Orbital-driven Mottness collapse in 1T-TaS2-xSex transition metal dichalcogenide

TL;DR

This study uncovers how orbital interactions drive the collapse of Mott insulating behavior in 1T-TaS2-xSex, revealing a transition from a Mott gap to a charge-transfer gap through combined STM and first-principles analysis.

Contribution

It demonstrates the orbital-driven mechanism behind the Mott insulator to metal transition in 1T-TaS2-xSex using STM and theoretical calculations.

Findings

Identification of localized and extended orbital textures.

Observation of continuous charge gap evolution.

Proposal of a new orbital-driven Mottness collapse mechanism.

Abstract

The vicinity of a Mott insulating phase has constantly been a fertile ground for finding exotic quantum states, most notably the high Tc cuprates and colossal magnetoresistance manganites. The layered transition metal dichalcogenide 1T-TaS2 represents another intriguing example, in which the Mott insulator phase is intimately entangled with a series of complex charge-density-wave (CDW) orders. More interestingly, it has been recently found that 1T-TaS2 undergoes a Mott-insulator-to-superconductor transition induced by high pressure, charge doping, or isovalent substitution. The nature of the Mott insulator phase and transition mechanism to the conducting state is still under heated debate. Here, by combining scanning tunneling microscopy (STM) measurements and first-principles calculations, we investigate the atomic scale electronic structure of 1T-TaS2 Mott insulator and its evolution to the metallic state upon isovalent substitution of S with Se. We identify two distinct types of orbital textures - one localized and the other extended - and demonstrates that the interplay between them is the key factor that determines the electronic structure. Especially, we show that the continuous evolution of the charge gap visualized by STM is due to the immersion of the localized-orbital-induced Hubbard bands into the extended-orbital-spanned Fermi sea, featuring a unique evolution from a Mott gap to a charge-transfer gap. This new mechanism of orbital-driven Mottness collapse revealed here suggests an interesting route for creating novel electronic state and designing future electronic devices.