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)(2)As-2 with the doping through the whole phase diagram with x approximate to 0.038, 0.047, 0.058, 0.074 and 0.10 (T-c ranged from 13 to 24 K) and by hole-doped (Ba1-xKx)Fe2As2 with doping close to optimal, with measured x approximate to 0.45 (T-c approximate to 28 K) and an underdoped sample with x approximate to 0.15 (T-c approximate to 19 K). The 1111 system was represented by single crystals of NdFeAs(O1-xFx) with nominal x=0.1 (T-c approximate to 43 K). All studied samples of both 122 systems show a robust power-law behavior, lambda(T) proportional to T-n, 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, gamma(lambda) equivalent to lambda(c)/lambda(ab), was found to decrease with increasing temperature, whereas the anisotropy of the coherence lengths, gamma(zeta) equivalent to xi(ab)/xi(c) = H-c2(perpendicular to c)/H-c2(parallel to 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. (C) 2009 Elsevier B.V. All rights reserved.