Is the gap of Ta2NiSe5 excitonic or lattice driven
In plain words
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 statement
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.
What would settle it
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 in the literature
Unverified note
Raman, photoemission and resonant x-ray studies published through 2025 still support both a dominant excitonic and a dominant structural instability.