Evidence for Two Distinct Anisotropies in the Oxypnictide Superconductors SmFeAsO0.8F0.2 and NdFeAsO0.8F0.2

Single crystals of the oxypnictide superconductors SmFeAsO_0.8F_0.2 and NdFeAsO_0.8F_0.2 with T _c in the range of 44 to 48 K were investigated by torque magnetometry. An analysis of the data in terms of a recently proposed model for the anisotropic magnetization in the superconducting state, treating the magnetic penetration depth anisotropy γ _λ differently than the upper critical field anisotropy γ _H , provides evidence that in the oxypnictide superconductors two distinct anisotropies are present. As a result γ _λ differs significantly in magnitude and in temperature dependence from γ _H , analogous to MgB₂ but with a reversed sign of slope. This scenario strongly suggests a new multi-band mechanism in the novel class of oxypnictide high-temperature superconductors.