Is superconducting $\mathrm{FeSe}1-x\mathrm{S}x$ in the BCS-BEC crossover regime
In plain words
In FeSe the energy needed to break a pair is comparable to the energy of the fastest electrons, which is unusual. This hints that its pairs are small and tightly bound, between ordinary weakly bound pairs and molecule-like bosons.
Precise statement
FeSe and FeSe1-xSx have $\Delta/E_{F}$ of order 0.3 to 1 on some pockets (Kasahara et al., PNAS 2014). Determine their place on the BCS-BEC crossover, parameterized by $1/(k_{F} \xi)$ per pocket ($\xi$ the pair size), by testing for preformed pairs above $T_{c}$ (pseudogap, large superconducting fluctuations), for the shape of the Bogoliubov dispersion (back-bending at $k_{F}$ or minimum at $k = 0$), and whether tetragonal FeSe1-xSx with $x > 0.17$ is BEC-like. An answer is a classification per pocket with consistent spectroscopic and thermodynamic evidence.
What would settle it
Photoemission of the Bogoliubov dispersion together with thermodynamic detection or exclusion of preformed pairs above $T_c$ in the same crystals.
Status in the literature
Photoemission around 2020 reported BEC-like dispersion in tetragonal $\mathrm{FeSe}1-x\mathrm{S}x$, while the expected large pair fluctuations above $T_c$ remain disputed.