Topological and Charge-Ordered Phases in Transition-Metal Dichalcogenides – Professor Phil King, School of Physics and Astronomy, University of St. Andrews, UK

April 17, 2019 @ 2:00 pm – 3:00 pm
Mott Seminar Room (531)
Cavendish Laboratory
Department of Physics
Dr Kaveh Delfanazari

Transition-metal dichalcogenides (TMDs) are renowned for their rich and varied properties. They range from metals and superconductors to strongly spin-orbit-coupled semiconductors and charge-density-wave systems, with their single-layer variants one of the most prominent current examples of two-dimensional materials beyond graphene.1,2 In this talk, I will focus on two aspects of TMD physics. First, I will show our spin- and angle-resolved photoemission measurements that demonstrate how both the 1T and 2H-structured TMDs are natural hosts of ladders of type-I and type-II bulk Dirac cones and topological surface states and resonances.3,4 These arise from the chalcogen p-orbital manifold as a very general consequence of their trigonal crystal field, and as such can be expected across a large number of compounds, opening routes to tune and ultimately exploit the topological physics of TMDs. Second, I will discuss the evolution of the electronic structure and charge density wave (CDW) phases of TiSe2 and VSe2 when their thickness is reduced to a single monolayer. Three-dimensionality is a core feature of the electronic structure and ordering wavevectors of both of these parent compounds. Nonetheless, by fabricating epitaxial monolayers using molecular-beam epitaxy, we show how their CDW phases not only persist, but are in fact strengthened, in the two-dimensional limit. In TiSe2, this can be attributed to a loss of kz-selectivity in band hybridisation at the CDW ordering instability.5 In VSe2, we show how a strong-coupling CDW-like instability in turn drives a metal-insulator transition in the monolayer, in contrast to small partial gap opening in bulk, which we argue removes a competing instability to ferromagnetism that is predicted for monolayer VSe2.6 Together, this work points to the delicate balance that can be realized between competing electronic, topological, and quantum many-body states and phases in bulk and monolayer transition-metal dichalcogenides.

This work was performed in close collaboration with O.J. Clark, M.D. Watson, A. Rajan, J. Feng, D. Biswas, M.S. Bahramy, and colleagues from the Universities of St Andrews, Tokyo, Oxford, Keil, Diamond, SNU, and NTNU.

1. Q. H. Wang et al., Nature Nano. 7, 669 (2012).
2. X. Xu et al., Nature Phys. 10, 343 (2014).
3. M.S. Bahramy, O.J. Clark et al., Nature Materials 17, 21 (2018).
4. O.J. Clark et al., Phys. Rev. Lett. 120, 156401 (2018).
5. M.D. Watson et al., Phys. Rev. Lett. 122, 076404 (2019).
6. J. Feng et al. Nano Lett. 18, 4493 (2018).