  Superconducting quantum interference device (SQUID) microscopy shows stripes
of increased diamagnetic susceptibility in underdoped, but not overdoped,
single crystals of Ba(Fe1-xCox)2As2. These stripes of increased diamagnetic
susceptibility are consistent with enhanced superfluid density on twin
boundaries. Individual vortices avoid pinning on or crossing the stripes, and
prefer to travel parallel to them. These results indicate a relationship
between superfluid density, local strain, and frustrated magnetism, and
demonstrate two mechanisms for enhancing critical currents.
