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arXiv · 1407.6059

Fate of the excitonic insulator in the presence of phonons

B. Zenker·H. Fehske·H. Beck
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Abstract

The influence of phonons on the formation of the excitonic insulator has hardly been analyzed so far. Recent experiments on $\rm Ta_2NiSe_5$, 1$T$-$\rm TiSe_2$, and $\rm TmSe_{0.45}Te_{0.55}$, 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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B. Zenker, H. Fehske, H. Beck. 2014-07-22. Fate of the excitonic insulator in the presence of phonons. https://doi.org/10.1103/physrevb.90.195118

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