On the theory of strong pinning in high-temperature superconducting films

Several pinning mechanisms proposed to explain extremely high critical current densities jc in c-oriented epitaxial YBaCuO films are analysed. It is concluded that only strong pin-breaking mechanisms, such as pinning by cores of edge dislocations threading the whole film and forming low-angle boundaries between single-crystal domains (at low and moderate fields) and the pinning by sparse large point defects, such as nanoscale inclusions of non-superconducting phases (at higher fields) are relevant. It is shown that available expressions for jc for the last mechanism have certain limits of applicability, such as unphysical divergence of jc with increasing anisotropy Γ leading to an essential overestimate even for YBaCuO with the moderate anisotropy Γ ∼ 7. A nature of such inconsistency and the ways of its overcoming are elucidated. Some aspects of the strong pinning theory are discussed. In the case of thin films it is shown that for the accurate description both tilt and shear elastic moduli should be taken into account as competing factors. The obtained area of vortex trapping on finite-length pinning centre describes crossover from 0D to 1D case and allows a unification of strong pinning theory for different types of defects