Temporal Evolution and Fluence Dependence of Band Structure in Photoexcited Ta₂Ni_0.9Co_0.1Se₅ Probed by Time- and Angle-Resolved Photoemission Spectroscopy

We have investigated the temporal evolution and fluence dependence of band structure in Ta2Ni0.9Co0.1Se5 by means of time-and angle-resolved photoemission spectroscopy (tr-ARPES). With the fluence of 3.00 mJ cm-2 and the delay time of 170 fs, the conduction band crossing the Fermi level (EF) and the valence band reaching EF are observed indicating the photoinduced semimetal phase. However, at the delay time of 70 fs, the valence band top does not reach EF creating a partial gap or a pseudogap, and the transition from the excitonic insulator state to the semimetal state is not completed. The conduction band crossing EF is observed even with the shorter delay time (down to 50-70 fs) and/or the lower fluence (down to 0.32 mJ cm-2). On the other hand, the valence band top stays well below EF up to the fluence of 1.39 mJ cm-2 even at the delay time of 160-170 fs. The different optical responses of the valence and conduction bands suggest a vital role of the lattice fluctuations pointed out by Nakano et al. [Phys. Rev. B 98 , 045139 (2018)] on the residual pseudogap at 70 fs after the photoexcitation.