On the causes of low critical current in twin film high-temperature superconductors

The influence of internal local and external demagnetization fields on the critical current density of inter-doublet Josephson weak bonds J_c of high-temperature superconducting YBCO samples is investigated using the oscillation differential technique of local approximation. In the zero-field and zero-field cooling with flux accumulation regimes for samples with different J_c and twin sizes d, the demagnetization fields of samples H_D1 and H_D2 have been measured. The values of: d; thermodynamic first critical magnetic fields of twins H_ic1; twin demagnetization fields H_{D tr}; density of intra-twin effective critical currents J_{c ef}; critical pinning currents J_{c p} and shielding Meissner critical currents J_{c M} are determined. It is shown that at H_ic1 fields the twins of large sizes “disintegrate” into a group of smaller twins with close demagnetizing factors. It is found that an increase in J_{c M}, J_{c ef}, and a decrease in d lead, on the one hand, to a decrease in J_c due to an increase in the demagnetization field of the sample H_D and H_{D tr} created by J_{c ef} and J_{c M}, and, on the other hand, to an increase in J_{c ef} and J_{c M} due to a decrease in d.