  The critical current (Jc) of highly twinned YBa2Cu3O7 films has been measured
as a function of temperature, magnetic field and angle. For much of the
parameter space we observe a strong suppression of Jc for fields in the twin
boundary (TB) directions; this is quantitatively modeled as
flux-cutting-mediated vortex channeling. For certain temperatures and fields a
cross-over occurs to a regime in which channeling is blocked and the TBs act as
planar pinning centers so that TB pinning enhances the overall Jc. In this
regime, intrinsic pinning along the TBs is comparable to that between the
twins.
