The usefulness of superconductors is dependent on engineering the pinning landscape to optimize critical currents. Columnar defect tracks created by the passage of high-energy ions are well known to contribute strongly to flux pinning in REBCO coated conductors. At lower energies the irradiation produces discontinuous tracks and somewhat isotropic pinning and at high energy produces correlated defect columns. Irradiation thus provides a useful tool for studying the optimization of the pinning landscape as defect populations of different natures are combined.
The pinning landscape can be explored through critical current changes with field, field angle and temperature, Jc(B, θ, T) but also n(B, θ, T), where n is the power law exponent in the current density-electric field relation, E = E0 (J/Jc)n. Jc and n are outcomes of the structural disorder in the sample and effects of thermal fluctuations. Characterisation of the pinning relies heavily on analysis of Jc but not so much interest has been shown in how n-values evolve. Many models of Jc have been developed but models of n-value are rare in the literature.
To understand the effects of mixed pinning landscapes we have irradiated (Y,Dy)BCO coated conductor tapes from AMSC with 50 MeV to 150 MeV Ag ions, and measured Jc and n-values at temperatures from 20 K to 77 K and fields up to 8 T at all field angles.
In comparison to Jc values, n-values are relatively constant with values mostly in the range of 10-30. Values change little with applied field and somewhat more strongly with temperature. With field angle, n-values show similar structural features to Jc(θ) with some maxima aligned with the irradiation angle. There are instances across the measured field and temperature ranges where n-values are correlated and anti-correlated with Jc values. As n-values arise from vortex behavior we argue they can be modelled using the same statistical distributions based on maximum entropy arguments we have previously used for Jc(θ) analysis. We use this analysis to show instances where n-values provide further useful insight into the nature of the pinning landscape.
Acknowledgement: This work is funded by the Royal Society of New Zealand Marden Fund MFP-VUW1805
Keywords: pinning, critical currents , n-value, ion irradiation