Mottness versus unit-cell doubling as the driver of the insulating state in 1T-TaS2
Christopher Butler1*, Masaro Yoshida1, Tetsuo Hanaguri1, Yoshihiro Iwasa1,2
1Center for Emergent Matter Science (CEMS), RIKEN, Wako, Saitama, Japan
2Department of Applied Physics, The University of Tokyo, Bunkyo, Tokyo, Japan
* Presenter:Christopher Butler,
The debate over the nature of the insulating ground state in 1T-TaS2 has persisted for decades, and only grown more intense recently. 1T-TaS2 exhibits two-dimensional (2D) charge order with a √13x√13 superstructure of ‘Star-of-David’ (SD) clusters. Naive non-interacting band theory seems to predict metallic behavior, because each cluster has an odd number of electrons (half-filling), and so it is usually thought that the insulating state must be driven by electron-electron interactions - a Mott insulator. Mott physics also lays the foundation for speculation that the ground state might be host to the elusive quantum spin liquid, an extraordinary phase of electronic matter in which spins refuse to order or freeze even at T = 0 K. However, this line of thinking largely neglects possible inter-layer effects, and recent calculations [1,2] indicate that for a particular inter-layer stacking pattern, in which layers of SD clusters are paired or dimerized, an insulating state can be realized without invoking Mott.
I will present scanning tunneling microscopy measurements which confirm the premise of these works by identifying two spectroscopically distinct surface terminations of the 3D charge order - the sign of a stacking pattern with two (dimerized) SDs per unit-cell. We determine the intrinsic inter-layer stacking vectors, and also identify an extrinsic stacking vector which renders the surface metallic, emphasizing the importance of inter-layer correlations for the electronic properties. We also find that at surfaces where the bulk dimerization of the SD clusters is broken, where we should therefore expect a metallic surface state, a small spectral gap can persist, which we interpret as a manifestation of Mottness.
If time allows, I will preview recent measurements of newly-discovered narrow spectral peaks at the onset of unoccupied states, with clear electron-phonon replicas. We speculatively recognize the underlying excitation as a doublon, a characteristic excitation of a Mott insulator.
Overall, these results help to elucidate the roles of both inter-layer interactions and Mott physics in this material’s mysterious insulating state, and its potentially useful metal-insulator transitions.

[1] T. Ritschel, H. Berger and J. Geck, Phys. Rev. B 98, 195134 (2018).
[2] S.-H. Lee, J. S. Goh and D. Cho, Phys. Rev. Lett. 122, 106404 (2019).

Keywords: Transition metal dichalcogenides (TMDCs), Charge density waves, Strongly correlated electrons, Mott insulators, Scanning tunneling microscopy/spectroscopy