Metal-Insulator transitions in generalized Hubbard models

TL;DR

This paper investigates how degeneracy, lattice structure, and frustration influence the Mott transition in Hubbard models, revealing that degeneracy allows transitions at various fillings and frustration raises the critical interaction Uc.

Contribution

It introduces a comprehensive analysis of the Mott transition in degenerate Hubbard models across different lattice geometries, incorporating Monte Carlo simulations and simple hopping arguments.

Findings

Degeneracy enables Mott transitions at any integer filling.

Frustration increases the critical Uc for the transition.

Results explain metallic and insulating behavior in doped Fullerides.

Abstract

We study the Mott transition in Hubbard models with a degenerate band on different 3-dimensional lattices. While for a non-degenerate band only the half-filled system may exhibit a Mott transition, with degeneracy there can be a transition for any integer filling. We analyze the filling dependence of the Mott transition and find that $U_c$ (the Hubbard interaction $U$ at which the transition takes place) decreases away from half-filling. In addition we can change the lattice structure of the model. This allows us to study the influence of frustration on the Mott transition. We find that frustration increases $U_c$, compared to bipartite systems. The results were obtained from fixed-node diffusion Monte Carlo calculations using trial functions which allow us to systematically vary the magnetic character of the system. To gain a qualitative understanding of the results, we have developed simple hopping arguments that help to rationalize the doping dependence and the influence of frustration on the Mott transition. Choosing the model parameters to describe the doped Fullerides, we can make contact with experiment and understand why some of the Fullerides are metals, while others, which according to density functional theory should also be metallic, actually are insulators.