Ta2NiSe5 : A candidate topological excitonic insulator with multiple band inversions

The electronic structures and topological properties of the orthorhombic and monoclinic phases of the quasi-one-dimensional excitonic insulator ${\mathrm{Ta}}₂{\mathrm{NiSe}}₅$ are investigated based on density functional theory. In contrast to a single parity or band inversion across the Fermi level in many topological insulators studied previously, there are multiple parity and band inversions with or without spin-orbit coupling in both phases of ${\mathrm{Ta}}₂{\mathrm{NiSe}}₅$, resulting in more complex and topologically nontrivial electronic structures. The Dirac cone type surface states of the low-temperature monoclinic phase are also obtained. In this paper, we demonstrate that ${\mathrm{Ta}}₂{\mathrm{NiSe}}₅$ is a promising candidate as a three-dimensional topological excitonic insulator.