Systematic evolution of electronic structure in monolayer TiXc₂ (Xc = S, Se, and Te)

The discovery of the quantum Hall effect in graphene has triggered intensive research to explore new two-dimensional materials with novel physical properties. Atomically-thin transition-metal dichalcogenides (TMDs) have attracted a great deal of attention because they show exotic quantum phenomena associated with two-dimensionality such as unconventional charge density waves (CDW), Mott insulator, and Ising superconductivity. Among many TMDs, titanium dichalcogenides TiXc₂ (Xc = S, Se, and Te) have been a target of intensive studies from the viewpoint of the interplay between CDW and dimensionality. In the bulk form, TiSe₂ is known to host a CDW with (2×2×2) periodicity, whereas both TiSe₂ and TiTe₂ show no indication of CDW. Surprisingly, a (2×2) CDW suddenly appears in TiTe₂ in a monolayer limit [1]. Although some previous angle-resolved photoemission spectroscopy (ARPES) studies reported this anomalous behavior in TiTe₂, its mechanism is still unclear. In addition, because of the technical barrier of synthesizing a high-quality crystalline film, it is also questionable whether monolayer TiS₂ undergoes a CDW transition. In this work, we have fabricated monolayer TiXc₂ (Xc = S, Se, and Te) on bilayer graphene by the unique combination of molecular beam epitaxy (MBE) and topotactic reaction, and studied their electronic states by ARPES. We will systematically compare the band structures of monolayer TiXc₂ and discuss the origin of CDW.

[1] K. Yanagizawa et al., Phys. Rev. Mater. 7, 104002 (2023).