Band hybridisation at the charge density wave transition in monolayer TiTe₂

In TiTe₂ a charge density wave (CDW) phase emerges at low temperature only in the monolayer (ML) configuration but not in the bulk¹. This is in contrast with its sister compound TiSe₂ in which a similar lattice instability occurs, largely independent of material thickness^2,3. The origin of the CDW transition that is stabilised in monolayer TiTe₂ remains unclear, while its semimetallic nature raises questions that challenge the standard pictures of CDW formation in this family. Tracking the electronic structure evolution in ML-TiTe₂ across the phase transition using angle-resolved photoemission spectroscopy (ARPES), we have identified spectroscopic signatures of a band hybridisation between the back-folded conduction and valence bands occurring below the critical temperature, which in turn leads to an electronic energy gain for the CDW transition to occur.
¹Chen et al., Nat. Commun., 2017, 8, 516
²Di Salvo et al., Phys. Rev. B, 1976, 14, 4321
³Chen et al., Nat. Commun., 2015, 6, 8943