Effect of irradiation-induced defects on the phase diagram of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$

We present a study of the changes of T$_{c}$ and of the upper critical field of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ that are induced by irradiation with 9 MeV protons and 1.4 GeV Pb-ions. Irradiation to a fluence of 2x10$^{15}$ protons/cm$^{2}$ creates sparse individual point defects and their clusters. These are sufficient to increase vortex pinning, but do not alter the phase diagram in a noticeable way. In contrast, heavy-ion irradiation to a dose matching field of 2 T induces a suppression of T$_{c}$ by $\sim $1K and a reduction of the jump of the specific heat, $\Delta $C, at T$_{c}$ by $\sim $50 {\%}. Furthermore, the upper critical field slopes increase to enormously high values of dH$_{c2}^{c}$/dT=-12.8 T/K and dH$_{c2}^{ab}$/dT=-21 T/K, corresponding to a low anisotropy of $\Gamma $ $\sim $ 1.6. Such behavior is not expected for a d-wave superconductor, but is consistent with pair-breaking by non-magnetic scattering centers in a superconductor with s$_{\pm }$ gap symmetry.