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 ${\mathrm{CsCa}}₂{\mathrm{Fe}}₄{\mathrm{As}}₄{\mathrm{F}}₂$ (${T}_c\ensuremath{\sim}29$ K) were investigated using ultrafast optical spectroscopy. A pseudogap (${\mathrm{\ensuremath{\Delta}}}_{\text{PG}}=3.3\ifmmode\pm\else\textpm\fi{}0.3$ meV) was observed to open below ${T}^*\ensuremath{\approx}60$ K, prior to the emergence of a superconducting gap [$\mathrm{\ensuremath{\Delta}}(0)=6.6\ifmmode\pm\else\textpm\fi{}0.4$ meV]. At high excitation fluence, a coherent ${A}_1g$ 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 ${\ensuremath{\lambda}}_{{A}_1g}\ensuremath{\approx}0.23$. These results provide insights into the interplay between the electron-phonon interactions, pseudogap, and the superconducting pairing mechanism in ${\mathrm{CsCa}}₂{\mathrm{Fe}}₄{\mathrm{As}}₄{\mathrm{F}}₂$.