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Title: Is the gap of Ta2NiSe5 excitonic or lattice driven
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.
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.
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.
Kind: phenomenon
Answer type: identification
References: (empty)
Literature status: contested
Status note: Raman, photoemission and resonant x-ray studies published through 2025 still support both a dominant excitonic and a dominant structural instability.
Posed since: (empty)