The A-15-type superconducting hydride La₄H₂₃: a nanograined structure with low strain, strong electron-phonon interaction, and a moderate level of nonadiabaticity

Abstract

For seven decades by A-15 superconductors we meant metallic A3B alloys (where A is a transition metal, and B is groups IIIB and IVB element) discovered by Hardy and Hulm (Phys. Rev. 89, 884 (1953)). Nb3Ge exhibited the highest superconducting transition temperature, Tc = 23 K, among these alloys. One of these alloys, Nb3Sn, is the primary material in modern applied superconductivity. Recently Guo et al (2024 National Science Reviews nwae149, https://doi.org/10.1093/nsr/nwae149) extended the family of superconductors where the metallic ions arranged in the beta tungsten (A-15) sublattice by observation of Tc,zero = 81 K in La4H23 phase compressed at P = 118 GPa. Despite the La4H23 has much lower Tc in comparison with near-room-temperature superconducting LaH10 phase (Tc,zero = 250 K at P ~ 200 GPa) discovered by Drozdov et al (Nature 569, 531 (2019)), the La4H23 holds the record high Tc within A-15 family. Cross et al (Phys. Rev. B 109, L020503 (2024)) confirmed the high-temperature superconductivity in the compressed La4H23. In this paper, we analyzed available experimental data measured in La4H23 and found that this superconductor exhibits nanograined structure, 5.5 nm ≤ D ≤ 35 nm, low crystalline strain, ε ≤ 0.003, strong electron-phonon coupling interaction, 1.5 ≤ λe-ph≤ 2.55, and moderate level of the nonadiabaticity, 0.18 ≤ ΘD/TF ≤ 0.22 (where ΘD is the Debye temperature, and TF is the Fermi temperature). We found that the derived ΘD/TF and Tc/TF values for the La4H23 phase are similar to the ones in MgB2, cuprates, pnictides, and near-room-temperature superconductors H3S and LaH10.