Temperature Dependent Anisotropy of Oxypnictide Superconductors Studied by Torque Magnetometry

Single crystals of different oxypnictide superconductors of the family ReFeAsO$_{1-x}$F$_{y}$ (Re $=$ Sm, Nd, Pr) with various carrier dopings and with masses $m\simeq100$ 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 $\gamma$ and the in-plane magnetic penetration depth $\lambda_{ab}$ were extracted. As an important result $\gamma$ was found to increase strongly as the temperature is decreased from $T_c$ down to low temperatures. This unconventional temperature dependence of $\gamma$ is similar to that observed in the two-band superconductor MgB$_2$ 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.