3D Unconventional Superconductivity in Bulk LaO

TL;DR

This study discovers intrinsic bulk LaO as a type-II superconductor with a record Tc of 12.7 K under pressure, revealing unconventional pairing mechanisms beyond traditional BCS theory, and highlights the effects of doping and pressure on its superconducting properties.

Contribution

The paper reports the synthesis of pure bulk LaO and demonstrates its intrinsic superconductivity with unprecedented Tc, revealing unconventional mechanisms influenced by pressure and doping.

Findings

Bulk LaO is an intrinsic superconductor with Tc up to 12.7 K under pressure.

Y doping enhances Tc by lattice contraction and increased electron carriers.

Pressure increases Tc despite reducing the density of states at EF, indicating unconventional pairing.

Abstract

Lanthanum-based compounds are cornerstones of superconductivity research, yet the La-5d orbitals typically remain empty spectator states far above the Fermi level (EF). 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 (TC) of ~6 K at ambient pressure. The bulk TC is further enhanced to 6.9 K in La1-xYxO 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 TC to a maximum of 12.7 K at 20 GPa, the highest TC 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 EF - 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 multi-pocket 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 TC