External and internal demanetizing fields as one of the main fundamental reasons for the low density of critical current in twinned YBa2Cu3O7-x high-temperature superconductors
RostamiH. R.1
1Fryazino Branch, Kotel’nikov Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Fryazino, Moscow oblast, Russia
Email: rostami@ms.ire.rssi.ru

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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 YBa2Cu3O7-x of samples with different Jc and twin size Jc are measured in the d and HD modes. The following parameters are determined: the twin size d, the thermodynamic first critical magnetic fields of twins Hic1, the demagnetizing fields of twins Hic1, the densities of intertwin effective critical currents HDtr, the critical pinning currents Jceff, and the Meissner screening critical currents Jcp. 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, HDtr and Jc suppress by ~ 70 about ~ 60%, respectively. According to the results obtained in the present study and the known literature data, d decreases with increasing d; 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. Keywords: twins, monodomains, crystallites, grains, sub- and nanocrystallites, demagnetizing fields, magnetic flux trapping, critical currents.
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