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

Sebastian Buchberger; Yann in 't Veld; Akhil Rajan; Philip A. E. Murgatroyd; Brendan Edwards; Bruno K. Saika; Naina Kushwaha; Maria H. Visscher; Jan Berges; Dina Carbone; Jacek Osiecki; Craig Polley; Tim Wehling; Phil D. C. King

doi:10.1103/9trc-9865

paper

Persistence of charge ordering instability to Coulomb engineering in the excitonic insulator candidate TiSe $_{2}$

arXiv:2506.01470 · doi:10.1103/9trc-9865

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 $_{2}$ .

15 pages including supplemental information, 5+4 figures

Persistence of charge ordering instability to Coulomb engineering in the excitonic insulator candidate TiSe2​

Abstract

Persistence of charge ordering instability to Coulomb engineering in the excitonic insulator candidate TiSe $_{2}$