Dissecting superconductivity in the Ruddlesden-Popper nickelates: The role of electron correlation and interlayer magnetic exchange

Abstract

The discovery of superconductivity in the Ruddlesden-Popper (RP) nickelates has opened a new chapter in the search for high superconducting transition temperatures ($T_\mathrm{c}$) materials. A central and puzzling feature of this family is the wide variation in $T_\mathrm{c}$ despite their common NiO$_2$ building blocks, as highlighted by the recent observation of superconductivity at $\sim$ 30 K in trilayer $\mathrm{La_4Ni_3O_{10}}$, significantly lower than 80 K reported in bilayer $\mathrm{La_3Ni_2O_7}$. Understanding the factors that control $T_\mathrm{c}$ in this family is therefore of paramount importance. Here, we use resonant inelastic x-ray scattering (RIXS) to investigate the electronic and magnetic excitations of $\mathrm{La_4Ni_3O_{10}}$ in direct comparison with its bilayer counterpart. Our results reveal a markedly different landscape. $\mathrm{La_4Ni_3O_{10}}$ exhibits a more itinerant character, evidenced by broader Ni $dd$ orbital excitations and a strong Ni 3$d$ fluorescence continuum, suggesting weaker electronic correlations than in the bilayer. Despite this, well-defined collective spin excitations persist, including dispersive acoustic and optical magnon branches alongside an incommensurate spin density wave. Using linear spin wave theory, we extract the interlayer superexchange interaction ($J_z$) to be $\sim$ 22 meV, much smaller than that in $\mathrm{La_3Ni_2O_7}$. The weaker correlation and reduced interlayer exchange together provide a consistent explanation for the substantially lower $T_\mathrm{c}$ in the trilayer compound. Our findings establish interlayer magnetic coupling and electronic correlation as key parameters governing superconductivity in layered nickelates and offer critical constraints for understanding the pairing mechanism in this emerging family.

Figures (4)

• Figure 1: Transport and magnetic susceptibility properties, XAS and the incident energy-dependent RIXS map in La$_4$Ni$_3$O$_{10}$.a Top view of the NiO$_2$ plane in La$_4$Ni$_3$O$_{10}$. The solid orange square represents the pseudo-tetragonal unit cell, and the dashed orange square represents the pseudo-orthorhombic $Bmab$ structure (The real space group is $P2_1/a$). b Schematic of the RIXS experimental geometry. c In-plane Brillouin zone for the pseudo-tetragonal unit cell. d Temperature dependence of resistance and magnetic susceptibility at ambient pressure. e, f$\pi$ polarized and $\sigma$ polarized XAS spectra of La$_4$Ni$_3$O$_{10}$ for Ni $L_3$-edge (e) and O $K$-edge (f), respectively. Here, the Ni $L_3$-edge XAS has been corrected by subtracting the La $M$-edge contribution below 852 eV. g, RIXS intensity map measured as a function of incident photon energy with $\pi$ polarisation. h Integrated RIXS spectra in g over the incident energy range of 851.8-853.4 eV (black curve). The corresponding spectra of $\mathrm{La_3Ni_2O_7}$ (red curve) from Ref. xchen2024 are overlaid for comparison.
• Figure 2: The dispersion of magnon in La$_4$Ni$_3$O$_{10}$.a RIXS intensity maps along three high-symmetry directions as indicated with red arrows in the insets. Measurements were taken at 20 K using $\sigma$ polarisation at the Ni $L_3$-edge of 852.3 eV. The red- and green-filled circles denote the undamped energies of the magnetic excitations, extracted from DHO fits, in this and all other panels of the figure. The error bars combine the uncertainty from fitting the RIXS spectra and the experimental energy resolution. b-d RIXS spectrum at representative projected in-plane momentum transfers. Intensities of some curves are multiplied by constant factors, as indicated. e$L$ dependence of RIXS spectra at the fixed $q_\parallel =$ (-0.31, 0). Black curves show the fitted RIXS spectra, and red filled circles indicate the fitted undamped magnon energies.
• Figure 3: The spin configuration and the calculated dispersion of magnon in La$_4$Ni$_3$O$_{10}$.a Schematic illustration of the in-plane exchange couplings $J_1-J_2-J_3$ and interlayer exchange coupling $J_z$ characterizing the effective Heisenberg Hamiltonian for La$_4$Ni$_3$O$_{10}$. To simplify the sketch only nickel cations are shown. b The SDW order of one trilayer as illustrated in the ref. jzhang2020b is shown. The SDW is only present in the top and bottom layers. c The spin configuration for the SDW order in one outer layer. Arrows represent the magnetic moments. The magnitude of each magnetic moment is reflected by the arrow length. d The experimental magnon dispersion $\epsilon_q$ (red filled circles) versus projected in-plane momentum transfer $q_\parallel$ along high symmetry directions at 20 K. The dispersion of magnon calculated by a $J_1-J_2-J_z$ Heisenberg model based on the SDW model shown in b is overlaid.
• Figure 4: SDW order in La$_4$Ni$_3$O$_{10}$.a Integrated SDW peak intensities as a function of projected momentum transfer ($q_\parallel$) along the ($H,H$) direction for $\pi$ and $\sigma$ polarisations. The integration is performed over an energy loss window of $\pm 20$ meV, corresponding to the energy resolution. The solid curves represent Lorentzian fits. $\sigma$-polarized data are scaled for clarity. b SDW peak intensities as a function of incident photon energy and polarization. c, d Polarimetric SDW data. The spectra are decomposed into $\pi\pi'$, $\pi\sigma'$, $\sigma\pi'$, and $\sigma\sigma'$ channels, corresponding to the incoming and outgoing polarization components, respectively. See Methods and Ref. xchen2024 for details. e Temperature dependence of the SDW peak. The solid curves represent Lorentzian fits. Intensities were integrated over an energy loss window of $\pm 20$ meV. f-h Temperature dependence of the SDW peak area (f), correlation length (g), and SDW wave vector position (h) obtained from the fits in e. Error bars indicate the standard deviation from Lorentzian fits.