Orbital excitations on the cusp of Mott-band insulator crossover in 1T-TaS₂

The nature of electron itinerancy is a fundamental descriptor of a solid-state material. In the case of insulators, band and Mott insulators represent two different regimes: the former can be described using spatially extended electron wavefunctions and well-defined quasiparticles, while the latter features localized electron wavefunctions with strong local correlations. Recently the nature of the insulator behavior has been under intense investigation in a number of 5d transition metal compounds. Here we perform resonant inelastic X-ray scattering studies of 1T-TaS₂ which has till recently been long considered a prototypical Mott insulator. We observed five electronic excitations arising from the interband transitions of the Ta 5d/5p orbitals and the S 3p ligand state at the Ta L₃ edge. These excitations cannot be explained within the framework of standard molecular orbital multiplet calculations that are based on a localized picture with strong electronic correlations. Instead, calculations from the band dispersions can reasonably capture the number and energies of the orbital excitations with small deviations. We argue that the insulating phase of 1T-TaS₂ is on the cusp of band-Mott insulator crossover and leans heavily towards the band insulator limit.