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This study constructs a microscopic theoretical model for the superconducting gap structure of bilayer nickel oxides, where a conduction band with dx²-y² symmetry coexists with localized d3z²-r² spins. Strong interlayer coupling leads to a singlet ground state of local magnetic moments, whose virtual singlet-triplet excitations (i.e., “triplons”) mediate pairing interactions between conduction electrons, thereby generating interband s±-wave pairing with opposite signs of the order parameters on the two bands (α and β). The theoretical results naturally explain key experimental observations: despite the smaller density of states of the α band, its superconducting gap is larger, and the gap exhibits significant momentum-space anisotropy arising from nonlocal Kondo coupling. These findings strongly support the triplon-mediated pairing mechanism as the microscopic origin of superconductivity in bilayer nickel oxides.

Using first-principles calculations and the singular-mode functional renormalization group method, we systematically investigate the effect of interlayer nickel-nickel distance on the ground state in La₃Ni₂O₇/LaAlO₃ thin films. The results show that a smaller interlayer distance leads to a C-type spin density wave (interlayer ferromagnetic coupling), while a larger interlayer distance yields a G-type spin density wave (interlayer antiferromagnetic coupling). Between these two phases, an s±-wave superconducting state emerges, dominated by pairing in the Ni 3d₃z²⁻ᵣ² orbital. This finding explains the origin of superconductivity observed in the thin films under ambient pressure and predicts that applying pressure will suppress the superconducting transition temperature until the system enters the C-type spin density wave. If confirmed experimentally, this prediction will provide deep insight into the nature of electronic correlations in this system, as the C-type spin density wave naturally emerges within the itinerant electron picture, whereas it is difficult to realize within the local magnetic moment picture (where interlayer spins remain antiferromagnetically coupled).

By combining density functional theory and random phase approximation, the effects of compressive and tensile strain on the superconductivity of 1313-La3Ni2O7 thin films are systematically investigated. A self-doping effect is found between the monolayer and trilayer blocks regardless of compressive or tensile strain, and it is most pronounced under tensile strain. Under compressive strain imposed by an LSAO substrate, even considering hole doping from strontium ion migration in the substrate, superconductivity is difficult to appear, consistent with experiments. However, under tensile strain from a KTO substrate, a band in the trilayer subsystem that originally did not cross the Fermi level shifts downward, giving rise to a small hole-type γ pocket at the M point, which is connected to a small electron-type σ pocket at the Γ point by a near-(π,π) wavevector. Random phase approximation calculations reveal that the trilayer subsystem can then form a stable s±-wave pairing state, with the order parameter reversing sign between these two pockets. Further analysis indicates that the size of the γ pocket is crucial for pairing, and an excessively large γ pocket suppresses superconductivity. This work predicts a strain-driven electronic structure reconstruction and proposes a design principle to realize superconductivity in 1313-La3Ni2O7 under ambient pressure through tensile strain engineering.

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This study developed an ultrafast magneto-pressure optical spectroscopy platform capable of operating simultaneously at pressures up to 40 GPa, magnetic fields up to 7 T, and temperatures as low as 5 K, and applied it to investigate the evolution of quasiparticle dynamics under magnetic pressure in the trilayer nickelate Pr₄Ni₃O₁₀. The experiments revealed a pronounced critical slowing down of quasiparticle relaxation near the charge density wave (CDW) transition, which disappears upon the application of pressure. At higher pressures, the low-temperature relaxation time instead becomes longer, consistent with initial superconducting correlation signatures. However, a magnetic field as high as 7 T hardly alters the relaxation behavior, and no vortex-induced pre-bottleneck dynamics—robustly observed in bulk superconducting control samples—was detected, suggesting that any superconducting state under the present pressure conditions is not bulk-like but rather filamentary or strongly inhomogeneous. This magneto-pressure ultrafast capability opens a new pathway for addressing unresolved issues of pressure-induced superconductivity and intertwined orders in correlated quantum materials.

