Vortex Pinning and Lower Critical Field Behavior of Overdoped Ba0.2K0. 8Fe2As2

We report on magneto-optical (MO) and Hall magnetometery measurements on single crystals of Ba$_{0.2}$K$_{0.8}$Fe$_2$As$_2$. The MO measurements demonstrate that the vortices prefer to stabilizing in the center of the sample upon penetration indicate of week pinning in these crystals. We determine the lower critical fields (H$_{c1}$) from the local magnetization using a Hall probe array. Below 2 K, an apparent sudden increase of H$_{c1}$ is observed which we associate with the surface barrier effect. The behavior of temperature dependence of Hc1 above 2 K can be understood in model of a nodal gap structure of overdoped Ba$_{0.2}$K$_{0.8}$Fe$_2$As$_2$.