Procedures for proper validation of record critical current density claims

Abstract

High critical current density Jc is generally the most important property of any superconductor used for applications. The critical current Ic is most frequently measured by transport 4 probe V-I techniques and the measured Ic is then divided by the cross-sectional area to give the critical current density Jc. This method is normally error-free. Occasionally however, Ic and Jc are computed from measurements of the magnetic moment and two recent papers (Goyal et al. 2024 Nat. Commun. 15, 6523 and Polichetti et al. 2024 arXiv:2410.09197) illustrate some of the perils of reporting properties of REBa2Cu3O7-{\delta} (REBCO) films grown on a coated conductor substrate using only magnetic evaluations. Correct data analysis is made difficult by the persistent use of cgs units in commercial instrument outputs and by misapplication of handbook Bean model formulae, which in this case led to an order of magnitude Jc overestimate, which of course renders the original world record claim invalid. The laudable attempt of the authors to explain their original mistake has unfortunately its own errors. Here we explain explicitly how errors are still present in the second paper and provide correct expressions for the conversion of magnetic moments to Jc values using Bean model expressions. An important conclusion is that record Jc values ought always to be validated by real transport measurements and by cross-reference to similar samples and their reported Jc values.