Ultrafast optical spectroscopy evidence of pseudogap and electron-phonon coupling in an iron-based superconductor KCa2Fe4As4F2

We use ultrafast optical spectroscopy to study the nonequilibrium quasiparticle relaxation dynamics of iron-based superconductor KCa$_2$Fe$_4$As$_4$F$_2$ with $T_c=33.5$ K. Our results reveal a possible pseudogap ($\Delta_{PG}$ = 2.4 $\pm$ 0.1 meV) below $T^*\approx 50$ K but prior opening superconducting gap ($\Delta_{SC}$(0) $\approx$ 4.3 meV). Measurements under high pump fluence real two distinct, coherent phonon oscillations 1.95 and 5.51 THz frequencies, respectively. The high-frequency $A_{1g}$(2) mode corresponds $c-$axis polarized vibrations FeAs planes nominal electron-phonon coupling constant $\lambda_{A_{1g}(2)}$ 0.194 0.02. findings suggest that is likely precursor superconductivity, may play an essential role in pairing KCa$_2$Fe$_4$As$_4$F$_2$.