Millimeter-wave surface impedance of optimally-doped Ba(Fe1−xCox)2As2single crystals

Abstract

Precision measurements of active and reactive components of in-plane microwave surface impedance were performed in single crystals of optimally-doped Fe-based superconductor Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ ($x=0.074$, ${T}_{c}=22.8$ K). Measurements in a millimeter wavelength range (${K}_{a}$ band, 35--40 GHz) were performed using whispering gallery mode excitations in the ultrahigh quality factor quasioptical sapphire disk resonator with YBa${}_{2}$Cu${}_{2}$O${}_{7}$ superconducting (${T}_{c}=90$ K) end plates. The temperature variation of the London penetration depth is best described by a power-law function, $\ensuremath{\Delta}\ensuremath{\lambda}(T)\ensuremath{\sim}{T}^{n}$, $n=2.8$, in reasonable agreement with radio-frequency measurements on crystals of the same batch. This power-law dependence is characteristic of a nodeless superconducting gap in the extended $s$-wave pairing scenario with a strong pair-breaking scattering. The quasiparticle conductivity of the samples, ${\ensuremath{\sigma}}_{1}(T)$, gradually increases with the decrease of temperature, showing no peak below or at ${T}_{c}$, in notable contrast with the behavior found in the cuprates. The temperature-dependent quasiparticle scattering rate was analyzed in a two-fluid model, assuming the validity of the Drude description of conductivity and generalized expression for the scattering rate. This analysis allows us to estimate the range of the values of a residual surface resistance from 3 to 6 $\mathrm{m}\ensuremath{\Omega}$.