  We numerically examine the flow of superconducting vortices in samples
containing square pinning arrays in which a band of pins is removed. When a
drive is applied at an angle with respect to the band orientation, we find that
the vortex depinning initiates in the pin-free channel. The moving vortices
form a series of quasi-one-dimensional shear bands that begin flowing in the
bulk of the pin-free channel, and the motion gradually approaches the edge of
the pinned region. The consecutive depinning of each shear band appears as a
series of jumps in the velocity-force curves and as sharp steps in the spatial
velocity profiles. When a constant drive is applied parallel to the pin-free
channel along with a gradually increasing perpendicular drive, the net vortex
velocity decreases in a series of steps that correspond to the immobilization
of bands of vortices, and in some cases the flow can drop to zero, creating a
field effect transistor phenomenon. These results should also be relevant to
other types of systems that exhibit depinning in the presence of inhomogeneous
pinning.
