Understanding irradiation damage in high-temperature superconductors for fusion reactors using high resolution X-ray absorption spectroscopy

Understanding the effects of fast neutrons on high-temperature superconductors is of growing importance as new compact fusion reactors rely on these materials to generate the high magnetic fields needed to confine the plasma. The critical temperature of the most promising candidate material for small-scale fusion devices, rare-earth barium cuprate, is known to decrease monotonically with radiation dose, indicating the generation of lattice defects everywhere in the material. Here, we use high-energy-resolution X-ray absorption spectroscopy to probe how the local environment around the copper atoms is influenced by point defects induced by He⁺ ion irradiation in the oxygen sublattice. Density functional theory calculations are used to interpret spectral features and we find clear evidence that ion irradiation significantly disrupts the bonding environment around the copper atoms in the copper-oxygen planes responsible for superconductivity in this compound. We propose the generation of a specific Frenkel defect that is consistent with our experimental results. Our results challenge previous assumptions in the literature that irradiation produces point defects only in the chain sites. In addition, we show that partial recovery is possible by annealing at modest temperatures, which may have implications for the operation of superconducting fusion magnets.