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(FeCo)As (, K). Measurements in a millimeter wavelength range ( band, 35–40 GHz) were performed using whispering gallery mode excitations in the ultrahigh quality factor quasioptical sapphire disk resonator with YBaCuO superconducting ( K) end plates. The temperature variation of the London penetration depth is best described by a power-law function, , , 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 -wave pairing scenario with a strong pair-breaking scattering. The quasiparticle conductivity of the samples, , gradually increases with the decrease of temperature, showing no peak below or at , 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 .
- Received 13 May 2012
DOI:https://doi.org/10.1103/PhysRevB.87.014506
©2013 American Physical Society