Layer- and substrate-dependent charge density wave criticality in 1T–TiSe₂

TiSe₂ exhibits an unconventional charge density wave (CDW) that has been associated with an excitonic insulator transition. Here we investigate how the CDW transition is changed for single to few layers compared to bulk TiSe₂. TiSe₂ grown by molecular beam epitaxy on HOPG- or MoS₂-substrates is characterized by variable temperature scanning tunneling microscopy and spectroscopy. We show that the CDW state persists for the monolayer but the transition temperature T _CDW is significantly increased compared to the bulk. Furthermore, T _CDW is strongly dependent on the substrate material. Within the model of an excitonic insulator phase for TiSe₂, the substrate dependence may be associated with variations of the excitonic binding energies by the dielectric properties of the substrate. Interestingly, for single layer TiSe₂ on HOPG we also observe peaks in the tunneling spectra below 50 K, which are tentatively assigned to coherence peaks of an excitonic condensate. The peaks are observed below T _CDW of ∼230 K, suggesting that an excitonic insulator induced CDW can exist without a phase coherent state. (paper)