Isotropic Superconductivity in Room-temperature Superconductor LaSc$_{2}$H$_{24}$

Abstract: The discovery of LaSc${2}$H${24}$ represents a milestone in the quest for room-temperature superconductivity, yet the microscopic mechanism underlying its superior performance remains unclear. Through a comprehensive revisit of theoretical calculations, we uncover a pivotal transition from the anisotropic two-gap superconductivity of LaH${10}$ to the isotropic single-gap superconductivity in LaSc${2}$H${24}$ upon the introduction of scandium, thereby enhancing the superconducting critical temperature ($T\mathrm{c}$). This enhancement is rooted in a critical dual role of Sc $3d$ electrons: i) the Sc-derived Jahn-Teller effect promotes hydrogen metallization via the elongation of specific interlayer H-H bonds and enhances electron-phonon coupling (EPC) through the softening of associated phonon modes; ii) Sc $3d$ electrons reconstruct the electronic structure into an MgB${2}$-like configuration, generating novel Sc-H-Sc $σ$- and $π$-bonding states with EPC strengths comparable to LaH${10}$. Crucially, the pronounced hybridization between Sc and the hydrogen cages effectively unifies these two contributions on the Fermi surface. This Sc-induced gap unification bridges the high-EPC H-H states with widespread Sc-H states, establishing an isotropic single-gap nature with a large overall EPC strength. Our findings identify this Sc-induced gap unification as the fundamental mechanism for achieving room-temperature superconductivity in LaSc${2}$H${24}$, offering a theoretical blueprint for the future design of superior superconducting hydrides.