Is the charge-density wave of 1T-TiSe2 an exciton condensate
In plain words
TiSe2 forms a charge-density wave (a periodic ripple of electron density and atom positions) below about 200 K. One experiment reported a softening electronic mode pointing to exciton condensation, while a 2025 calculation concluded that lattice fluctuations alone open the gap.
Precise statement
1T-TiSe2 forms a commensurate $2x2x2$ charge-density wave at $T_{\mathrm{CDW}} \sim 200\,\mathrm{K}$. Determine whether the transition is driven by an excitonic instability between the Se 4p hole pocket and Ti 3d electron pockets (soft plasmon at the ordering wavevector in momentum-resolved electron energy-loss spectroscopy, Kogar et al. 2017) or by an electron-phonon (band Jahn-Teller) instability, quantified by the $T_{\mathrm{CDW}}$ obtained with and without the electron-hole ladder vertex at fixed lattice. An answer is the driving instability with $T_{\mathrm{CDW}}$ computed in both channels.
What would settle it
An independent momentum-resolved measurement of the electronic collective mode through $T_{\mathrm{CDW}}$ together with a many-body calculation reproducing $T_{\mathrm{CDW}}$ from one channel alone.
Status in the literature
Unverified note
A 2025 quasiparticle self-consistent many-body calculation (Pashov et al., npj Computational Materials) concluded that TiSe2 is a band insulator produced by lattice fluctuations, against the 2017 soft-plasmon interpretation.