Is the gap of Ta_2NiSe_5 excitonic or lattice driven? · revision history

Revision 2

Accepted · core change · record

Changes against revision 1:

Title · changed
Revision 1: Is the gap of Ta2NiSe5 excitonic or lattice driven
Revision 2: Is the gap of $\mathrm{Ta_2NiSe_5}$ excitonic or lattice driven?
Plain · changed
Revision 1: 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.
Revision 2: $\mathrm{Ta_2NiSe_5}$ 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.
Precise · changed · core
Revision 1: 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.
Revision 2: $\mathrm{Ta_2NiSe_5}$ 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.
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)

Decisions