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

Superconducting stiffness ρ_s and coherence length ξ are usually determined by measuring the penetration depth λ of a magnetic field and the upper critical field H_c2 of a superconductor (SC), respectively. However, in magnetic SC, e.g. some of the 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 Fe_1+ySe_xTe_1-x with x~0.5 and y~0 (FST), we measure 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. ρ_s and ξ are determined from London’s equation j=- ρ_sA and its range of validity. This method is particularly accurate at temperatures close to the critical temperature T_c. We find weaker ρ_s and longer ξ than existing literature reports, and critical exponents which agree better with expectations based on the Ginzburg–Landau theory.