We report microwave surface impedances of FeSe$_{0.4}$Te$_{0.6}$ single crystals measured at 12, 19, and 44 GHz. The penetration depth exhibits a power law behavior, $delta lambda_L=lambda_L (T)-lambda_L (0) propto CT^n$ with an exponent $nsimeq 2$,
which is considered to result from impurity scattering. This behavior is consistent with $spm$-wave pairing symmetry. 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.
The Hall effect is investigated in thin-film samples of iron-chalcogenide superconductors in detail. The Hall coefficient (RH) of FeTe and Fe(Se1-xTex) exhibits a similar positive value around 300 K, indicating that the high-temperature normal state
is dominated by hole-channel transport. FeTe exhibits a sign reversal from positive to negative across the transition to the low-temperature antiferromagnetic state, indicating the occurrence of drastic reconstruction in the band structure. The mobility analysis using the carrier density theoretically calculated reveals that the mobility of holes is strongly suppressed to zero, and hence the electric transport looks to be dominated by electrons. The Se substitution to Te suppresses the antiferromagnetic long-range order and induces superconductivity instead. The similar mobility analysis for Fe(Se0.4Te0.6) and Fe(Se0.5Te0.5) thin films shows that the mobility of electrons increases with decreasing temperature even in the paramagnetic state, and keeps sufficiently high values down to the superconducting transition temperature. From the comparison between FeTe and Fe(Se1-xTex), it is suggested that the coexistence of itinerant carriers both in electron and hole channels is indispensable for the occurrence of superconductivity.
