  Transport characteristics of nano-sized superconducting strips and bridges
are determined by an intricate interplay of surface and bulk pinning. In the
limiting case of a very narrow bridge, the critical current is mostly defined
by its surface barrier, while in the opposite case of very wide strips it is
dominated by its bulk pinning properties. Here we present a detailed study of
the intermediate regime, where the critical current is determined, both, by
randomly placed pinning centers and by the Bean-Livingston barrier at the edge
of the superconducting strip in an external magnetic field. We use the
time-dependent Ginzburg-Landau equations to describe the vortex dynamics and
current distribution in the critical regime. Our studies reveal that while the
bulk defects arrest vortex motion away from the edges, defects in their close
vicinity promote vortex penetration, thus suppressing the critical current. We
determine the spatial distribution of the defects optimizing the critical
current and find that it is in general non-uniform and asymmetric: the barrier
at the vortex-exit edge influence the critical current much stronger than the
vortex-entrance edge. Furthermore, this optimized defect distribution has a
more than 30% higher critical current density than a homogeneously disorder
superconducting film.
