Paper 2D-ThP4
Controlling Charge Density Wave Transition in Monolayer TiSe₂

Thursday, November 2, 2017, 6:30 pm, Room Central Hall

Layered transition metal dichalcogenides (TMDCs) have been studied for their charge density wave (CDW) and superconductivity transitions. TiSe₂ is interesting because traditional explanations for CDW formation based on electron phonon coupling described by Fermi surface nesting does not apply. Instead an unconventional excitonic mechanism has been invoked. The well-established dependence of the exciton binding energy on the number of layers in TMDCs implies that the CDW transition in TiSe₂ should also be layer dependent. Furthermore, the dielectric properties of the substrate for monolayer TiSe₂ should exert further control over the excitonic properties and thus the CDW transitions in TiSe₂. Here we investigate mono- to few-layer TiSe₂ films grown by molecular beam epitaxy on HOPG or MoS₂ substrate by variable temperature scanning tunneling spectroscopy. We show that the band gap opening and CDW transition temperature in monolayer is strongly increased compared to bulk TiSe₂ and correlates with the expected exciton binding energy dependence. Interestingly, we also observe phase coherence peaks in monolayer TiSe₂. The phase coherence occurs, however, at a much lower temperature than the CDW transition, suggesting that formation of an excitonic condensate is possible but is not essential for CDW formation.