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 the iron-based superconductor KCa2Fe4As4F2 with Tc = 33.5 K. Our results reveal a possible pseudogap (ΔPG = (2.4 ± 0.1) meV) below T* ≈ 50 K but prior to the opening of a superconducting gap (ΔSC(0) ≈ (4.3 ± 0.1) meV). Measurements under high pump fluence reveal two distinct, coherent phonon oscillations with 1.95 and 5.51 THz frequencies, respectively. The high-frequency A1g(2) mode corresponds to the c-axis polarized vibrations of FeAs planes with a nominal electron-phonon coupling constant $${\lambda _{{A_{1g}}(2)}} = 0.194 \pm 0.02$$ . Our findings suggest that the pseudogap is likely a precursor of superconductivity, and the electron-phonon coupling may play an essential role in the superconducting pairing in KCa2Fe4As4F2.