Insights Into Radiation Damage in YBa$_2$Cu$_3$O$_{7-δ}$ From Machine-Learned Interatomic Potentials

TL;DR

This work addresses predicting radiation damage in YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) under fusion-relevant neutron irradiation, where oxygen deficiency is pervasive. It introduces two machine-learned interatomic potentials, ACE and tabGAP, trained against DFT data to model high-energy collision cascades across varied oxygen contents. The results show near-DFT accuracy for quasi-static drag, threshold displacement energies, and liquid structure, with large-scale 5 keV cascades predicting higher peak and final defect counts than previous empirical potentials, including Cu-O divacancies, and substoichiometric studies revealing only weak dependence of total damage on oxygen content. Fusion-relevant 300 keV cascades reproduce amorphous cores of a few nanometres, consistent with coherence-length scales observed experimentally, supporting the use of ACE and tabGAP for predictive, scalable radiation-damage modeling in YBCO and informing HTS-tape design for fusion environments.

Abstract

Accurate prediction of radiation damage in YBa$_2$Cu$3$O${7-δ}$ (YBCO) is essential for assessing the performance of high-temperature superconducting (HTS) tapes in compact fusion reactors. Existing empirical interatomic potentials have been used to model radiation damage in stoichiometric YBCO, but fail to describe oxygen-deficient compositions, which are ubiquitous in industrial Rare-Earth Barium Copper Oxide conductors and strongly influence superconducting properties. In this work, we demonstrate that modern machine-learned interatomic potentials enable predictive modelling of radiation damage in YBCO across a wide range of oxygen stoichiometries, with higher fidelity than previous empirical models. We employ two recently developed approaches: an Atomic Cluster Expansion (ACE) potential and a tabulated Gaussian Approximation Potential (tabGAP). Both models accurately reproduce Density Functional Theory (DFT) energies, forces, and threshold displacement energy distributions, providing a reliable description of atomic-scale collision processes. Molecular dynamics simulations of 5 keV cascades predict enhanced peak defect production and recombination relative to a widely used empirical potential, indicating different cascade evolution. By explicitly varying oxygen deficiency, we show that total defect production depends only weakly on stoichiometry, offering insight into the robustness of radiation damage processes in oxygen-deficient YBCO. Finally, fusion-relevant 300 keV cascade simulations reveal amorphous regions with dimensions comparable to the superconducting coherence length, consistent with electron microscopy observations of neutron-irradiated HTS tapes. These results establish machine-learned interatomic potentials as efficient and predictive tools for investigating radiation damage in YBCO across relevant compositions and irradiation conditions.

Figures (13)

• Figure 1: (a) Unit cell of YBCO with displacement vectors used for QSD overlaid as arrows in pink. Oxygen is red, barium green, yttrium yellow, and copper is blue. (b) QSD simulations, showing energies and forces (x, y and z components) for displacement of each symmetrically unique atom in YBCO. The lines are data from the potentials, and the traingles are DFT data.
• Figure 2: TDE distributions for the O1 atom at 25 K and 360 K. The distributions are each composed of 50 average TDEs ($E_{d,ave}^{av}$). Each $E_{d,ave}^{av}$ is determined from a different starting equilibration. The results are compared for the ACE, tabGAP and Gray potentials. A DFT $E_{d,ave}^{av}$ value dickson2025threshold is also overlaid for each temperature as a dashed black line. The dotted lines in the centre of each distribution denote the average of the 50 $E_{d,ave}^{av}$ values obtained for each potential ($\langle E_{d,ave}^{av} \rangle$).
• Figure 3: Radial distribution functions for each pair in YBCO at a density of 6.8 $gcm^{-3}$.
• Figure 4: Number of vacancies of each element type for 5 keV Ba PKA collision cascades as a function of time. The lines are the averages of all cascades, and the shaded areas are the standard deviations.
• Figure 5: Defect count (antisites, interstitials, vacancies) against oxygen content in YBCO. The points are mean values for each stoichiometry and the error bars represent the standard deviation of the values. Data is shown from both ACE and tabGAP.