Microwave impedance properties of single crystal Ba(Fe1−XCoX)2As2

The authors reported out microwave surface impedance measurements of the optimally doped single crystal Ba(Fe_1-xCo_x)₂As₂ (x=0.074) with critical temperature Tc=22.8K and found the power-law exponent 2.8 in temperature dependence of a field penetration depth. The obtained dependence is similar to however noticeably distinguished from radiofrequency range measurements. Thus, conclusion that nodal states are excluded in this material in favor of the extended s± pairing symmetry having a node less order parameter, becomes even more justified. The temperature dependence [λ(0)/λ(T)]² calculated for two temperature laws, namely, for a power law Δλ(T) ~ T2.8 and exponential law for superconductor with a small gap Δ=0.73 can be an additional confirmation of the above mentioned conclusion. However the fact of proximity of two temperature laws indicates that the additional and systematic studies are necessary for establishing the unambiguous conclusion concerning temperature dependence of field penetration depth and superfluid density.