Prominent stress-driven anomalies in superconductivity of yttrium hexahydride

Yttrium hexahydride is a top superconducting superhydride, but its measured ${T}_{c}$ data are widely scattered and well below theoretical predictions. These prominent anomalies remain unresolved despite major efforts, impeding the understanding of this distinct class of superb superconductors. Here, we show that nonhydrostatic stresses, which are omnipresent in high-pressure experiments but omitted in prior theoretical studies, cause large deviations of ${T}_{c}$ from hydrostatic values, accounting for the strong scattering of experimental data and, joint with an anharmonic correction, reconciling the diverging theoretical and experimental ${T}_{c}$ results. Analysis of stress-induced phonon and charge responses unveils the underlying microscopic mechanisms. These findings offer a viable solution to the notable superconductivity anomalies in ${\mathrm{YH}}_{6}$, and the insights have broad implications for other superconducting superhydrides and, more generally, superconductors under extreme compression.