  The high $T_{c}$ cuprate superconductors are noted for their anisotropic
layered structure, certain of these materials indeed tend toward the limit of a
Lawrence-Doniach superconductor. However, YBa$_2$Cu$_3$O$_{7-\delta}$ has a
smaller anisotropy than would be expected from its interlayer spacing. This is
due to the cuprate chains in the structure. To investigate the influence of the
chain oxygen on transport properties critical current versus applied field
angle measurements were performed on fully oxygenated and de-oxygenated
YBa$_2$Cu$_3$O$_{7-\delta}$ thin films and optimally oxygenated
Y$_{0.75}$Ca$_{0.2}$Ba$_2$Cu$_3$O$_{7-\delta}$ thin films. The films were grown
on 10$^{\circ}$ mis-cut SrTiO$_3$ substrates to enable the intrinsic vortex
channelling effect to be observed. The form of the vortex channelling minimum
observed in field angle dependent critical current studies on the films was
seen to depend on film oxygenation. The vortex channelling effect is dependent
on a angular dependent cross-over to a string-pancake flux line lattice. The
results obtained appear to be consistent with the prediction of Blatter et al.
[Rev. Mod. Phys., 66 (4): 1125 (1994)] that increased superconducting
anisotropy leads to the kinked string-pancake lattice existing over a smaller
angular range.
