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Coherent order parameter oscillations in the ground state of the excitonic insulator Ta2NiSe5

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Manske,  D.
Department Quantum Many-Body Theory (Walter Metzner), Max Planck Institute for Solid State Research, Max Planck Society;

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Takagi,  H.
Department Quantum Materials (Hidenori Takagi), Max Planck Institute for Solid State Research, Max Planck Society;

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Kaiser,  S.
Department Solid State Spectroscopy (Bernhard Keimer), Max Planck Institute for Solid State Research, Max Planck Society;
Former Research Groups, Max Planck Institute for Solid State Research, Max Planck Society;

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Citation

Werdehausen, D., Takayama, T., Höppner, M., Albrecht, G., Rost, A. W., Lu, Y., et al. (2018). Coherent order parameter oscillations in the ground state of the excitonic insulator Ta2NiSe5. Science Advances, 4(3): eaap8652.


Cite as: https://hdl.handle.net/21.11116/0000-000E-D886-7
Abstract
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 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.