Critical currents in vicinal YBa$_2$Cu$_3$O$_{7-δ}$ films
Most measurements of critical current densities in YBa$_2$Cu$_3$O$_{7-δ}$ thin films to date have been performed on films where the \textit{c}-axis is grown normal to the film surface. With such films, the analysis of the dependence of $j_c$ on the magnetic field angle is complex. The effects of extrinsic contributions to the angular field dependence of $j_c$, such as the measurement geometry and disposition of pinning centres, are convoluted with those intrinsically due to the anisotropy of the material. As a consequence of this, it is difficult to distinguish between proposed FLL structure models on the basis of angular critical current density measurements on \textit{c}-axis films. Films grown on mis-cut (vicinal) substrates have a reduced measurement symmetry and thus provide a greater insight into the critical current anisotropy. In this paper previous descriptions of the magnetic field angle dependence of $j_c$ in YBa$_2$Cu$_3$O$_{7-δ}$ are reviewed. Measurements on YBa$_2$Cu$_3$O$_{7-δ}$ thin films grown on a range of vicinal substrates are presented and the results interpreted in terms of the structure and dimensionality of the FLL in YBa$_2$Cu$_3$O$_{7-δ}$. There is strong evidence for a transition in the structure of the flux line lattice depending on magnetic field magnitude, orientation and temperature. As a consequence, a simple scaling law can not, by itself, describe the observed critical current anisotropy in YBa$_2$Cu$_3$O$_{7-δ}$. The experimentally obtained $j_c(θ)$ behaviour of YBCO is successfully described in terms of a kinked vortex structure for fields applied near parallel to the \textit{a-b} planes.