AC susceptibility harmonic response in YBCO single crystals with different oxygen contents

Study of higher-order harmonics of complex ac susceptibility in YBa2Cu3O7−δ thin films by the mutual inductive method

“…These tests also support our previous arguments given for the third harmonic that the unexplainable effects may imply that the power law (6) for the current-voltage characteristics in the flux creep is not valid for the range of temperatures treated in this experiment. We have seen that the whole structure of even harmonics is well reproduced but perhaps a change in the material law [19], or a magnetic field dependence of J c or U c may take place [16,31]. However, in this article we did not include this models since there are parameters that we cannot measure, like, for instance, the characteristic length ξ of the flux lattice line.…”

“…One of the advantages of the power law (6) is that the problem reduces to the manipulation of only two parameters, that is, U c0 and J c0 . The initial values for U c0 and J c0 were taken from the literature [9,16,19] (see next section), however, once that the first computation is given, we fixed one parameter, say J c0 , and manipulate the other. By comparison with the experimental results we obtained the best values of U c0 for each sample.…”

“…The nonlinear magnetic response of the superconductor when flux pinning is present, is reflected in the presence of higher harmonics. Great efforts have been made to understand the physics implied in these harmonics not only for superconductors in bulk [12,13,14,15,16,17,18] but also for superconducting thin films [9,19]. The third harmonic χ 3 has shown to be an indicative of flux penetration into the sample and has been embraced as a criterion for the determination of the critical current density J c [5,6,7,9].…”

“…The third harmonic χ 3 has shown to be an indicative of flux penetration into the sample and has been embraced as a criterion for the determination of the critical current density J c [5,6,7,9]. In a similar fashion, the decrease in the value of higher harmonics has been related to a reduction of the oxygen contents in YB 2 C 3 O 7−δ crystals [16]. Other works [9] have suggested that models based solely on the critical state model [1,20] cannot reproduce the experimental data of the higher harmonics due to thermal effects like flux creep.…”