Quantum Structural Renormalization and Anharmonic Stabilization of Superconductivity in P4/mmm YScH8

Abstract

The recent synthesis of P4/mmm-YScH$_{8}$ at 191.7 GPa with a superconducting critical temperature Tc of 113 K has attracted considerable interest in the study of ternary superhydrides. Here we systematically evaluate the effects of quantum and anharmonic motion of ions in P4/mmm-YScH$_{8}$ using the stochastic self-consistent harmonic approximation (SSCHA). We find that these effects renormalize the crystal structure and lower the dynamical stability threshold pressure from $\sim$140 GPa to $\sim$84 GPa, a reduction of about 40%. Furthermore, neglecting these effects causes a significant overestimation of the $T_{c}$, for instance by $\sim$20 K at 190 GPa. Our prediction of $T_{c}$ at 190 GPa is 113 K ($\mu^{*}$= 0.13), close to the experimental observation. Analysis of the phonon density of states and Eliashberg spectral function further reveals site-dependent contributions to superconductivity. These results establish that a consistent treatment of nuclear quantum motion and anharmonicity is essential for accurately predicting superconductivity in compressed ternary hydrides.