{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"20a4ddfb76d0dc0e99cf291ac984c626946aff243ba90c31caa6f60e5be6a7a8","created":"2026-10-03T07:17:59Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"dc0ad0b8a50d311c13cd09b08a8776ddca1eae604c10c5304e72f3e54e0bc0e7","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"identification","assisted_by":[],"external_id":"cm.excitonic-insulators.tise2-cdw-origin","kind":"phenomenon","literature_status":"contested","n":"1","parents":[],"plain":"TiSe2 forms a charge-density wave (a periodic ripple of electron density and atom positions) below about 200 K. One experiment reported a softening electronic mode pointing to exciton condensation, while a 2025 calculation concluded that lattice fluctuations alone open the gap.","posed_since":"","precise":"1T-TiSe2 forms a commensurate $2x2x2$ charge-density wave at $T_{\\mathrm{CDW}} \\sim 200\\,\\mathrm{K}$. Determine whether the transition is driven by an excitonic instability between the Se 4p hole pocket and Ti 3d electron pockets (soft plasmon at the ordering wavevector in momentum-resolved electron energy-loss spectroscopy, Kogar et al. 2017) or by an electron-phonon (band Jahn-Teller) instability, quantified by the $T_{\\mathrm{CDW}}$ obtained with and without the electron-hole ladder vertex at fixed lattice. An answer is the driving instability with $T_{\\mathrm{CDW}}$ computed in both channels.","problem_ref":null,"references":"","settled_by":"An independent momentum-resolved measurement of the electronic collective mode through $T_{\\mathrm{CDW}}$ together with a many-body calculation reproducing $T_{\\mathrm{CDW}}$ from one channel alone.","status_note":"A 2025 quasiparticle self-consistent many-body calculation (Pashov et al., npj Computational Materials) concluded that TiSe2 is a band insulator produced by lattice fluctuations, against the 2017 soft-plasmon interpretation.","title":"Is the charge-density wave of 1T-TiSe2 an exciton 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