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 tau(-1) cannot be accurately described within the framework of standard Drude module in the popular approximation of omega tau << 1, where omega is the signal frequency. To account for the discrepancy, we have extended the classical Drude model for the case of arbitrary values of omega tau, and obtained the expression for tau(-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 tau(-1) in superconducting Ba(Fe1-xCox)(2)As-2 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 omega tau << 1 gives results different from those obtained without this restriction, i.e., incorrect results. (C) 2013 AIP Publishing LLC.