Coherent order parameter oscillations in the ground state of the excitonic insulator Ta2NiSe5
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The excitonic insulator is an intriguing electronic phase of condensed excitons. A prominent candidate is the small bandgap semiconductor Ta2NiSe5, 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 Ta2NiSe5. Using nonlinear excitations with short laser pulses, we identify a phonon-coupled state of the condensate that can be understood as a novel amplitudemode. 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.
Werdehausen , D , Takayama , T , Höppner , M , Albrecht , G , Rost , A W , Lu , Y , Manske , D , Takagi , H & Kaiser , S 2018 , ' Coherent order parameter oscillations in the ground state of the excitonic insulator Ta 2 NiSe 5 ' Science Advances , vol. 4 , no. 3 , eaap8652 . DOI: 10.1126/sciadv.aap8652
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DescriptionS.K. acknowledges support by the Ministerium für Wissenschaft, Forschung und Kunst Baden-Württemberg through the Juniorprofessuren-Programm and a fellowship by the Daimler und Benz Stiftung.
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