Nanostructural Defects and Critical Current Densities in High-Quality $\hbox{Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{8+{\rm X}} $ Epitaxial Thin Films Prepared by MOCVD

Nanostructural defects were investigated by transmission electron microscopy in c-axis-oriented epitaxial Bi ₂ Sr ₂ CaCu ₂ O _8+X (Bi2212) thin films with high T _c (R = 0) ≥ 83 K and high transport critical current density ofJ _c >10 ¹⁰ A/m ² 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.