Band insulator to Mott insulator transition in 1T-TaS$_2$

TL;DR

This study reveals that 1T-TaS$_2$ transitions from a band insulator with interlayer dimerization at low temperatures to a Mott insulator at higher temperatures, clarifying its electronic phase behavior.

Contribution

The paper provides direct experimental evidence distinguishing the band insulating and Mott insulating states in 1T-TaS$_2$, highlighting the temperature-dependent transition and the underlying electronic mechanisms.

Findings

Low-temperature ground state is a band insulator with interlayer dimerization.

A narrow temperature window exhibits a Mott insulating state.

Results challenge the focus on quantum magnetism at low temperatures in 1T-TaS$_2$.

Abstract

1T-TaS$_2$ undergoes successive phase transitions upon cooling and eventually enters an insulating state of mysterious origin. Some consider this state to be a band insulator with interlayer stacking order, yet others attribute it to Mott physics that support a quantum spin liquid state.Here, we determine the electronic and structural properties of 1T-TaS$_2$ using angle-resolved photoemission spectroscopy and X-Ray diffraction. At low temperatures, the 2$\pi$/2c-periodic band dispersion, along with half-integer-indexed diffraction peaks along the c axis, unambiguously indicates that the ground state of 1T-TaS$_2$ is a band insulator with interlayer dimerization. Upon heating, however, the system undergoes a transition into a Mott insulating state, which only exists in a narrow temperature window. Our results refute the idea of searching for quantum magnetism in 1T-TaS$_2$ only at low temperatures, and highlight the competition between on-site Coulomb repulsion and interlayer hopping as a crucial aspect for understanding the material's electronic properties.