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

C J Butler,Y Iwasa,T Hanaguri,M Yoshida

doi:10.1038/s41467-020-16132-9

C J Butler, Y Iwasa + Show 2 more

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https://doi.org/10.1038/s41467-020-16132-9

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Abstract

If a material with an odd number of electrons per unit-cell is 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 the unit-cell to include an even number of orbitals, the nature of the insulating state is ambiguous. Here, scanning tunnelling microscopy 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 can drive gap formation. We also observe the collapse of Mottness at an extrinsically re-stacked termination, demonstrating that the microscopic mechanism of insulator-metal transitions lies in degrees of freedom of inter-layer stacking.

Highlights

If a material with an odd number of electrons per unit-cell is insulating, Mott localisation may be invoked as an explanation
Samples were cleaved, transferred to the scanning tunnelling microscopy (STM) and measured at temperatures far below the transition temperature at which the C-charge density wave (CDW) sets in, and the bulk structure of the CDW should be preserved such that measurements on a large number of cleaved surfaces may show evidence of the ACAC pattern
We tentatively attribute the appearance of these two forms of density of states (DOS) to the surfaces created by the two cleavage planes of the bulk stacking pattern

Summary

Introduction

If a material with an odd number of electrons per unit-cell is 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. 1234567890():,; The origin of the spectral gap in many insulating materials is difficult to determine because as well as the simple band theoretic criterion of a completely filled valence band, electron–phonon interactions, strong electronic correlations[1,2] and other mechanisms generally can coexist and may all play some role This is true in the decades-old charge density wave (CDW) compound 1T-TaS2, for which the debate over the nature of the low-temperature insulating state has only intensified in recent years[3,4,5,6,7]. From this foundation it has been suggested that, since a Mott state in

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