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Using first-principles calculations and large-scale dynamic cluster quantum Monte Carlo simulations, this work systematically investigates the effect of electron doping on the superconducting properties of two-orbital bilayer models for three representative systems: bulk La₃Ni₂O₇ under ambient pressure and at 15 GPa, as well as the La₃Ni₂O₇:La₃Al₂O₇ heterostructure. The results show that electron doping universally enhances s±-wave pairing superconductivity, with the heterostructure exhibiting the highest superconducting transition temperature in the underdoped region, even exceeding that of bulk samples under 15 GPa pressure. Further analysis reveals an inter-orbital synergistic mechanism: pairing on the d_{z²} orbital induces pairing on the d_{x²-y²} orbital, which gradually dominates at low temperatures, forming a two-orbital collaborative superconducting instability. This conclusion is validated by simulations with two different cluster sizes. This study provides a theoretical prediction for enhanced superconductivity in electron-doped Ruddlesden-Popper phase nickelates and proposes the heterostructure as a feasible experimental pathway, awaiting future experimental verification.

Polycrystalline samples of La₃₋ₓNaₓNi₂O₇₊δ with various sodium doping concentrations were synthesized via a solid-state method, and their structural, thermal, magnetic, and electrical transport properties were systematically investigated using X-ray diffraction, thermogravimetric analysis, and measurements of magnetic susceptibility and electrical resistivity. X-ray diffraction analysis revealed that when the sodium doping level x ≥ 0.075, the samples undergo a structural transition from the ‘327’ Amam phase to the ‘4310’ Bmab phase, accompanied by gradual lattice expansion. Resistivity measurements indicated that sodium doping significantly enhances metallicity while slightly suppressing the density wave transition temperature; applying external pressure further suppresses the density wave transition, yet the low-temperature insulating behavior remains insensitive to pressure. These findings demonstrate that hole doping introduced by substituting sodium for lanthanum effectively modulates competing electronic phases in layered nickelates, providing crucial experimental evidence for understanding the roles of elemental substitution and carrier doping in stabilizing high-pressure superconducting phases.

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Evidence for charge and spin density waves in single crystals of La₃Ni₂O₇ and La₃Ni₂O₆

Using ultrafast terahertz spectroscopy, we performed optical-pump terahertz-probe experiments on an infinite-layer samarium nickel oxide thin film with Tc = 20 K to measure the temperature-dependent photoconductivity. Under weak excitation, the photoinduced destruction of the superfluid density is proportional to the equilibrium superfluid density and decreases linearly with increasing temperature, consistent with clean-limit d-wave pairing. From this linear relationship, the superconducting gap was extracted to be 2.5 meV, yielding 2Δ/kTc ≈ 3, indicating the system is in the weak coupling regime. Furthermore, independent estimates of the ratio of the mean free path to the coherence length (l/ξ) give approximately 1.5, further confirming clean-limit behavior. These results demonstrate that nickel oxide superconductors can realize a clean superconducting state and reveal a close similarity in pairing mechanism to cuprate high-temperature superconductors.

The recently discovered superconductivity with critical temperature 𝑇𝑐 up to 80 K in the double-layer Nickelate La3⁢Ni2⁢O7−𝛿 under pressure has drawn great attention. Here, we report the positive muon spin relaxation (𝜇+⁢SR) study of polycrystalline La3⁢Ni2⁢O6.92 under ambient pressure. Zero-field 𝜇+⁢SR experiments reveal the existence of magnetic order in La3⁢Ni2⁢O6.92 with 𝑇𝑁=154 K. The weak transverse field 𝜇+⁢SR measurements reveal the bulk nature of magnetism. In addition, a small quantity of oxygen deficiencies can greatly broaden the internal magnetic field distribution sensed by muons.

Evidence of Spin Density Waves in La₃Ni₂O₇−δ