Weak emergence in the angular dependence of the critical current density of the high temperature superconductor coated conductor REBCO

Abstract

Extensive critical current density Jc measurements are reported as a function of magnetic field B, temperature T, angle θ between the applied field and the surface of the tape, and strain ϵ, on a REBCO coated conductor. The strain, ϵpeak (B, T, θ), at which Jc (ϵapp) is maximised, is a function of B, T, and θ, which is consistent with weakly emergent behaviour. It is described by the chain model that considers competition between twinned domains with different crystallographic orientations and opposing responses under an applied uniaxial strain. Detailed effective upper critical field B∗c2 (T, ϵapp,θ = 0) data are presented that show universal temperature and strain scaling. They lead to an accurate flux pinning relation for the volume pinning force, Fp ∝ Fp,maxbp(1-b)q, where b = B/B∗c2 (T, ϵapp,θ = 0) and p and q are constants, and are used to help parameterise the scaling behaviour of the angular Jc data more accurately in those cases where B∗c2 cannot be measured directly. We derive approximate analytic in-field expressions that explain how the fraction, f, of a-domains amongst the a- and b-domains affects the strain dependence of the critical parameters and conclude that in our tape, f = 0.4, and the strain at which Jc is the same in both domains is ϵJcA=JcB = 0.15%. We report a sharp peak in Jc as the applied field approaches alignment with the ab-plane and the unusual result that with it, a suppression of the index of transition N also occurs. We find that the effective upper critical field B∗c2 increases as the field angle approaches the ab-plane significantly faster than any available theoretical model for the upper critical field Bc2. In addition, we conclude that a weak-emergence description is not limited to high temperature superconductors, but also describes some low-temperature superconductors.