Effect of proton irradiation on superconductivity in optimally doped BaFe2(As1-xPx)2 single crystals

Irradiation with 4 MeV protons was used to systematically introduce defects in single crystals of the iron-arsenide superconductor BaFe2(As1-xPx)(2), x = 0.33. The effect of disorder on the low-temperature behavior of the London penetration depth lambda(T) and transition temperature T-c was investigated. In nearly optimally doped samples with T-c similar to 29 K, signatures of a superconducting gap with nodes were observed. Contrary to previous reports on electron-irradiated crystals, we do not see a disorder-driven lifting of accidental nodes, and we observe that proton-induced defects are weaker pair breakers than electron-induced defects. We attribute our findings to anisotropic electron scattering caused by proton irradiation defects.