Vortex pinning by natural linear defects in thin films of YBa₂Cu₃O_7-δ

The behavior of the superconducting current density j _s(B,T) and the dynamical relaxation rate Q(B,T) of YBa₂Cu₃O_7-δ thin films exhibits a number of features typical for strong pinning of vortices by growth induced linear defects. At low magnetic fields j_s(B) and Q(B) are constant up to a characteristic field B*, that is directly proportional to the linear defect density n_disl. The pinning energy U_c(B=0)≈600 K can be explained by half-loop excitations determining the thermal activation of vortices at low magnetic fields. Extending the Bose glass theory [D. R. Nelson and V. M. Vinokur, Phys. Rev. B 48, 13 060 (1993)], we derive a different expression for the vortex pinning potential ε_r(R), which is valid for all defect sizes and describes its renormalization due to thermal fluctuations. With this expression we explain the temperature dependence of the true critical current density j_c(O,T) and of the pinning energy U_c(0,T) at low magnetic fields. At high magnetic fields μ₀H≫B* the current density experiences a power law behavior j_s(B)∼B^α with α≈-0.58 for films with low n_disl and α≈-0.8 to -1.1 for films with high n_disl. The pinning energy in this regime, U_c(high B)≈60-200 K is independent of magnetic field, but depends on the dislocation density. This implies that vortex pinning is still largely determined by the linear defects, even when the vortex density is much larger than the linear defect density. Our results show that natural linear defects in thin films form an analogous system to columnar tracks in irradiated samples. There are, however, three essential differences: (i) typical matching fields are at least one order of magnitude smaller, (ii) linear defects are smaller than columnar tracks, and (iii) the distribution of natural linear defects is nonrandom, whereas columnar tracks are randomly distributed. Nevertheless the Bose glass theory, that has successfully described many properties of pinning by columnar tracks, can be applied also to thin films. A better understanding of pinning in thin films is thus useful to put the properties of irradiated samples in a broader perspective.