Ultrafast optical evidence of coexisting density waves in bilayer nickelate La₃Ni₂O₇

The discovery of high-temperature superconductivity in La3⁢Ni2⁢O7 at pressures above 14 GPa has spurred extensive research efforts. Yet, fundamental aspects of the superconducting phase, including the possibility of a filamentary character, are currently subjects of controversial debates. Conversely, a crystal structure with NiO6 octahedral bilayers stacked along the 𝑐-axis direction was consistently posited in initial studies on La3⁢Ni2⁢O7. Here, we reassess this structure in optical floating zone-grown La3⁢Ni2⁢O7 single crystals that show signs of filamentary superconductivity. Employing scanning transmission electron microscopy and single-crystal x-ray diffraction under high pressures, we observe multiple crystallographic phases in these crystals, with the majority phase exhibiting alternating monolayers and trilayers of NiO6 octahedra, signifying a profound deviation from the previously suggested bilayer structure. Using density functional theory, we disentangle the individual contributions of the monolayer and trilayer structural units to the electronic band structure of La3⁢Ni2⁢O7, providing a firm basis for advanced theoretical modeling and future evaluations of the potential of the monolayer-trilayer structure for hosting superconductivity.在高于 14 GPa 的压力下， La3⁢Ni2⁢O7 中高温超导性的发现激发了广泛的研究。然而，超导相的基本性质，包括其丝状特征的可能性，目前仍存在争议。相反，在对 La3⁢Ni2⁢O7 的早期研究中，人们一直假设其晶体结构为沿 𝑐 轴方向堆叠的 NiO6 八面体双层。本文中，我们重新评估了光学浮区法生长的 La3⁢Ni2⁢O7 单晶中的这种结构，这些单晶表现出丝状超导的迹象。利用扫描透射电子显微镜和高压下的单晶 X 射线衍射技术，我们观察到这些晶体中存在多种晶相，其中主要相由 NiO6 八面体的单层和三层交替构成，这表明其与先前提出的双层结构存在显著偏差。利用密度泛函理论，我们分离出单层和三层结构单元对 La3⁢Ni2⁢O7 电子能带结构的各个贡献，为高级理论建模和未来评估单层-三层结构承载超导性的潜力提供了坚实的基础。

We performed systematic electronic Raman scattering studies on single crystals of the bilayer nickelate La₃Ni₂O₇ under hydrostatic pressures up to 16.51 GPa, tracing the evolution of the spin-density-wave (SDW) state with pressure. The results show that both the SDW gap energy and the SDW transition temperature generally increase with increasing pressure, while the dimensionless coupling ratio 2Δ_SDW/(k_B T_SDW) remains approximately 7.5, indicating robust strong-coupling character of the SDW state. Concurrently, the SDW Raman peak progressively broadens with pressure, suggesting that the long-range SDW ordering coherence gradually weakens. This work reveals an unusual pressure evolution in which the SDW energy scale is enhanced while the SDW state becomes increasingly incoherent, thereby providing spectroscopic constraints on magnetic correlations relevant to high-temperature superconductivity in bilayer nickelates.

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This study employs first-principles path-integral molecular dynamics to comprehensively incorporate nuclear quantum many-body effects, constructing free energy surfaces and revealing a lattice quantum disordered (LQD) phase in H3S and La3Ni2O7. By comparing classical and quantum phase boundaries, the LQD phase forms a triangular region in the pressure–temperature phase diagram, with its left boundary precisely aligning with the transition temperature of the left flank of the superconducting dome; notably, the maximum temperature Tcmax of the LQD phase coincides with the maximum superconducting Tc, and the isotope effect is accurately captured. These findings demonstrate that superconductivity on the dome’s left flank originates from a transition from a low-symmetry phase to the quantum disordered phase, with the superconducting state residing entirely within the high-symmetry phase, thereby refuting the two-phase interpretation. The research establishes the LQD phase as a unified framework that not only elucidates the critical role of lattice quantum many-body effects in unconventional superconductivity but also opens new pathways for predicting higher-Tc superconductors and explaining anomalous condensed-matter phenomena.