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 (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

💡 Research Summary

The authors report the first definitive demonstration of intrinsic superconductivity in bulk lanthanum monoxide (LaO), a material that has long been elusive due to synthetic difficulties. Using a high‑pressure (5 GPa) and high‑temperature (1573 K) route, they obtained phase‑pure rock‑salt (Fm‑3m) LaO crystals. Ambient‑pressure measurements reveal type‑II superconductivity with an onset transition temperature (TC) of ≈6 K, confirmed by both magnetic susceptibility (nearly 100 % shielding) and resistivity (metallic normal state, sharp drop at TC).

Partial substitution of La by the smaller Y³⁺ ion (La1‑xYxO, x = 0.05 and 0.10) introduces chemical pressure: the lattice contracts (a = 5.140 Å for x = 0.05, 5.129 Å for x = 0.10) and the electron carrier concentration rises from 2.5 × 10²² cm⁻³ to 3.3 × 10²² cm⁻³, as shown by Hall measurements. Correspondingly, TC increases to 6.9 K for the x = 0.10 sample, and the upper critical field reaches ≈3.85 T, giving a coherence length of ~93 Å.

The most striking result is the pressure dependence of TC. In a diamond‑anvil cell, the authors measured resistivity up to 53 GPa. TC rises sharply from 6 K to 11.8 K by 2.5 GPa, then continues to increase more gradually, peaking at 12.7 K around 20 GPa. Between 20 and 25 GPa a plateau appears, after which TC slowly declines. This dome‑shaped TC(P) curve is reminiscent of many unconventional superconductors and is opposite to the behavior observed in tensile‑strained LaO thin films, where only lattice expansion stabilizes superconductivity.

First‑principles density‑functional theory (DFT) calculations reveal the microscopic origin of this unconventional pressure response. Under isotropic compression, the crystal‑field splitting of La 5d states is altered, enhancing La‑5d/O‑2p hybridization and increasing inter‑orbital hopping. The Fermi surface evolves from a simple pocket to a three‑dimensional multi‑pocket topology, providing nesting conditions favorable for spin and orbital fluctuations. Although the calculated density of states at the Fermi level decreases with pressure—an effect that would suppress TC in a conventional electron‑phonon (BCS) scenario—the strengthened p‑d hybridization and the emergence of multi‑band character promote pairing mediated by electronic fluctuations. The authors therefore argue that bulk LaO is a rare example of a 5d‑based, three‑dimensional, non‑BCS superconductor.

Comparing bulk and film results, the study highlights that anisotropic tensile strain (in films) primarily modifies crystal‑field anisotropy and reduces cubic symmetry, whereas isotropic compression (in bulk) preserves the NaCl symmetry while boosting covalency and orbital mixing. This dichotomy explains why chemical/physical pressure enhances TC in bulk while tensile strain is required for films.

In summary, the work establishes: (i) bulk LaO is an intrinsic type‑II superconductor with TC≈6 K; (ii) Y‑doping (chemical pressure) raises TC to 6.9 K; (iii) hydrostatic pressure drives TC up to a record 12.7 K for La monochalcogenides, forming a dome‑shaped TC(P) phase diagram; (iv) DFT shows that pressure‑induced La‑5d/O‑2p hybridization and a three‑dimensional multi‑pocket Fermi surface favor spin/orbital‑fluctuation‑mediated pairing, contradicting a conventional phonon mechanism. These findings provide a solid platform for designing higher‑TC rare‑earth monochalcogenide superconductors.