Hubbard model on a triangular lattice at finite temperatures

TL;DR

This study uses the strong coupling diagram technique to identify three finite-temperature phases of the half-filled Hubbard model on a triangular lattice, revealing temperature-dependent spin excitations and phase transitions.

Contribution

It introduces a finite-temperature phase diagram for the Hubbard model on a triangular lattice, highlighting the emergence of spin excitations and bound states not captured at zero temperature.

Findings

Identifies three phases: metal, Mott insulator, and an intermediate phase with Fermi-level peak.

Shows the Mott gap is filled by a Fermi-level peak at finite temperatures.

Links spin excitations to the Pomeranchuk effect at finite temperatures.

Abstract

Using the strong coupling diagram technique, we find three phases of the half-filled isotropic Hubbard model on a triangular lattice at finite temperatures. The weak-interaction ($U\lesssim5t$) and strong-interaction ($U\gtrsim9t$) phases are similar to those obtained by zero-temperature methods -- the former is a metal without perceptible spin excitations; the latter is a Mott insulator with the 120$^\circ$ short-range spin ordering. Zero-temperature approaches predict a nonmagnetic insulating spin-liquid phase sandwiched between these two regions. In our finite-temperature calculations, the Mott gap in the intermediate phase is filled by the Fermi-level peak, which is a manifestation of the bound states of electrons with pronounced spin excitations. We relate the appearance of these excitations at finite temperatures to the Pomeranchuk effect.