On the determination of the quasiparticle scattering rate in unconventional superconductors by microwave surface impedance
As found from numerous microwave experiments on the unconventional Fe-based superconductors, the temperature dependence of the quasiparticle scattering rate τ−1 cannot be accurately described within the framework of standard Drude module in the popular approximation of ωτ ≪ 1, where ω is the signal frequency. To account for the discrepancy, we have extended the classical Drude model for the case of arbitrary values of ωτ, and obtained the expression for τ−1 as a function of experimentally measurable quantities, namely the real and imaginary parts of the microwave surface impedance. We then show the temperature dependence of τ−1 in superconducting Ba(Fe1-xCox)2As2 single crystal pnictide derived from the Ka-band surface impedance measurements within the framework of the modified expression. The measurements indicate the extent to which assumption of ωτ ≪ 1 gives results different from those obtained without this restriction, i.e., incorrect results.