Magnetic penetration-depth measurements of a suppressed superfluid density of superconducting Ca0.5Na0.5Fe2As2 single crystals by proton irradiation

Abstract

We report on the dramatic effect of random point defects, produced by proton irradiation, on the superfluid density ρ s in superconducting Ca 0.5Na 0.5Fe 2As 2 single crystals. The magnitude of the suppression is inferred from measurements of the temperature-dependent magnetic penetration depth λ(T) using magnetic force microscopy. Our findings indicate that a radiation dose of 2×1016 cm -2 produced by 3 MeV protons results in a reduction of the superconducting critical temperature T c by approximately 10%. In contrast, ρ s(0) 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 λ(T/T c)/λ(0) exhibit similar temperature dependences before and after irradiation. This may be due to a combination of the highly disordered nature of Ca 0.5Na 0.5Fe 2As 2 with large intraband and simultaneous interband scattering as well as the s ±-wave nature of short coherence length superconductivity. © 2012 American Physical Society.