Abstract

Single crystals of the oxypnictide superconductors SmFeAsO0.8F0.2 and NdFeAsO0.8F0.2 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.