Critical Fields and Vortex Pinning in Overdoped Ba0.2k0.8fe2as2

We determine the upper and lower critical fields, the penetration depth and the vortex pinning characteristics of single crystals of overdoped Ba0.2K0.8Fe2As2 with T-c similar to 10 K. We find that bulk vortex pinning is weak and vortex dynamics to be dominated by the geometrical surface barrier. The temperature dependence of the lower critical field, H-c1, displays a distinctive upturn at low temperatures, which is suggestive of two distinct superconducting gaps. Furthermore, the penetration depth, lambda, varies linearly with temperature below 4 K indicative of line nodes in the superconducting gap. These observations can be well described in a model based on a multiband nodal superconducting gap.