Stripes of increased diamagnetic susceptibility in underdoped superconducting Ba(Fe(1-x)Cox)(2)As-2 single crystals: Evidence for an enhanced superfluid density at twin boundaries

Superconducting quantum interference device microscopy shows stripes of increased diamagnetic susceptibility in the superconducting state of twinned, orthorhombic, underdoped crystals of Ba(Fe1-xCox)(2)As-2, but not in tetragonal overdoped crystals. These stripes are consistent with enhanced superfluid density on twin boundaries.