Direct measurement of the temperature dependence of the in-plane magnetic penetration depth in optimally doped BaFe2(As1−xPx)2 single crystals

Measurements of the temperature dependence of the in-plane magnetic penetration depth, Δλab(T)=λab(T)−λab(0), are presented in the isovalently substituted iron pnictide BaFe2(As1−xPx)2 near optimal doping (Tc ∼ 28K). The data were obtained directly from the shielding magnetic susceptibility of thin single crystals (∼ 20 µm thickness along the crystals c-axis) under magnetic fields parallel to the ab layers. Complications associated to flux penetration were avoided by using fields in the 10−4 T range, which ensured that the samples were well inside the Meissner region. At low temperatures (T < 0.25Tc) our data confirm the linear temperature dependence of λab(T) observed by using other procedures, and that is consistent with a nodal superconducting order parameter. By using values for λab(0) in the literature we obtained the temperature dependence up to Tc of the superfluid density, ns(T)∝1/λab2(T). It is found that samples with slightly different Tc values present a significantly different qualitative behavior of ns(T).