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 . https://doi.org/10.1126/sciadv.aap8652
Copyright © 2018 The Authors. This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.
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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