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

Superconducting stiffness (Formula presented.) and coherence length (Formula presented.) are usually determined by measuring the penetration depth (Formula presented.) of a magnetic field and the upper critical field (Formula presented.) 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 Fe (Formula presented.) Se (Formula presented.) Te (Formula presented.) with (Formula presented.) and (Formula presented.) (FST), we measured both quantities with the Stiffnessometer technique. In this technique, one applies a rotor-free vector potential (Formula presented.) to a superconducting ring and measures the current density (Formula presented.) via the ring’s magnetic moment (Formula presented.). (Formula presented.) and (Formula presented.) are determined from London’s equation, (Formula presented.), and its range of validity. This method is particularly accurate at temperatures close to the critical temperature (Formula presented.). We find weaker (Formula presented.) and longer (Formula presented.) than existing literature reports, and critical exponents which agree better with expectations based on the Ginzburg–Landau theory.