  The excitonic insulator is an intriguing electronic phase of quasi-condensed
excitons. A prominent candidate is the small bandgap semiconductor Ta2NiSe5, in
which excitons are believed to undergo a BEC-like transition. But experimental
evidence for the existence of a coherent condensate in this material is still
missing. A direct fingerprint of such a state would be the observation of its
collective modes, which are equivalent to the Higgs- and Goldstone-modes in
superconductors. Here we report evidence for the existence of a coherent
amplitude response in the excitonic insulator phase of Ta2NiSe5. Using
non-linear excitations with short laser pulses we identify a phonon-coupled
state of the condensate that can be understood as a coupling of its electronic
Higgs-mode to a low frequency phonon. The Higgs-mode contribution substantiates
the picture of an electronically driven phase transition and characterizes the
transient order parameter of the excitonic insulator as a function of
temperature and excitation density.
