search for optical signatures of the excitonic insulator phase*

The excitonic insulator (EI) is an interesting phase of matter where electron-hole pairs form a Bose-Einstein-like condensate. We report the low energy dynamics of Ta₂NiSe₅, a candidate EI material with T_c = 328 K. This is accomplished using ultrafast near-infrared pump-broadband THz probe spectroscopy. Our broadband probe enables simultaneous measurement of electron and phonon dynamics relevant to the EI state. We will compare the dynamics in with its sister compound Ta₂NiS₅, an isoelectric semiconductor, with a view towards disentangling dynamics associated with conventional semiconducting behavior (and the monoiclinic-orthorhombic structural transition) from pure EI order parameter dynamics.