Local internal demagnetizing fields as one of the main fundamental reasons for the low density of critical current in twinned YBa2Cu3O7 − 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 J c of intertwin Josephson weak links in YBCO HTSC samples. Demagnetizing fields H D1 and H D2 of samples with different J c and twin size d are measured in the zero field cooling and zero field cooling with magnetic flux accumulation modes. The following parameters are determined: the twin size d, the thermodynamic first critical magnetic fields of twins Hi c1, the demagnetizing fields of twins H Dtr, the densities of intertwin effective critical currents J ceff, the critical pinning currents J cp, and the Meissner screening critical currents J cg. It is shown that, at the fields Hi c1, large twins are stepwise decomposed into a group of smaller twins with close demagnetizing factors. It is found that an increase in J cg and J ceff and a decrease in d lead, on the one hand, to a decrease in J c due to an increase in H D and H Dtr produced by J ceff and J cg, and, on the other hand, to an increase in J ceff and J cg due to a decrease in d. For samples with large J c and d, as well as for samples with low values of J c and d in a field of about 8 × 10−2 T, H D and H Dtr suppress J c by about 70% and 60%, respectively. According to the results obtained in the present study and the known literature data, J c decreases with increasing J cg; in this case, a decrease in d leads to a decrease, rather than an increase, in J c. Since the weak links and low values of J c are fundamental properties of HTSCs, the literature data should be corrected with regard to the important facts on the effect of H D and H Dtr on J c.