Do excitonic insulators exist?

When Coulomb interaction between holes and electrons is not screened well in a semimetal or a narrow gap semiconductor, the holes and electrons may undergo coherent formation of excitons and the system may fall in an excitonic insulator. We have studied the electronic structure of Ta₂NiSe₅ which is one of the candidates for the excitonic insulator. The Ni 2p core level photoemission suggests that Ni 3d band is partly unoccupied indicating unusual hybridization between the Ni 3d valence band and the Ta 5d conduction band. By means of state-of-the-art time-resolved ARPES, we have realized a photoinduced phase transition from the possible excitonic insulator phase to a semimetal phase in Ta₂NiSe₅ and related systems. The ultrafast dynamics of the photoinduced phase transition indicates that Ta₂NiSe₅ can be viewed as an excitonic insulator with direct band gap. (author)