Harmonic ac susceptibilities of high temperature superconductors

Abstract

By numerically solving the flux creep equation, we have investigated the effects of dc field Bd, ac field amplitude Bac and frequency f on the harmonic ac susceptibilities of high temperature superconductors. It is found that the peak height in |χn(T)| = √χ′n 2(T)+χ″n 2(T) curves depends strongly on Bd, Bac and f, especially when Bd is not too much larger than Bac. This result cannot be explained by the critical state model, which predicts a constant peak height. When the condition Bd≫Bac is satisfied, the peak height in odd harmonics becomes constant while even harmonics approaches zero, the critical state model can then be used. However, in order to account for the frequency dependence, magnetic relaxation effects must be taken into account, which introduce the frequency as an independent parameter into the model.