Journal article

Fate of the excitonic insulator in the presence of phonons

  • Zenker, B. Institut für Physik, Ernst-Moritz-Arndt-Universität Greifswald, Germany
  • Fehske, H. Institut für Physik, Ernst-Moritz-Arndt-Universität Greifswald, Germany
  • Beck, Hans Département de Physique and Fribourg Center for Nanomaterials, Université de Fribourg, Switzerland
Published in:
  • Physical Review B. - 2014, vol. 90, no. 19, p. 195118
English The influence of phonons on the formation of the excitonic insulator has hardly been analyzed so far. Recent experiments on Ta2NiSe5,1T−TiSe2, and TmSe0.45Te0.55, being candidates for realizing the excitonic-insulator state, suggest, however, that the underlying lattice plays a significant role. Employing the Kadanoff-Baym approach we address this issue theoretically. We show that owing to the electron-phonon coupling a static lattice distortion may arise at the excitonic instability. Most importantly such a distortion will destroy the acoustic phase mode being present if the electron-hole pairing and condensation is exclusively driven by the Coulomb interaction. The absence of off-diagonal long-range order, when lattice degrees of freedom are involved, challenges that excitons in these materials form a superfluid condensate of Bose particles or Cooper pairs composed of electrons and holes.
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