Orbital-selective Band Hybridisation at the Charge Density Wave Transition in Monolayer TiTe$_2$

TL;DR

This study uncovers how orbital-selective band hybridisation drives the charge density wave transition in monolayer TiTe$_2$, highlighting the role of electronic dimensionality in collective state emergence.

Contribution

It reveals the orbital-selective hybridisation mechanism at the CDW transition in monolayer TiTe$_2$ and explains its absence in bulk due to three-dimensional effects.

Findings

Orbital-selective band hybridisation occurs at the CDW transition.

The energy gain from hybridisation drives the CDW in monolayer TiTe$_2$.

Bulk TiTe$_2$ suppresses this energy gain due to 3D band effects.

Abstract

An anomalous $(2\times2)$ charge density wave (CDW) phase emerges in monolayer 1T-TiTe$_2$ which is absent for the bulk compound, and whose origin is still poorly understood. Here, we investigate the electronic band structure evolution across the CDW transition using temperature-dependent angle-resolved photoemission spectroscopy. Our study reveals an orbital-selective band hybridisation between the backfolded conduction and valence bands occurring at the CDW phase transition, which in turn leads to a significant electronic energy gain, underpinning the CDW transition. For the bulk compound, we show how this energy gain is almost completely suppressed due to the three-dimensionality of the electronic band structure, including via a $k_z$-dependent band inversion which switches the orbital character of the valence states. Our study thus sheds new light on how control of the electronic dimensionalilty can be used to trigger the emergence of new collective states in 2D materials.