Controlling Charge Density-Wave States in Single-Layer Transition-Metal Dichalcogenides

Control over materials thickness down to the single-atom scale has emerged as a powerful tuning
parameter for manipulating not only the single-particle band structures of solids, but increasingly
also their interacting electronic states and phases. A particularly attractive materials system in which
to explore this is the transition-metal dichalcogenides, both because of their naturally-layered van der
Waals structures as well as the wide variety of materials properties which they are known to host. Yet,
how their interacting electronic states and phases evolve when thinned to the single-layer limit
remains a key open question in many such systems. Here, we use angle-resolved photoemission to
investigate the electronic structure and charge density wave (CDW) phases of monolayer TiSe2, TiTe2,
and VSe2 epitaxial thin films grown by molecular-beam epitaxy [1]. Three-dimensionality is a core
feature of the electronic structure of all of these parent compounds, but we show how their CDW
phases not only persist, but are strengthened, in the monolayer limit. In TiSe2, we observe a strongcoupling and orbital-selective CDW [2], necessarily without a kz-selectivity in band hybridisation
that is of key importance for the bulk instability [3], while TiTe2 is driven into a charge-ordered
phase in the monolayer which is not stable in the bulk at all. In VSe2, we show how the monolayer
hosts a much stronger-coupling CDW instability than for the bulk compound, which in turn drives a
metal-insulator transition, removing a competing instability to ferromagnetism [4]. We show how
ferromagnetism can, however, be re-established via proximity coupling [5]. Together, these studies
point to the delicate balance that can be realized between competing interacting states and phases in
monolayer transition-metal dichalcogenides, and suggest new strategies for controlling these.
This work was performed in close collaboration with M.D. Watson, A. Rajan, K. Underwood, J. Feng,
T. Antonelli, D. Biswas, W. Rahim, D.O. Scanlon, G. Vinai, G. Panaccione and colleagues from the
Universities of St Andrews, Oxford, Keil, UCL, Diamond, Elettra, and SOLEIL.

References
[1] Rajan et al., Phys. Rev. Materials 4, 014003 (2020).
[2] Watson et al., 2D Materials in press (DOI: 10.1088/2053-1583/abafec)
[3] Watson et al., Phys. Rev. Lett. 122, 076404 (2019).
[4] Feng et al., Nano Lett. 18, 4493 (2018).
[5] Vinai et al., Phys. Rev. B 101, 035404 (2020).