Direct observation of the flux line lattice in Al-doped YBa₂Cu₃O_7−δ

The high-resolution Bitter pattern technique has been used to study the flux line lattice (FLL) in Al-doped YBa₂Cu₃O_7−δ (YBCO) single crystals with 4% of Cu atoms substituted for Al. In contrast to undoped YBCO, the FLL was isotropic and no effects of vortex interaction with twin boundaries were found which is consistent with the extremely small size of the twin domains ≈30 Å in these crystals brought about by Al-doping. In addition, the FLL consisted of misoriented domains inside which an undisturbed regular vortex arrangement was seen, whose dimensions rapidly increased with the magnetic field. This behaviour testifies to the FLL being collectively pinned by weak pinning sites such as vacancies, small defect clusters, etc. Proceeding from the dimensions of these domains, we estimated the critical current density for our crystals to be j_c≈3x10² A/cm², two orders of magnitude lower than the typical value for undoped YBCO single crystals, but of the same order of magnitude as for Fe-doped YBCO.