Magnetic penetration depth and flux-flow resistivity measurements on NaFe_0.97Co_0.03As single crystals

We measured the surface impedance of NaFe_1-xCo_xAs (x ≈ 0.03, optimally doped) single crystals under finite magnetic fields. At low temperatures (T < 0.5T_c), the penetration depth of these crystals was increased as λ(T) - λ(0) = A × Tⁿ with an exponent n ≈ 2, indicating the realization of the gapless superconductivity. The flux-flow resistivity, ρ_f, behaved similarly to the Bardeen-Stephen prediction, ρ_f/ρ_n = B/B _c2. However, the electronic state inside the vortex core was not so dirty. By comparing the results with those of LiFeAs_1-xP_x and SrFe₂(As_0.7P_0.3)₂, it has been clarified that the gap anisotropy dominates the gradient of ρ_f(B) .