Exploring the Charge Density Wave phase of 1$T$-TaSe$_2$: Mott or Charge-transfer Gap?

TL;DR

This study combines experimental and theoretical methods to show that the electronic gap in 1$T$-TaSe$_2$'s CDW phase arises from band structure changes, not Mott physics, challenging previous interpretations.

Contribution

It provides evidence that the charge density wave phase alone explains the electronic gap, without requiring Mott correlation effects, revising the understanding of this material's insulating behavior.

Findings

Confirmed the existence of a state above $E_F$ linked to the band gap.

Demonstrated the gap is due to CDW-induced band structure modifications.

Supported by density functional theory calculations.

Abstract

1$T$-TaSe$_2$ is widely believed to host a Mott metal-insulator transition in the charge density wave (CDW) phase according to the spectroscopic observation of a band gap that extends across all momentum space. Previous investigations inferred that the occurrence of the Mott phase is limited to the surface only of bulk specimens, but recent analysis on thin samples revealed that the Mott-like behavior, observed in the monolayer, is rapidly suppressed with increasing thickness. Here, we report combined time- and angle-resolved photoemission spectroscopy and theoretical investigations of the electronic structure of 1$T$-TaSe$_2$. Our experimental results confirm the existence of a state above $E_F$, previously ascribed to the upper Hubbard band, and an overall band gap of $\sim 0.7$ eV at $\overline{\Gamma}$. However, supported by density functional theory calculations, we demonstrate that the origin of this state and the gap rests on band structure modifications induced by the CDW phase alone, without the need for Mott correlation effects.