Cooperative Exciton-Phonon Bose-Einstein Condensation in an Excitonic Insulator

ORAL

Abstract

The excitonic insulator is an exotic phase of matter in which excitons spontaneously form and collectively undergo Bose-Einstein condensation. Recently, increasing evidence has shown that this ground state is stabilized in the layered transition metal chalcogenide Ta2NiSe5 (TNS). Distinctive signature of exciton condensation is the pronounced flattening of the valence band top with decreasing temperature, signaling the opening of an additional many-body gap, as well as a coherent amplitude-like response observed in optical pump-probe data. Due to its direct bandgap, TNS is believed to realize the pure excitonic insulator state, free from the complications of coexisting density-wave orders or strong coupling to other degrees of freedom. Here, we reveal that a cooperative exciton-phonon mechanism lies instead at the origin of the condensate in TNS. Specifically, we use time- and angle-resolved photoemission spectroscopy to show that the vibrational degrees of freedom play a crucial role in the photoinduced melting of the exciton Bose-Einstein condensate. Our results open new routes towards the selective manipulation of the excitonic insulating state via specific modes of the crystal lattice.

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Presenters

  • Edoardo Baldini

    Physics, Massachusetts Institute of Technology, Massachusetts Institute of Technology, Department of Physics, Massachusetts Institute of Technology

Authors

  • Edoardo Baldini

    Physics, Massachusetts Institute of Technology, Massachusetts Institute of Technology, Department of Physics, Massachusetts Institute of Technology

  • Yao Wang

    Harvard University, Department of Physics, Harvard University, Physics, Harvard University

  • Alfred Zong

    Physics, Massachusetts Institute of Technology, Department of Physics, Massachusetts Institute of Technology, Massachusetts Institute of Technology

  • Changmin Lee

    Physics, Massachusetts Institute of Technology, Massachusetts Institute of Technology, Department of Physics, Massachusetts Institute of Technology

  • Debanjan Chowdhury

    Physics, MIT, Department of Physics, Massachusetts Institute of Technology, Massachusetts Institute of Technology

  • Yangfan Lu

    Department of Physics, University of Tokyo

  • Tomohiro Takayama

    Max Planck Institute for Solid State Research, University of Tokyo (Japan)

  • Hidenori Takagi

    Department of Advanced Materials, University of Tokyo, Max Planck Institute for Solid State Research, Physics, University of Tokyo, University of Tokyo (Japan)

  • Eugene Demler

    Harvard University, Department of Physics, Harvard University, Physics, Harvard University

  • Nuh Gedik

    Physics, Massachusetts Institute of Technology, Massachusetts Inst of Tech-MIT, Massachusetts Institute of Technology, Department of Physics, Massachusetts Institute of Technology