{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"c36145b4536e12588da1a0a6c503ae213b4cea91919145acc57d58daac319ab9","created":"2026-10-03T07:17:59Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"d50183d3a0aff2b1c181a11556a0962579fa401587d6a9256ba4efc50aadae7b","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":"1","parents":[],"plain":"Ta2NiSe5 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":"Ta2NiSe5 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":null,"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 Ta2NiSe5 excitonic or lattice 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