Critical current density: Measurements vs. reality

Institute for Superconducting and Electronic Materials, University of Wollongong - Northfields Avenue, Wollongong, NSW 2522, Australia

^(a) pan@uow.edu.au

Received: 8 May 2013
Accepted: 25 June 2013

Abstract

Different experimental techniques are employed to evaluate the critical current density (J_c), namely transport current measurements and two different magnetisation measurements forming quasi-equilibrium and dynamic critical states. Our technique-dependent results for superconducting YBa₂Cu₃O₇ (YBCO) film and MgB₂ bulk samples show an extremely high sensitivity of J_c and associated interpretations, such as irreversibility fields and Kramer plots, which lose meaning without a universal approach. We propose such approach for YBCO films based on their unique pinning features. This approach allows us to accurately recalculate the magnetic-field–dependent J_c obtained by any technique into the J_c behaviour, which would have been measured by any other method without performing the corresponding experiments. We also discovered low-frequency–dependent phenomena, governing flux dynamics, but contradicting the considered ones in the literature. The understanding of these phenomena, relevant to applications with moving superconductors, can clarify their dramatic impact on the electric-field criterion through flux diffusivity and corresponding measurements.