Selection of the offset-criterion voltage parameter and its relation to the second differential of a superconductor's voltage-current curve

A relatively rapid characterisation method that gives valuable information about the superconducting transport behaviour is the calculation of the second differential of the superconductors E-I curve. This curve can be interpreted as a distribution of critical current values in a superconducting sample, as resulting from factors such as sausaging, cracks, phase content, and the distribution of magnetic flux-pinning forces. Here, we present measurements on samples of various qualities and with various metallic sheath materials, and the effect these factors have on the shape of the E-I curve and its second differential. The relation to the offset critical current criterion is discussed in terms of intrinsic material properties and the selection of the criterion voltage parameter, E-c(offset).