Temperature Dependent Anisotropy of Oxypnictide Superconductors Studied by Torque Magnetometry

Single crystals of different oxypnictide superconductors of the family ReFeAsO_1-xF_y (Re = Sm, Nd, Pr) with various carrier dopings and with masses m∼100 ng have been investigated by means of torque magnetometry. We present most recent data, obtained by using highly sensitive piezoresistive torque sensors from which the superconducting anisotropy parameter γ and the in-plane magnetic penetration depth λ_ab were extracted. As an important result γ was found to increase strongly as the temperature is decreased from T_c down to low temperatures. This unconventional temperature dependence of γ is similar to that observed in the two-band superconductor MgB₂ and cannot be explained within the classical Ginzburg-Landau model. This scenario strongly suggests a new multi-band mechanism in the novel class of oxypnictide high-temperature superconductors.