Substitution-dependent orbital character of the electronic structure in Ta₂NiSe₅ and derivatives

Ni L-edge x-ray absorption (XAS) was used to study the electronic structure of the Ta₂NiSe₅ family, including substitutions on the Ni site (by Co) and on the Se site (by S). This is a “candidate” for the yet elusive realization of an excitonic insulator (EI). The polarization dependence of XAS indicates the orbital character of the Ni 3d states involved in the temperature-driven structural transitions (one for each composition) that were determined and studied by high-resolution x-ray diffraction. In the complex interplay between electronic structure, spatial structure, and orbital character we observe a systematic evolution with substitution, moving between phase-space regions with somewhat insulating, small-gap, or slightly semimetallic character. Taken together, the results may augment our understanding of the electronic and structural properties conducive for EI behavior.

We acknowledge the KIT Light Source, the KNMFi, and the European Synchrotron Radiation Facility (ESRF) for provision of synchrotron radiation facilities.