Study of the upper critical magnetic field Hc2 of YBa2Cu3O7-δ compounds

Careful resistive measurements of the upper critical field H(c2) have been made on high quality polycrystalline YBa2Cu3O7-delta samples in magnetic fields up to 12 T. The data have been fitted on the form H(c2) infinity (T(c)-T)alpha to compare with theoretical models. A cusp is found in the H(c2) (T) curve for both of the samples. Different exponents-alpha are obtained depending on the temperature range used in the fitting procedure.