Anomalous temperature dependence of the superfluid density caused by a dirty-to-clean crossover in superconducting FeSe_0.4Te_0.6 single crystals

We report microwave surface impedances of FeSe_0.4Te _0.6 single crystals measured at 12, 19, and 44 GHz. The penetration depth exhibits a power-law behavior, δλ_L= λ_L(T)-λ_L(0) CTn with an exponent n2, which is considered to result from impurity scattering. The temperature dependence of the superfluid density largely deviates from the behavior expected in the BCS theory. We believe that this deviation is caused by the crossover from the dirty regime near T_c to the clean regime at low temperatures, which is supported by the rapid increase of the quasiparticle scattering time obtained from the microwave conductivity. We also believe that the previously published data of the superfluid density can be interpreted in this scenario.