., ferromagnetic layers stacked antiferromagnetically͒ observed in neutron scattering experiments on Na x CoO 2 . 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 a "bad metal" with incoherent quasiparticles at temperatures T Ͼ 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 also shows that the one band Hubbard model on a triangular lattice is not enough to describe the unusual properties of Na x CoO 2 and is used to identify the simplest relevant model that captures the essential physics in Na x CoO 2 . 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.

Ionic Hubbard model on a triangular lattice forNa0.5CoO2,Rb0.5Powell1, Merino2, McKenzie3 2009Phys. Rev. BSelf Cite9020View full textAdd to dashboardCite

“…Above 51 K, it is weakly temperature dependent with values of a few m⍀ cm, 4 characteristic of a "bad" or incoherent metal which does not have well-defined quasiparticle excitations. 2 Below 51 K, the resistivity increases significantly consistent with an activated form with energy gap of about 10 meV. 4 Although, it is usually stated that the transition at 51 K is a "metal-insulator" transition, we stress that it is really above 51 K Na 0.5 CoO 2 is a bad metal and below 51 K the experimental evidence is not all consistent with the claim that Na 0.5 CoO 2 is an insulator ͑see Sec.…”

“…Thus an effective Hamiltonian for A x CoO 2 is the ionic Hubbard model. 2 In addition to the interest in A x CoO 2 the ionic Hubbard model is of significant interest in its own right. The ionic Hubbard model on half-filled bipartite lattices has attracted interest because it undergoes a transition from a Mott insulator to a band insulator.…”

“…Therefore, for simplicity and definiteness, we specialize to the Hamiltonian relevant to x = 0.5. This model has two sublattices with different site energies, ⑀ i = ⌬ / 2 for A sites and ⑀ i =−⌬ / 2 for B sites, 2 and with the sublattices arranged in stripes as shown in Fig. 1͑b͒.…”

“…2 In principle, each of these different ͕⑀ i ͖ corresponds to a different Hamiltonian. Therefore, for simplicity and definiteness, we specialize to the Hamiltonian relevant to x = 0.5.…”

“…Further, many of these features seem to have a natural explanation in terms of the ionic Hubbard model. 2,23,24 We have previously studied the quarter-filled ionic Hubbard model with a stripe potential by exactly diagonalizing small clusters. 23,24 We found a complicated interplay between the charge and spin degrees of freedom and between insulating and metallic states.…”