Interplay of electron-phonon coupling, pseudogap, and superconductivity in CsCa${}_2$Fe${}_4$As${}_4$F${}_2$ studied using ultrafast optical spectroscopy

The quasiparticle relaxation dynamics of the iron-based superconductor CsCa${}_2$Fe${}_4$As${}_4$F${}_2$ ($T_c$ $\sim$ 29 K) were investigated using ultrafast optical spectroscopy. A pseudogap (${\mathrm{\Delta }}_{PG}$ $\approx$ 3.3 meV) was observed to open below $T^{\ast }$ $\approx$ 60 K, prior to the emergence of a superconducting gap ($\mathrm{\Delta }$ $\approx$ 6.6 $\pm$ 0.4 meV). At high excitation fluence, a coherent A1g phonon mode at 5.49 THz was identified, exhibiting deviations from anharmonic behavior below $T_c$. The electron-phonon coupling constant for this mode was estimated to be ${\lambda }_{A_{1g}}$ $\approx$ 0.23. These results provide insights into the interplay between the electron-phonon interactions, pseudogap, and the superconducting pairing mechanism in CsCa${}_2$Fe${}_4$As${}_4$F${}_2$.