Responsibility of small defects for the low radiation tolerance of coated conductors

Rare-earth-barium-copper-oxide based coated conductors exhibit a relatively low radiation robustness compared to e.g. Nb₃Sn due to the d-wave symmetry of the order parameter, rendering impurity scattering pair breaking. The type and size of the introduced defects influence the degrading effects on the superconducting properties; thus the disorder cannot be quantified by the number of displaced atoms alone. In order to develop degradation mitigation strategies for radiation intense environments, it is relevant to distinguish between detrimental and beneficial defect structures. Gadolinium-barium-copper-oxide based samples irradiated with the full TRIGA Mark II fission reactor spectrum accumulate a high density of point-like defects and small clusters due to n - γ capture reactions of gadolinium. This leads to a 14–15 times stronger degradation of the critical temperature compared to samples shielded from slow neutrons. At the same time both irradiation techniques lead to the same degradation behavior of the critical current density as function of the transition temperature Jc(Tc). Furthermore, annealing the degraded samples displayed the same Tc recovery rates, indicating the universality of the defects responsible for the degradation. Since the primary knock on atom of the n - γ reaction as well as the recoil energy is known, we used molecular dynamics simulations to calculate which defects are formed in the neutron capture process and density functional theory to assess their influence on the local density of states. The defects found in the simulation were mainly single defects as well as clusters consisting of Oxygen Frenkel pairs, however, more complex defects such as Gd Cu antisites occurred as well.