Peak effect in optimally dopedp-type single-crystal Ba0.5K0.5Fe2As2studied by ac magnetization measurements

Abstract

We have used the ac magnetic susceptibility to investigate the vortex state in an optimally doped $p$-type Ba${}_{0.5}$K${}_{0.5}$Fe${}_{2}$As${}_{2}$ single crystal under various ac and dc fields. A peak effect is observed in the temperature dependence of the in-phase ac susceptibility, indicating an order-disorder transition on the vortex phase diagram. The peak effect displays an anomalous history effect compared with other type-II superconductors, which we ascribe to the strong pinning existing in the material. We observe the development of a small dissipation peak at the temperature ${T}_{p2}$ slightly below the peak effect region. Similar to the peak effect boundary, ${T}_{p2}$ delimits a region in the $H\ensuremath{-}T$ phase diagram which is independent on the ac field amplitude. We argue that this small peak may arise from the softening of the vortex lattice, leading to a collective pinning of the whole vortex lattice. This effect assists and further enhances the peak effect occurring in the Ba${}_{0.5}$K${}_{0.5}$Fe${}_{2}$As${}_{2}$ superconductor.