Superconducting Stiffness and Coherence Length of FeSe_0.5Te_0.5 Measured in a Zero-Applied Field

Superconducting stiffness rho(s) and coherence length xi are usually determined by measuring the penetration depth lambda of a magnetic field and the upper critical field H-c2 of a superconductor (SC), respectively. However, in magnetic SC, which is iron-based, this could lead to erroneous results, since the internal field could be very different from the applied one. To overcome this problem in Fe1+ySexTe1-x with x similar to 0.5 and y similar to 0 (FST), we measured both quantities with the Stiffnessometer technique. In this technique, one applies a rotor-free vector potential A to a superconducting ring and measures the current density j via the ring's magnetic moment m. rho(s) and xi are determined from London's equation, j = -rho(s)A, and its range of validity. This method is particularly accurate at temperatures close to the critical temperature T-c. We find weaker rho(s) and longer xi than existing literature reports, and critical exponents which agree better with expectations based on the Ginzburg-Landau theory.