Direct measurement of the temperature dependence of the magnetic penetration depth in Ba(Fe1−xNix)2As2 superconductors

The temperature dependence of the in-plane magnetic penetration depth lambda(ab) of Ba(Fe1-xNix)(2)As-2 single crystals is determined directly from the shielding magnetic susceptibility, measured in the Meissner region with the field parallel to the ab layers. The doping levels studied cover the underdoped, optimally doped and overdoped regimes. At temperatures below 0.5T(c) a well-defined power-law behavior lambda(ab)(T) - lambda(ab) (0) = AT(n) (with n approximate to 2.5) is observed. At lower temperatures (T < 0.3T(c)) the data are still consistent with n = 2 and A alpha T-c(-3), as predicted by the strong pair-breaking scenario proposed by Gordon et al (2010 Phys. Rev. B 81, 180501(R)). The temperature dependence of the superfluid density rho(s) alpha lambda(-2)(ab) presents a marked positive curvature just below T-c, which is a sign of two-gap superconductivity. The analysis of rho(s) (T) in terms of a two-gap model allowed estimation of parameters like the in-band and inter-band couplings, the relative weight of each band, and their dependence on the doping level. A comparison with rho(s) (T) data obtained by using other techniques in compounds with a similar composition is also presented.