Time-resolved ARPES study on Ta₂NiSe₅

Ta₂NiSe₅ has been long proposed as an excitonic insulator, in which remarkable band flattening below the structural transition temperature at 328 K, as a result of the strong Coulomb attractive interactions between the conduction and valence bands. Recently, photo-excitation has been intensively used to explore non-equilibrium phases. Our previous result also confirms that Ta₂NiSe₅ is an excitonic insulator from the characteristic dynamical behavior [1]. In this work, we have used time-resolved ARPES (TARPES) to investigate the photo-excited non-equilibrium phases in Ta₂NiSe₅. We have discovered that intense photo-excitation induce a transient semimetalic state, which could not be realized under equilibrium. In order to gain insight on the mechanism of this phase transition, we have pursued the TARPES measurements with better signal-to-noise ratio enabled by use of the higher repetition rate laser system. We have observed oscillating behaviors corresponding to the coherent phonons in the ARPES intensity maps, which allowed us to identify which phonon mode is relevant to the metallization.