Impact of Co or Cu Substitution for Ni on the Electronic Structure of Ta₂NiSe₅Studied by Band Structure Calculations

We have investigated the electronic structure of Ta2Ni1-xCoxSe5 and Ta2Ni1-xCuxSe5 by means of band structure calculations. Ta2NiSe5 is one of the excitonic insulator candidates, and its electronic properties can be controlled by chemical substitution for Ni. In the Co doped systems with x ≤ 0.125, since a narrow Co 3d band is formed around the Fermi level, energy shift of the Ta 5d conduction and Ni 3d valence bands by the Co doping is considerably small. This is consistent with the photoemission result reported by Mitsuoka et al. [J. Phys. Soc. Jpn. 89, 124703 (2020)]. For x ≥ 0.25, the Co 3d band becomes wider, and the valence and conduction bands move to the higher energy side with increasing x. In Ta2CoSe5, Ta valence and Co valence are close to +5 (d0) and 0 (d9) respectively. In the Cu doped systems, the Cu 3d band is located well below the Fermi level and is almost fully occupied by electrons. The Cu substitution partially suppresses the Ni-Ta hybridization and causes band shift to the lower energy side by electron doping to the Ta 5d band. In Ta2CuSe5, Ta valence and Cu valence are close to +4.5 (d0.5) and +1 (d10) respectively.