Mottness versus unit-cell doubling as the driver of the insulating state in 1T-TaS2

TL;DR

This study investigates whether Mott localization or unit-cell doubling primarily causes the insulating state in 1T-TaS2, using STM and spectroscopy to distinguish their effects and reveal the role of interlayer stacking.

Contribution

The paper demonstrates that Mott localization alone can induce the insulating state in 1T-TaS2 despite unit-cell doubling effects, clarifying the microscopic mechanism behind the insulator-metal transition.

Findings

Mott localization is sufficient for gap formation in 1T-TaS2.

Two distinct charge order terminations are observed, indicating different stacking patterns.

Interlayer stacking degrees of freedom influence the insulator-metal transition.

Abstract

If a material with an odd number of electrons per unit cell turns out to be insulating, Mott localisation may be invoked as an explanation. This is widely accepted for the layered compound 1T-TaS2, which has a low-temperature insulating phase comprising charge order clusters with 13 unpaired orbitals each. But if the stacking of layers doubles up the unit cell to include an even number of orbitals, the nature of the insulating state is ambiguous. Here, scanning tunnelling microscopy (STM) reveals two distinct terminations of the charge order in 1T-TaS2, the sign of such a double-layer stacking pattern. However, spectroscopy at both terminations allows us to disentangle unit-cell doubling effects and determine that Mott localisation alone is enough to drive gap formation. We also observe the collapse of Mottness at an extrinsically restacked termination, demonstrating that the microscopic mechanism of insulator-metal transitions lies in degrees of freedom of interlayer stacking.