{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"67d88c5baf4e26bd1068984f78a10d6885868cad234c7f8e30ae07e8185cca17","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5d3ad13082197792588981e48b806f9e2cdedf14bb14dc067eb0e1bdf4ef7a5d","ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b"],"salt":"39a149894e420ac0661c77d0b5d790e4e67a16ca66d364dfd0fded1206deb6d1","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"identification","assisted_by":[],"external_id":"cm.excitonic-insulators.ta2nise5-gap-origin","kind":"phenomenon","literature_status":"contested","n":"2","parents":[],"plain":"$\\mathrm{Ta_2NiSe_5}$ becomes insulating below about 326 K, and at the same temperature its crystal distorts slightly. It is unclear whether electron-hole attraction or the lattice distortion opens the gap.","posed_since":"","precise":"$\\mathrm{Ta_2NiSe_5}$ has an orthorhombic-to-monoclinic transition at $T_c \\sim 326\\,\\mathrm{K}$ with valence-band flattening and a gap of order 0.1 eV seen in photoemission; the excitonic and structural order parameters have the same symmetry and so coexist. Determine which instability drives the transition, i.e. whether a purely electronic (excitonic) susceptibility diverges at $T_c$ with the lattice frozen, or whether the cooperative zone-center phonon distortion is primary, and what fraction of the low-temperature gap each produces. An answer is an identified primary order parameter with a quantitative partition of the gap.","problem_ref":"5d3ad13082197792588981e48b806f9e2cdedf14bb14dc067eb0e1bdf4ef7a5d","references":"","settled_by":"Observation or exclusion of a soft electronic collective mode at $T_{c}$ by Raman or resonant inelastic x-ray scattering, combined with a frozen-lattice many-body computation of the electronic susceptibility.","status_note":"Raman, photoemission and resonant x-ray studies published through 2025 still support both a dominant excitonic and a dominant structural instability.","title":"Is the gap of $\\mathrm{Ta_2NiSe_5}$ excitonic or lattice 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