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

Abstract Measurements of the temperature dependence of the in-plane magnetic penetration depth, Δ λ a b ( T ) = λ a b ( T ) − λ a b ( 0 ) , are presented in the isovalently substituted iron pnictide BaFe 2 (As 1 − x P x ) 2 near optimal doping ( T c ∼ 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 T c ) our data confirm the linear temperature dependence of λ ab ( T ) observed by using other procedures, and that is consistent with a nodal superconducting order parameter. By using values for λ ab (0) in the literature we obtained the temperature dependence up to T c of the superfluid density, n s ( T ) ∝ 1 / λ a b 2 ( T ) . It is found that samples with slightly different T c values present a significantly different qualitative behavior of n s ( T ).