Sebastian Buchberger, Yann in ’t Veld, Akhil Rajan, Philip A. E. Murgatroyd, Brendan Edwards, Bruno Kenichi Saika, Naina Kushwaha, Maria H. Visscher, Jan Berges, Dina Carbone, Jacek Osiecki, Craig Polley, Tim O. Wehling, P. D. C. King
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 ultraclean van der Waals heterostructures of monolayer (ML) TiSe 2 /graphite and ML TiSe 2 / h -BN. We observe how the modified substrate screening environment drives a renormalization of the quasiparticle band gap of the TiSe 2 layer, signifying its susceptibility to Coulomb 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 .