Photoinduced phase transition and associated time scales in the excitonic insulator Ta2NiSe5

We investigate the nonequilibrium electronic structure and characteristic timescales in a candidate excitonic insulator, Ta2NiSe5, using time- and angle-resolved photoemission spectroscopy. Following a strong photoexcitation, the band gap closes transiently within 100 fs, i.e., on a timescale faster than the typical lattice vibrational period. Furthermore, we find that the characteristic time associated with the rise of the photoemission intensity above the Fermi level decreases with increasing excitation strength, while the relaxation time of the electron population towards equilibrium shows an opposite behavior. We argue that these experimental observations can be consistently explained by an excitonic origin of the band gap in the material. The excitonic picture is supported by microscopic calculations based on the nonequilibrium Green's function formalism for an interacting two-band system. We interpret the speedup of the rise time with fluence in terms of an enhanced scattering probability between photoexcited electrons and excitons, leading to an initially faster decay of the order parameter and the inclusion of electron-phonon coupling at a semiclassical level changes only the quantitative aspects of the proposed dynamics, while the qualitative features remain the same.