Controlling the charge density wave transition in single-layer TiTe_2xSe_2(1−x) alloys by band gap engineering

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 (CDW) and excitonic insulator states. We explore this approach for the controversial CDW material monolayer (ML) TiSe₂ by engineering its narrow band gap to the semimetallic limit of ML-TiTe₂. Using molecular beam epitaxy, we demonstrate the growth of ML-TiTe_2xSe_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.