Observation of orbital order in the Van der Waals material 1T-TiSe2

TL;DR

This study provides direct experimental evidence of orbital order in the layered Van der Waals material 1T-TiSe2, highlighting the role of orbital degrees of freedom in its electronic properties.

Contribution

First resonant X-ray scattering observation of orbital order in a weakly correlated, quasi-two-dimensional Van der Waals material, supported by first-principles calculations.

Findings

Orbital order exists in 1T-TiSe2, a weakly correlated layered material.

Charge redistribution, lattice displacements, and orbital order are interconnected.

Orbital degrees of freedom are crucial in understanding Van der Waals materials.

Abstract

Besides magnetic and charge order, regular arrangements of orbital occupation constitute a fundamental order parameter of condensed matter physics. Even though orbital order is difficult to identify directly in experiments, its presence was firmly established in a number of strongly correlated, three-dimensional Mott insulators. Here, reporting resonant X-ray scattering experiments on the layered Van der Waals compound $1T$-TiSe$_2$, we establish the emergence of orbital order in a weakly correlated, quasi-two-dimensional material. Our experimental scattering results are consistent with first-principles calculations that bring to the fore a generic mechanism of close interplay between charge redistribution, lattice displacements, and orbital order. It demonstrates the essential role that orbital degrees of freedom play in TiSe$_2$, and their importance throughout the family of correlated Van der Waals materials.