- 16 Sep 2022
Claude Monney I What is the role of electronic correlations in 1T-TaS2 ?
Department of Physics, University of Fribourg, 1700 Fribourg, Switzerland
Tantalum disulphide (1T-TaS2) is a layered material that hosts a series of charge density wave (CDW) phases at temperatures below ~350K, and an insulating commensurate CDW (CCDW) phase below ~165 K. In 1976, already, it was recognized that the particular rearrangement of atoms in so-called Star-of-David within the CCDW phase might lead to a Mott phase due to the presence of strong electronic correlations and small electronic bandwidth of the Ta 5d band . Since then, more evidence in favour of a Mott phase has accumulated in the literature. However, recently, this picture has been challenged by the proposal that interlayer hybridization is in fact responsible for the insulating CCDW phase . What is then the role of electronic correlations in this material?
In this talk, I will present recent scanning tunnelling microscopy and spectroscopy (STM/STS) and angle resolved photoelectron spectroscopy (ARPES) measurements on 1T-TaS2 surfaces combined with advanced electronic structure calculations (GW+EDMFT) showing actually that both pictures, the Mott correlations and the interlayer hybridization, are leading to band gap formation and thus the insulating character of the CCDW phase in 1T-TaS2 .
In a second part, I will show how strain can be employed to modify the ground state of 1T-TaS2 as probed by ARPES . As a consequence of strain, a bandwidth-driven insulator-metal transition is observed upon reducing temperature. The presence of an emergent quasi-particle peak at the Fermi level further supports that the system hosts Mott correlations.
In a third part, I will discuss the occurrence of a metallic phase in pristine 1T-TaS2 samples. Indeed, recent investigations have revealed the existence of a metastable metallic phase accessible from the CCDW phase by applying a laser- or current pulse [5,6]. Here STM/STS measurements on surfaces not exposed to such pulses show the same metallic electronic structure and topographic features as the pulse induced phase over distances of hundreds of nanometres . We provide evidence of a change in top layer stacking in this metallic phase. This finding indicates a strong influence of the interlayer stacking on the electronic structure of the material.
All in all, the recent literature and our studies depict the layered material 1T-TaS2 as a complex material for which a subtle combination of electronic correlations and interlayer stacking leads to a rich phase diagram.
This work is supported by the Swiss National Science Foundation.
 Tosatti and Fazekas, Journal de Physique 37, C4-165 (1976).
 Ritschel et al., Phys. Rev. B98, 195134 (2018).
 Petocchi et al., Phys. Rev. Lett. 129, 016402 (2022).
 Nicholson et al., arXiv, 2204.05598 (2022).
 Stojchevska et al., Science 344, 177 (2014).
 Vaskivskyi et al., Nat. Commun. 7, 1 (2016).
 B. Salzmann et al., submitted.