Local internal demagnetizing fields as one of the main fundamental reasons for the low density of critical current in twinned YBa₂Cu₃O_{7 − x} HTSCs

Abstract

An oscillatory differential method of local approximation is applied to study the effect of local internal and external demagnetizing fields on the density of the critical current Jc of intertwin Josephson weak links in YBCO HTSC samples. Demagnetizing fields HD1 and HD2 of samples with different Jc and twin size d are measured in the ZFC and ZFCMFA modes. The following parameters are determined: the twin size d, the thermodynamic first critical magnetic fields of twins Hic1, the demagnetizing fields of twins HDtr, the densities of intertwin effective critical currents Jceff, the critical pinning currents Jcp, and the Meissner screening critical currents Jcg. It is shown that, at the fields Hic1, large twins are stepwise decomposed into a group of smaller twins with close demagnetizing factors. It is found that an increase in Jcg and Jceff and a decrease in d lead, on the one hand, to a decrease in Jc due to an increase in HD and HDtr produced by Jceff and Jcg, on the other hand, to an increase in Jceff and Jcg due to a decrease in d. For samples with large Jc and d, as well as for samples with low values of Jc and d in a field of about 810-2 T, HD and HDtr suppress Jc by about 70% and 60%, respectively. According to the results obtained in the present study and the known literature data, Jc decreases with increasing Jcg; in this case, a decrease in d leads to a decrease, rather than an increase, in Jc. Since the weak links and low values of Jc are fundamental properties of HTSCs, the literature data should be corrected with regard to the important facts on the effect of HD and HDtr on Jc.