Daily Overview: Today’s highlights focus on deepening the understanding of the electronic structure of mixed Ruddlesden-Popper nickelates. In [1], the superfluid density of infinite-layer Nd1-xSrxNiO2 was systematically measured using the mutual inductance method, revealing a weak superfluid stiffness with a square-root dependence on Tc. Unexpectedly, a strong coupling between Nd 4f magnetic moments and the superfluid was found to significantly suppress the superfluid density, suggesting that the interplay between magnetic order and the superconducting phase limits Tc. [2] constructed a microscopic theory of triplon-mediated superconducting pairing in bilayer nickel oxides, explaining the experimentally observed features of the α-band—namely, its small density of states but larger energy gap—as well as the momentum-space anisotropy of the gap. This provides strong support for triplon mediation as the pairing mechanism in this system. [3] investigated the spin-density wave (SDW) transition in trilayer Pr4Ni3O10 through oxygen isotope substitution and pressure experiments, finding that the isotope shift does not vary with pressure. This indicates that the SDW transition is primarily driven by electronic correlations rather than lattice dynamics, in contrast to the doping-enhanced isotope effect in cuprates, providing key constraints for understanding the electronic origin of density-wave order in nickelates and its relationship with superconductivity. [4] employed quantum Monte Carlo simulations with a multi-orbital model incorporating interstitial s orbitals to reproduce experimental features of infinite-layer nickelates, such as the persistence of an electron pocket at 20% doping and strong renormalization of d-orbital dispersion. The study also found that s orbitals significantly enhance short-range antiferromagnetic correlations, revealing the decisive role of multi-orbital strong correlation effects on low-energy electronic states and spin correlations. These studies deepen the understanding of nickel-based superconducting systems from multiple dimensions, including superfluid response, pairing mechanisms, the origin of density waves, and multi-orbital electronic structure. arXiv submission processing window: 2026-03-24 00:00 to 2026-03-24 00:00 UTC.

1. Evolution of the Superfluid Density in Infinite-Layer Nickelates

Summary: This paper systematically measures the superfluid density of the infinite-layer nickel-based superconductor Nd1-xSrxNiO2 within the doping superconducting dome using the mutual inductance method. The results show that the superfluid stiffness is weak and exhibits an approximate square-root relationship with the superconducting transition temperature Tc. Additionally, a strong interaction between the Nd 4f magnetic moments and the superfluid is observed, leading to a significant suppression of the superfluid density at low temperatures, with an effect far beyond simple paramagnetic explanations, suggesting a coupling between magnetic order and the superconducting phase. These findings indicate that superconducting phase fluctuations play an important role in limiting Tc and reveal an unexpectedly strong coupling between rare-earth magnetic ions and the superfluid.

2. Triplon-mediated pairing and the superconducting gap structure in bilayer nickelates

Summary: 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.

3. Pressure-Invariant Isotope Effect as Evidence for Electronically Driven Intertwined Order in Pr$_4$Ni$_3$O$_{10}$

Summary: This study utilized muon spin rotation spectroscopy to investigate the effect of oxygen isotope substitution (16O/18O) on the spin density wave (SDW) transition in the trilayer Ruddlesden-Popper nickelate Pr4Ni3O10. Under ambient pressure, the SDW transition temperatures for the 16O and 18O samples were 158.04 K and 159.81 K, respectively, exhibiting a finite isotope shift. Under hydrostatic pressure, the transition temperatures for both isotopes decreased linearly at nearly identical rates (approximately -4.9 K/GPa), resulting in an essentially pressure-independent isotope shift. This pressure-independent isotope effect indicates that the SDW transition primarily originates from electronic correlations rather than lattice dynamics. Combined with recent inelastic X-ray scattering results that revealed no phonon softening, this study supports a novel mechanism of intertwined charge density wave and spin density wave order stabilized by strong spin interactions in trilayer Ruddlesden-Popper nickelates. This finding contrasts with the doping-enhanced isotope effect observed in cuprates and provides critical constraints for understanding the electronic origin of density wave order and its relationship with superconductivity in nickelates.

4. Role of interstitial $s$ orbital in a model of infinite-layer nickelates

Summary: This study employs the determinant quantum Monte Carlo method to simulate the low-energy electronic structure of infinite-layer nickelates by adding a gap s orbital with three-dimensional dispersion to the three-orbital Emery model. Large-scale calculations reveal that strong correlation effects significantly reduce the electron pocket induced by the gap s orbital, yet the pocket persists at 20% hole doping, with a size comparable to ARPES experimental observations; the d_{x^2-y^2} orbital dispersion undergoes strong renormalization, and the weak dispersion along the k_z direction agrees with experiments. Furthermore, compared to the conventional three-orbital model, the introduction of the s orbital markedly enhances short-range antiferromagnetic correlations. These results highlight the crucial role of strong correlation and multi-orbital effects in determining the low-energy electronic states and spin correlations of infinite-layer nickelates, indicating that interaction-driven many-body physics must be treated within a realistic multi-orbital framework.