Persistence of charge ordering instability to Coulomb engineering in the excitonic insulator candidate TiSe$_2$

TL;DR

This study investigates whether excitons drive the charge-density-wave transition in TiSe$_2$ by using dielectric screening in heterostructures, finding that excitons are not essential for the phase transition.

Contribution

It introduces a novel Coulomb engineering approach to test exciton involvement in TiSe$_2$'s phase transition, challenging the excitonic insulator hypothesis.

Findings

Substrate screening modifies TiSe$_2$ band gap

Electronic structure evolution is temperature-independent

Excitons are not necessary for the CDW transition

Abstract

TiSe$_2$ has long been considered one of the best candidate materials to host the elusive excitonic insulator (EI) phase. However, a finite coupling to the lattice can generically be expected, while a lack of "smoking-gun" signatures for the importance of the electron-hole interaction in driving the phase transition has rendered it challenging to distinguish the EI from the conventional charge-density-wave (CDW) phase. Here, we demonstrate a new approach, exploiting the susceptibility of excitons to dielectric screening. We combine mechanical exfoliation with molecular-beam epitaxy to fabricate ultra-clean van der Waals heterostructures of monolayer (ML-)TiSe$_2$/graphite and ML-TiSe$_2$/hBN. We observe how the modified substrate screening environment drives a renormalisation of the quasi-particle band gap of the TiSe$_2$ layer, signifying its susceptibility to Coloumb engineering. The temperature-dependent evolution of its electronic structure, however, remains unaffected, indicating that excitons are not required to drive the CDW transition in TiSe$_2$.