Excitonic Insulator State inTa2NiSe5Probed by Photoemission Spectroscopy

Abstract

We report on a photoemission study of Ta2NiSe5 that has a quasi-one-dimensional structure and an insulating ground state. Ni 2p core-level spectra show that the Ni 3d subshell is partially occupied and the Ni 3d states are heavily hybridized with the Se 4p states. In angle-resolved photoemission spectra, the valence-band top is found to be extremely flat, indicating that the ground state can be viewed as an excitonic insulator state between the Ni 3d-Se 4p hole and the Ta 5d electron. We argue that the high atomic polarizability of Se plays an important role to stabilize the excitonic state.

Competition between excitonic charge and spin density waves: Influence of electron-phonon and Hund's rule couplings

“…In these systems, the valence and conduction bands are formed by orbitals located on different atoms. For example, in 1T -TiSe 2 , the 4p orbitals of Se ions account for the valence bands and the 3d orbitals of Ti ions account for the conduction bands [7][8][9][10][11][12][13][14] , and in Ta 2 NiSe 5 , the 3d orbitals of Ni ions form the valence bands and the 5d orbitals of Ta ions form the conduction bands [21][22][23][24] . Hund's rule coupling, acting between electrons on different orbitals of a single ion and favoring the spin-triplet excitons, is therefore negligible.…”

“…The condensation of spin-triplet excitons was also predicted to occur in the proximity of the spin-state transition, 18 of which Pr 0.5 Ca 0.5 CoO 3 is an example. 19,20 Likewise, the structural phase transition of the layered chalcogenide Ta 2 NiSe 5 has been attributed to a spin-singlet EI [21][22][23][24] . The spin-density-wave (SDW) state of iron-pnictide superconductors has sometimes been argued to be of the excitonic origin as well [25][26][27][28] .…”

Magnetic Field Effects in a Correlated Electron System with Spin-State Degree of Freedom — Implications for an Excitonic Insulator —

“…[15][16][17] The EI state has been studied since 1960s in the narrow gap semiconductors and semimetals, [18][19][20][21][22] and is recently reexamined from the modern viewpoints. 23,24) The EI state is expected to be realized, when the exciton binding energy exceeds the band gap energy. Since the LS and HS states in the cobalt oxides are identified as a band insulator and a Mott insulator, respectively, a narrow-gapped state is expected around a crossover between the two spin states.…”

Existence of excitonic insulator phase in the extended Falicov-Kimball model: SO(2)-invariant slave-boson approach

“…Actual materials experimentally studied from the viewpoint of the EI are numbered. One recent example is quasi-one-dimensional Ta 2 NiSe 5 with highly polarizable Se, where angle-resolved photoemission spectra reveal an extreme valence-band top flattening indicating that the ground state might be viewed as an EI 19 . At present, the transition-metal dichalcogenide 1T -TiSe 2 seems to be the only candidate for a low-temperature phase transition to the EI without the influence of any external parameters other than the temperature.…”