Effect of ion irradiation on surface morphology and superconductivity of BaFe2(As1−xPx)2 films

We have irradiated epitaxial thin films of BaFe2(As1−xPx)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.