Coherent order parameter oscillations in the ground state of the excitonic insulator Ta<sub>2</sub>NiSe<sub>5</sub>.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 29740599.
- Also identified by DOI 10.1126/sciadv.aap8652 and PMC identifier 5938280.
- Licence recorded as CC BY-NC.
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Abstract
The excitonic insulator is an intriguing electronic phase of condensed excitons. A prominent candidate is the small bandgap semiconductor Ta<sub>2</sub>NiSe<sub>5</sub>, in which excitons are believed to undergo a Bose-Einstein condensation-like transition. However, direct 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. We report evidence for the existence of a coherent amplitude response in the excitonic insulator phase of Ta<sub>2</sub>NiSe<sub>5</sub>. Using nonlinear excitations with short laser pulses, we identify a phonon-coupled state of the condensate that can be understood as a novel amplitude mode. The condensate density 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.