Ultrafast dynamics and coherent order parameter oscillations under photo-excitation in the excitonic insulator Ta_2NiSe_5

摘要

The excitonic insulator (EI) is an intriguing phase of condensed excitons undergoing a Bose-Einstein-Condensation (BEC)-type transition. A prominent candidate has been identified in Ta_2NiSe_5. Ultrafast spectroscopy allows tracing the coherent response of the El condensate directly in the time domain. Probing the collective electronic response we can identify fingerprints for the Higgs-amplitude equivalent mode of the condensate. In addition we find a peculiar coupling of the El phase to a low frequency phonon mode. We will discuss the transient response on multiple energies scales ranging from the exciton dynamics to the coherent THz response of the gap.