Effect of ion irradiation on surface morphology and superconductivity of BaFe2(As(1-x)P(x))2 films

We have irradiated with Au ions (having an energy of 250 MeV) epitaxial thin films of BaFe2(As(1-x)P(x))2 with x @ 0.2 (optimal doping), grown by MBE on MgO single-crystal substrates. We have studied the effects of irradiation (for three different fluences, F=2.4×10^11 cm^-2, 4.8×10^11 cm^-2 and 7.3×10^11 cm^-2) 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 smoothing – accompanied by the appearance of localized defects and cracks – and then to a completely disordered surface. However, the resistivity increases only slightly and the critical temperature decreases very little (i.e. by about 2%) going from the pristine film to the most-irradiated one. The latter result is confirmed by local measurements of the critical temperature carried out by point-contact Andreev reflection spectroscopy.