Critical fields and vortex pinning in overdopedBa0.2K0.8Fe2As2

Abstract

We determine the upper and lower critical fields, the penetration depth and the vortex pinning characteristics of single crystals of overdoped $\mathrm{B}{\mathrm{a}}_{0.2}{\mathrm{K}}_{0.8}\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ with ${T}_{c}\ensuremath{\sim}10\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. 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, \ensuremath{\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.