We report on the dramatic effect of random point defects, produced by proton irradiation, on the superfluid density ${\rho }_s$ in superconducting Ca${}_{0.5}$Na${}_{0.5}$Fe${}_2$As${}_2$ single crystals. The magnitude of the suppression is inferred from measurements of the temperature-dependent magnetic penetration depth $\lambda \left(T\right)$ using magnetic force microscopy. Our findings indicate that a radiation dose of $2\times {10}^{16}$ cm${}^{-2}$ produced by 3 MeV protons results in a reduction of the superconducting critical temperature $T_c$ by approximately 10$\%$. In contrast, ${\rho }_s\left(0\right)$ is suppressed by approximately 60$\%$. This breakdown of the Abrikosov-Gorkov theory may be explained by the so-called “Swiss cheese model,” which accounts for the spatial suppression of the order parameter near point defects similar to holes in Swiss cheese. Both the slope of the upper critical field and the penetration depth $\lambda (T/T_c)/\lambda \left(0\right)$ exhibit similar temperature dependences before and after irradiation. This may be due to a combination of the highly disordered nature of Ca${}_{0.5}$Na${}_{0.5}$Fe${}_2$As${}_2$ with large intraband and simultaneous interband scattering as well as the $s^{\pm }$-wave nature of short coherence length superconductivity.

• Received 21 June 2012

DOI:https://doi.org/10.1103/PhysRevB.86.144509