Effects of Anharmonicity on Superconducting Y-Ca-H Systems

Electron-phonon coupling calculations generally employ the harmonic and Born-Oppenheimer approximations within the framework of density functional theory. Although these approximations are appropriate for most systems, they can be insufficient for metal hydrides that are susceptible to anharmonic effects due to the lightness of hydrogen atoms. Anharmonic effects in metal hydrides can reshape the crystal structure, renormalize the phonon dispersion spectrum, eliminate or even induce dynamic instabilities, and alter superconducting properties. To incorporate anharmonic effects, the Stochastic Self-Consistent Harmonic Approximation (SSCHA) was used, and to offset the computational costs associated with SSCHA calculations, it was combined with machine learning interatomic potentials that were retrained via active learning for each system. This procedure has been applied to study the effects of anharmonicity on the superconducting properties of select and previously predicted Y-Ca-H compounds, along with some of their binary hydride constituents. The question of whether or not anharmonic effects can reduce the dynamic stabilization pressure of these metal hydrides has also been investigated.