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Existence of excitonic insulators was theoretically predicted in 1960s. They can be realized if conduction and valence bands in a semiconductor or semimetal are hybridized by the electron-hole attractive Coulomb interaction. The electron-hole hybridization by the attractive Coulomb interaction can be viewed as a Bose-Einstein condensation of bounded electron-hole pairs or excitons. In 2009, a layered transition-metal chalcogenide Ta₂NiSe₅ was proposed to be an excitonic insulator. This has been confirmed by various experimental techniques. In particular, a photoinduced phase transition with 100 fs time scale from the excitonic insulating state to a semimetal state is one strongest proof. In the present study, the photoinduced energy shift of conduction and valence bands (electron and hole bands) of Ta₂Ni_0.9Co_0.1Se₅(which is also an excitonic insulator) is investigated by time-resolved angle-resolved photoemission spectroscopy. In a canonical excitonic insulator, valence and conduction bands are expected to shift simultaneously to close the energy gap between them owing to the screening effect of the photoinduced carriers. However, in Ta₂Ni_0.9Co_0.1Se₅, the energy shift of the valence band is delayed than that of the conduction band. This observation indicates that the electron-hole symmetry (which is assumed in the canonical theory for excitonic insulators) is broken and that the mechanism of the electron-hole hybridization is complicated in the layered transition-metal chalcogenide. We speculate that the electron-lattice interaction and electronic correlation in the valence band of Ni 3d and Se 4p orbitals provide the residual and transient energy gap during the photoinduced phase transition to the semimetal state.

(written by T. Mizokawa on behalf of all authors.)