Investigation of n-Values With Applied Field and Temperature in Coated Conductors Ion Irradiated With 50 to 150 MeV Ag Ions

We investigate changes to the pinning landscape in ion-irradiated coated conductors through comparisons of critical current and n -values as a function of field, field angle and temperature, where n is the power law exponent in the current density-electric field relation, ${\bm{E\ }} = {{\bm{E}}}_{\bm{c}}\ {({{\bm{J}}/{{\bm{J}}}_{\bm{c}}})}^{\bm{n}}$ . Many models of J _c have been developed but models of n -value are rare in the literature. The n -value can be interpreted as the sensitivity of E to J or in the context of a maximum entropy analysis n can be related to a constraint on the vortex system. We investigated samples of (Y,Dy)BCO coated conductor tapes from AMSC irradiated with 50 to 150 MeV Ag ions, and measured J _c and n -values at temperatures from 20 to 77 K and fields up to 8 T at all field angles. As n -values arise from vortex pinning behavior we model the angle dependence using the maximum entropy distributions we have previously used for J _c (θ) analysis. Fitting with these distributions shows the n -values are the outcome of the combined defect structures interacting with vortices. The changes in n -value with field and temperature are not simply correlated with J _c values, for example, n -values often have a broad c -axis centered peak with no correlation to a peak in J _c . At higher temperatures n -values generally have no peak centered on the ab -plane. Irradiation along the c -axis changes n -values in a broad angular range. The n -value is sometimes interpreted as a measure of the energy barrier associated with thermal creep, but from our observations we propose that it is a measure of the dimensionality of the vortex system.