Anomalous orbital structure in two-dimensional titanium dichalcogenides

Banabir Pal,Fangdi Wen,Yanwei Cao,A T N’Diaye,Xiaoran Liu,M Kareev,P Shafer,E Arenholz,J Chakhalian

doi:10.1038/s41598-018-37248-5

Abstract

Generally, lattice distortions play a key role in determining the electronic ground states of materials. Although it is well known that trigonal distortions are generic to most two dimensional transition metal dichalcogenides, the impact of this structural distortion on the electronic structure and topological properties has not been understood conclusively. Here, by using a combination of polarization dependent X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) and atomic multiplet cluster calculations, we have investigated the electronic structure of titanium dichalcogenides TiX2 (X = S, Se, Te), where the magnitude of the trigonal distortion increase monotonically from S to Se and Te. Our results reveal the presence of an anomalously large crystal field splitting. This unusual kind of crystal field splitting is likely responsible for the unconventional electronic structure of TiX2 compounds and ultimately controls the degree of the electronic phase protection. Our findings also indicate the drawback of the distorted crystal field picture in explaining the observed electronic ground state and emphasize the key importance of trigonal symmetry, metal-ligand hybridization and electron-electron correlations in defining the electronic structures at the Fermi energy.

Highlights

The realization of numerous exotic electronic phases of graphene[1,2,3] and the relentless tendency to miniaturization of silicon-based electronics[4] have ignited exhaustive research in a wide range of two dimensional (2D) layered materials
Surprisingly very little is know about the topological phases (TPs) stability subjected to lattice distortions associated with trigonal symmetry as trigonal distortions could be used to either enter a topological phase, or exit a topological phase with a trigonal distortion[22,23,24,25]
The problem is vividly illustrated by the case of TiSe2 which undergoes the transition into a chiral charge density wave (CDW) state[33,34] and further into a conventional CDW resulting in dramatic renormalization of electronic and structural properties

Summary

Results and Discussion

These findings clearly suggest that purely ionic picture which is normally very efficient in capturing the excitation spectra of Ti4 + derived states in oxide systems is inadequate in explaining the origin of XLD signal in dichalcogenides. All these discrepancies clearly establish that conventional crystal field picture fails to explain the observed features in these system contrary to the Ti4+ based oxide materials The excellent agreement between theory and the XAS spectra suggests the importance of the band-like description including trigonal lattice deformations, electron-electron correlations and metal-ligand covalency to elucidate the electronic structure of transition metal dichalcogenides

Methods

Author Contributions

Additional Information