Effect of ion irradiation on surface morphology and superconductivity of BaFe₂(As_1−xP_x)₂ films

Highlights: • Epitaxial films of BaFe₂(As,P)₂ at optimal doping on MgO substrate have been irradiated by 250 MeV Au ions with different fluences. • Irradiation induces a partial relaxation of the in-plane tensile stress typical of the pristine films. • The residual resistivity increases less than linearly with fluence, tending to saturate; the overall increase is about 60%, but the critical temperature decreases by only 2%. • These results indicate that the substrate and the reduced dimensionality of the films (as compared to the case of single crystals) play an important role in their response to irradiation. - Abstract: We have irradiated epitaxial thin films of BaFe₂(As_1−xP_x)₂ with x ≃ 0.2 (optimal doping) with Au ions having an energy of 250 MeV. We have used two different fluences, Φ₁ = 2.4 × 10¹¹ cm⁻² and Φ₂ = 7.3 × 10¹¹ cm⁻², and we have studied the effects of irradiation on the surface morphology, on the resistivity and on the critical temperature. We have found that irradiation progressively destroys the very clear and interconnected growth terraces typical of the pristine surface, leading first to their smoothening – accompanied by the appearance of localized defects – and then to a completely disordered surface. The residual resistivity increases by almost 60%, but the critical temperature decreases very little (i.e. by about 2%) on going from the pristine film to the most irradiated one. The possible role of the substrate in these results is discussed.