Magnetic penetration depth and flux-flow resistivity measurements on NaFe0.97Co0.03As single crystals

• We measure the microwave-surface impedance under finite magnetic fields.
• The penetration depth indicates the realization of the gapless superconductivity.
• NaFe0.97Co0.03As single crystals have the moderately clean ore.
• The flux-flow resistivity of NaFe0.97Co0.03As shows Bardeen–Stephen like behavior.

We measured the surface impedance of NaFe1−xCoxAs (x ≈ 0.03, optimally doped) single crystals under finite magnetic fields. At low temperatures (T < 0.5Tc), 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, ρf, behaved similarly to the Bardeen–Stephen prediction, ρf/ρn = B/Bc2. However, the electronic state inside the vortex core was not so dirty. By comparing the results with those of LiFeAs1−xPx and SrFe2(As0.7P0.3)2, it has been clarified that the gap anisotropy dominates the gradient of ρf(B).