Quantum Phase Transitions in the Hubbard Model on Triangular Lattice

TL;DR

This study uses the path-integral renormalization group method to analyze quantum phase transitions in the half-filled Hubbard model on a triangular lattice, revealing a sequence of phase changes including a nonmagnetic insulator phase.

Contribution

It introduces a new iteration scheme for PIRG and provides evidence for an intermediate nonmagnetic insulator phase in the Hubbard model on a triangular lattice.

Findings

First-order transition from metal to NMI at Uc1~7.7t

Second-order transition from NMI to magnetic order at Uc2~10t

Location of real material in the NMI phase near transition

Abstract

We investigate the quantum phase transitions in the half-filled Hubbard model on the triangular lattice by means of the path-integral renormalization group (PIRG) method with a new iteration scheme proposed recently. It is found that as the Hubbard interaction $U$ increases, the paramagnetic metallic state undergoes a first-order phase transition to a nonmagnetic insulating (NMI) state at $U_{c1}\sim 7.7t$, which is followed by another first-order transition to a $120^\circ$ N\'eel ordered state at $U_{c2}\sim 10t$, where $t$ is the transfer integral. Our results ensure the existence of the intermediate NMI phase, and resolve some controversial arguments on the nature of the previously proposed quantum phase transitions. We find that $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ is located in the NMI phase close to the metal-insulator transition point.