The chicken and egg question in excitonic insulators
1. Nature of symmetry breaking at the excitonic insulator transition: Ta2NiSe5
Authors: G. Mazza, M. Rösner, L. Windgätter, S. Latini, H. Hübener, A. J. Millis, A. Rubio, and A. Georges
Phys. Rev. Lett. 124, 197601 (2020); DOI: 10.1103/PhysRevLett.124.197601
2. Nonlinear spectroscopy of collective modes in excitonic insulator
Authors: D. Golež, Z. Sun, Y. Murakami, A. Georges, and A. J. Millis
arXiv:2007.09749
Recommended with a commentary by Atsushi Fujimori, Waseda University
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This commentary may be cited as:
DOI: 10.36471/JCCM_August_2020_01
https://doi.org/10.36471/JCCM_August_2020_01
I really enjoyed this clear and insightful summary of the latest on the excitonic vs. lattice controversy in Ta2NiSe5. In my opinion there is a more fundamental question we should also be trying to answer: Under what circumstances does a Bose condensate of excitons behave like a superfluid, and when is it an insulator? Many manuscripts offer conflicting accounts of this point. Some make arguments about the electron-phonon interaction and gapping of collective modes, as above. Others give platitudes about the requirement of separate particle number conservation for both electrons and holes. It would be nice to have a universal explanation based on fundamental principles that also clarifies the relationship between spontaneous excitonic phases in materials like Ta2NiSe5 and TiSe2, and those in artificial devices like quantum Hall and graphene bilayers (in which, one should note, the electron-phonon interaction is not “small”).