Mott transition from the non-analyticity of the one-body reduced density-matrix functional

TL;DR

This paper develops a new approach within one-body reduced density-matrix functional theory to explicitly model the Mott transition in the Hubbard model, revealing non-analytic behavior linked to the transition.

Contribution

It introduces a variational ansatz-based 1RDMF for the multi-orbital Hubbard model that captures Mott physics and the nature of the transition, including its order.

Findings

Non-analytic behavior in 1RDMF at integer filling indicates Mott transition.

Nonzero Hund exchange influences the transition's order, making it first-order.

The approach accurately models Mott and Hund physics in strongly correlated systems.

Abstract

One-body reduced density-matrix functional (1RDMF) theory has yielded promising results for small systems such as molecules, but has not addressed quantum phase transitions such as the Mott transition. Here we explicitly execute the constrained search within a variational ansatz to construct a 1RDMF for the multi-orbital Hubbard model with up to seven orbitals in the thermodynamic limit. The variational ansatz is the \mathcal{N}=3 ansatz of the variational discrete action theory (VDAT), which can be exactly evaluated in d=\infty. The resulting 1RDMF exactly encapsulates the \mathcal{N}=3 VDAT results, which accurately captures Mott and Hund physics. We find that non-analytic behavior emerges in our 1RDMF at fixed integer filling, which gives rise to the Mott transition. We explain this behavior by separating the constrained search into multiple stages, illustrating how a nonzero Hund exchange drives the continuous Mott transition to become first-order. Our approach creates a new path forward for constructing an accurate 1RDMF for strongly correlated electron materials.