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 [1]. 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 [2]. 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 [3].

In a second part, I will show how strain can be employed to modify the ground state of 1T-TaS2 as probed by ARPES [5]. 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 [7]. 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.

[1] Tosatti and Fazekas, Journal de Physique 37, C4-165 (1976).

[2] Ritschel et al., Phys. Rev. B98, 195134 (2018).

[3] Petocchi et al., Phys. Rev. Lett. 129, 016402 (2022).

[4] Nicholson et al., arXiv, 2204.05598 (2022).

[5] Stojchevska et al., Science 344, 177 (2014).

[6] Vaskivskyi et al., Nat. Commun. 7, 1 (2016).

[7] B. Salzmann et al., submitted.