Coherent Order Parameter Oscillations in the Ground State of the   Excitonic Insulator Ta2NiSe5
  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. 
Daniel Werdehausen, Tomohiro Takayama, Marc H\"oppner, Gelon Albrecht,   Andreas W. Rost, Yangfan Lu, Dirk Manske, Hidenori ...
cond-mat.str-el