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 (E_F). 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 (T_C) of ∼6 K at ambient pressure. The bulk T_C is further enhanced to 6.9 K in La_1-xY_xO at x = 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 T_C to a maximum of 12.7 K at 20 GPa, the highest T_C in lanthanum monochalcogenides LaX (X = 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 E_F─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 T_C.