Critical current density and ac harmonic voltage generation in YBa2Cu3O7-δ

Abstract

The temperature and field dependence of harmonics in voltage V n = V n − iV n using the screening technique have been measured for YBa 2 Cu 3 O 7−δ superconducting thin films. Using the Sun model we obtained the curves for the temperature-dependent critical current density J c (T ). In addition, we applied the criterion proposed by Acosta et al. to compute J c (T ). Also, we made used of the empirical law J c ∝ (1 − T /T c ) n as an input in our calculations to reproduce experimental harmonic generation up to the fifth harmonic. We found that most models fit well the fundamental voltage but higher harmonics are poorly reproduced. Such behavior suggests the idea that higher harmonics contain information concerning complex processes like flux creep or thermally assisted flux flow.

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

“…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.…”

“…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.…”

“…On the other hand, we can introduce the temperature dependence of the magnetic susceptibility via B m . If the critical state is present, thus a linear relation between B m and J c , from equation (7), can be assumed [9]. The parameter r is close to 1 whenever T is more than 10 K below T c and ∼ 3.5 for temperatures very close to T c [9].…”

“…Since the pioneer works of Bean [1] the analysis of the harmonics of the complex ac susceptibility χ n = χ n − iχ n has become one of the most common methods used for the characterization of high-temperature superconductors [3,4]. Conventionally, the ac susceptibility technique [2] is used in most physics laboratories; however, the inductive method has shown to be an alternative and highly efficient method not only to study the electromagnetic properties and the vortex dynamics in superconducting thin films but also some important aspects of the models [5,6,7,8,9]. The physical information contained in the harmonics of susceptibility have been extensively studied [7,8,10,11].…”

“…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]. 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].…”