Coherent Order Parameter Oscillations in the Ground State of the Excitonic Insulator Ta2NiSe5

TL;DR

This paper provides experimental evidence of collective excitations, including a Higgs-like mode, in the excitonic insulator Ta2NiSe5, demonstrating a coherent order parameter response driven by electronic interactions.

Contribution

It reports the first observation of a Higgs-like collective mode in Ta2NiSe5, linking it to the excitonic condensate and its phase transition.

Findings

Identification of a phonon-coupled Higgs-like mode

Evidence of a transient order parameter in Ta2NiSe5

Coupling between electronic condensate and lattice vibrations

Abstract

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.