Anisotropic London penetration depth and superfluid density in single crystals of iron-based pnictide superconductors

In- and out-of-plane magnetic penetration depths were measured in three iron-based pnictide superconducting systems. The “122” system was represented by electron-doped Ba(Fe1−xCox)2As2 with the doping through the whole phase diagram with x≈0.038, 0.047, 0.058, 0.074 and 0.10 (Tc ranged from 13 to 24K) and by hole-doped (Ba1−xKx)Fe2As2 with doping close to optimal, with measured x≈0.45 (Tc≈28K) and an underdoped sample with x≈0.15 (Tc≈19K). The “1111” system was represented by single crystals of NdFeAs(O1−xFx) with nominal x=0.1 (Tc≈43K). All studied samples of both 122 systems show a robust power-law behavior, λ(T)∝Tn, with the sample-dependent exponent n=2–2.5, which is indicative of unconventional pairing. This scenario could be possible either through scattering in a S± state or due to nodes in the superconducting gap. In the Nd-1111 system, the interpretation of the results is complicated by magnetism of the rare-earth ions. For all three systems, the anisotropy ratio, γλ≡λc/λab, was found to decrease with increasing temperature, whereas the anisotropy of the coherence lengths, γξ≡ξab/ξc=Hc2⊥c/Hc2‖c, has been found to increase (both opposite to the trend in two-band MgB2). The overall anisotropy of the pnictide superconductors is small, in fact much smaller than that of the cuprates (except YBa2Cu3O7−x (YBCO)). The 1111 system is about two times more anisotropic than the 122 system. Our data and analysis suggest that the iron-based pnictides are complex superconductors in which a multiband three-dimensional electronic structure and strong magnetic fluctuations play important roles.