  The excitonic insulator is an intriguing correlated electron phase formed of
condensed excitons. A promising candidate is the small band gap semiconductor
Ta2NiSe5. Here we investigate the quasiparticle and coherent phonon dynamics in
Ta2NiSe5 in a time resolved pump probe experiment. Using the models originally
developed by Kabanov et al. for superconductors, we show that the material's
intrinsic gap can be described as almost temperature independent for
temperatures up to about 250 K to 275 K. This behavior supports the existence
of the excitonic insulator state in Ta2NiSe5. The onset of an additional
temperature dependent component to the gap above these temperatures suggests
that the material is located in the BEC-BCS crossover regime. Furthermore, we
show that this state is very stable against strong photoexcitation, which
reveals that the free charge carriers are unable to effectively screen the
attractive Coulomb interaction between electrons and holes, likely due to the
quasi one-dimensional structure of Ta2NiSe5.
