Anomalous phonon behavior in superconducting CaKFe4As4 : An optical study

Abstract

The temperature dependence of $ab$-plane optical conductivity of ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$ has been measured below and above its superconducting transition temperature ${T}_{c}\ensuremath{\simeq}35.5$ K. In the normal state, analysis with the two-Drude model reveals a $T$-linear scattering rate for the coherent response, which suggests strong spin-fluctuation scattering. Below the superconducting transition, the optical conductivity below 120 ${\mathrm{cm}}^{\ensuremath{-}1}$ vanishes, indicating nodeless gap(s). The Mattis-Bardeen fitting in the superconducting state gives two gaps of ${\mathrm{\ensuremath{\Delta}}}_{1}\ensuremath{\simeq}9$ meV and ${\mathrm{\ensuremath{\Delta}}}_{2}\ensuremath{\simeq}14$ meV, in good agreement with recent angle-resolved photoemission spectroscopy (ARPES) results. In addition, around 255 ${\mathrm{cm}}^{\ensuremath{-}1}$, we observe two different infrared-active Fe-As modes with obvious asymmetric lineshape, originating from strong coupling between lattice vibrations and spin or charge excitations. Considering a moderate Hund's rule coupling determined from spectral weight analysis, we propose that the strong fluctuations induced by the coupling between itinerant carriers and local moments may affect the phonon mode, and the electron-phonon coupling through the spin channel is likely to play an important role in the unconventional pairing in iron-based superconductors.