3D Unconventional Superconductivity in Bulk LaO.

Lanthanum-based compounds are cornerstones of superconductivity research, yet the La 5d orbitals typically remain empty spectator states far above the Fermi level (). While superconductivity has been induced in LaO up to 5.37 K in tensile epitaxy films, the intrinsic ground state of the bulk phase has remained controversial mostly due to synthetic challenges, with early reports suggesting a metallic nature. Here we report the high-pressure and high-temperature synthesis of pure bulk rock-salt LaO and unveil its intrinsic type-II superconductivity with a transition temperature () of ∼6 K at ambient pressure. The bulk is further enhanced to 6.9 K in LaYO at = 0.10, where Y doping leads to lattice contraction (chemical pressing) and a remarkable increase in electron carrier concentration. Strikingly, applying physical pressure further enhances the to a maximum of 12.7 K at 20 GPa, the highest in lanthanum monochalcogenides La = S, Se, Te, and O to date. This pressure dependence is diametrically opposed to the behavior observed in films, and occurs despite a pressure-induced reduction in the density of states at ─a trend that sharply contradicts the conventional phonon-mediated BCS mechanism. Our first-principles calculations reveal that compressive strain modifies the crystal field splitting to enhance La 5d/O 2p hybridization, fostering a three-dimensional multipocket Fermi surface favorable for spin/orbital fluctuation-mediated pairing. This work clarifies the intrinsic superconductivity of bulk LaO and provides a foundation for designing new rare-earth-based superconductors with higher .