Controlling the Charge Density Wave Transition in Single-Layer TiTe_2xSe_2(1–x) Alloys by Band Gap Engineering

Abstract

Closing the band gap of a semiconductor into a semimetallic state gives a powerful potential route to tune the electronic energy gains that drive collective phases like charge density waves (CDWs) and excitonic insulator states. We explore this approach for the controversial CDW material monolayer (ML) TiSe 2 by engineering its narrow band gap to the semimetallic limit of ML-TiTe 2 . Using molecular beam epitaxy, we demonstrate the growth of ML-TiTe 2x Se 2(1−x) alloys across the entire compositional range and unveil how the (2 × 2) CDW instability evolves through the normal state semiconductor−semimetal transition via in situ angle-resolved photoemission spectroscopy. Through model electronic structure calculations, we identify how this tunes the relative strength of excitonic and Peierls-like coupling, demonstrating band gap engineering as a powerful method for controlling the microscopic mechanisms underpinning the formation of collective states in two-dimensional materials.

Chemical Trends of the Bulk and Surface Termination-Dependent Electronic Structure of Metal-Intercalated Transition Metal Dichalcogenides

“…Transition metal dichalcogenides (TMDs) have become the subject of intensive research, owing to the diverse properties they host. − These include metallic and insulating electronic structures, several notable examples of spin-valley locking, − collective phenomena such as charge density waves and superconducting states, − and a myriad of topological phases. − Intercalating magnetic transition metal ions into the van der Waals gap of 2H-TMDs has emerged as a powerful method to further stabilize novel magnetic states and textures, where the intercalant species nominally act as local magnetic moments. , At critical concentrations, they occupy periodic sites that break the centrosymmetry of the host compounds. This allows for the presence of antisymmetric Dzyaloshinskii–Moriya exchange interactions, which underpin the formation of a host of noncollinear magnetic orders. − …”