Nanostructural Defects and Critical Current Densities in High-Quality Epitaxial Thin Films Prepared by MOCVD

Nanostructural defects were investigated by transmission electron microscopy in c-axis-oriented epitaxial Bi₂Sr₂CaCu₂O_8+X (Bi2212) thin films with high T_c(R = 0) ≥ 83 K and high transport critical current density ofJ_c >10¹⁰ A/m² at 10 K. We observed misfit dislocations at twin boundaries, dislocations associated with stacking faults parallel to the a-b plane, and antiphase boundaries. We compared the magnetic-field dependent J_c values at various temperatures in two Bi2212 films. About 110-nm-thick film A showed lower J_c than ~280-nm-thick film B at low temperatures (≤ 20 K) or in low magnetic fields (≤ 0.1 T). However, the J_c of film B decreased more rapidly in high magnetic fields, leading to a crossover of the two J_c-B curves at temperatures of 30 K and above. This is probably because film B contained a higher density of antiphase boundaries, which caused stronger pinning and higher J_c values at low temperatures but may have caused enhanced thermally activated motion of pancake vortices that eventually resulted in lower Jc values in high magnetic fields at moderate temperatures.