Study of the relationship between the V-I curve and the flux dynamics in superconductors

The equivalence of the experimental V-I characteristic (V proportional to I -m) and the material E-j curve (E proportional to j(n)) was studied. We sho w numerically that only when the current I is larger than I-hom, the fully penetrating current, can the V-I curve be equivalent to the E-j one and thu s be used to determine the material parameter n, whereas if I < I-hom, j is inhomogeneous and the V-I curve is not appropriate for use in probing the properties of the sample. The inhomogeneity of j can be checked by simply m easuring the voltage relaxation curve at a given I. Furthermore, it is show n that I-hom varies with dI/dt and n. The dependence of I-hom on dI/dt indi cates that current cannot go directly into a homogeneous region in practica l transport measurements. Moreover, we argue that ail of the V-I curves wit h different values of the flux-creep barrier exponent <mu> merge at large c urrent. Since current is probably inhomogeneous in the small-current region , the V-I curve might not be appropriate for use in studying the properties of the sample in this region. Therefore, the V-I curve may not be appropri ate for determining mu in the U-j relation at all sensitively.