Ultrafast Optical Spectroscopy Evidence of Pseudogap and Electron-Phonon Coupling in an Iron-Based Superconductor KCa${}_2$Fe${}_4$As${}_4$F${}_2$

Abstract

We use ultrafast optical spectroscopy to study the nonequilibrium quasiparticle relaxation dynamics of the 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 $T^{\ast }\approx 50$ K but prior to the opening of a superconducting gap (${\Delta }_{SC}$(0) $\approx$ 4.3 $\pm$ 0.1 meV). Measurements under high pump fluence real two distinct, coherent phonon oscillations with 1.95 and 5.51 THz frequencies, respectively. The high-frequency $A_{1g}$(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 KCa${}_2$Fe${}_4$As${}_4$F${}_2$.

6 pages, 3 figure

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