Millimeter-wave surface impedance of optimally-doped Ba(Fe1-xCox)2As2 single crystals

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(Fe1-xCox)(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 YBa2Cu2O7 superconducting (T-c = 90 K) end plates. The temperature variation of the London penetration depth is best described by a power-law function, Lambda lambda(T) similar to 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, 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 m Omega. DOI: 10.1103/PhysRevB.87.014506