Electronic structure and enhanced charge-density wave order of monolayer VSe2

We gratefully acknowledge support from The Leverhulme Trust (Grant Nos. RL-2016-006 and PLP-2015-144), The Royal Society, The Engineering and Physical Sciences Research Council, UK, for support under grant Nos. EP/I031014/1, EP/M023958/1, EP/P020151/1, and EP/M028771/1, and the International Max-Planck Partnership for Measurement and Observation at the Quantum Limit. OJC and KU acknowledge EPSRC for PhD studentship support through grant Nos. EP/K503162/1 and EP/L015110/1. IM acknowledges PhD studentship support from the IMPRS for the Chemistry and Physics of Quantum Materials. LBD acknowledges studentship support from EPSRC and the Science and Technology Facilities Council (UK).How the interacting electronic states and phases of layered transition-metal dichalcogenides evolve when thinned to the single-layer limit is a key open question in the study of two-dimensional materials. Here, we use angle-resolved photoemission to investigate the electronic structure of monolayer VSe2 grown on bi-layer graphene/SiC. While the global electronic structure is similar to that of bulk VSe2, we show that, for the monolayer, pronounced energy gaps develop over the entire Fermi surface with decreasing temperature below Tc = 140 ± 5 K, concomitant with the emergence of charge-order superstructures evident in low-energy electron diraction. These observations point to a charge-density wave instability in the monolayer which is strongly enhanced over that of the bulk. Moreover, our measurements of both the electronic structure and of x-ray magnetic circular dichroism reveal no signatures of a ferromagnetic ordering, in contrast to the results of a recent experimental study as well as expectations from density-functional theory. Our study thus points to a delicate balance that can be realised between competing interacting states and phases in monolayer transition-metal dichalcogenides.Peer reviewe