Contrasting Spin Excitations in Octahedral and Square-Planar n=8 Ruddlesden-Popper Nickelates

Abstract

The discovery of superconductivity in reduced square-planar nickelates marked a major advance in identifying structural and electronic analogs to the high-$T_c$ cuprates. The more recent observation of superconductivity in parent Ruddlesden-Popper (RP) octahedral nickelates with a clear difference in electron count with respect to cuprates raises new questions about the nature of superconductivity across these related but distinct nickelate families. Here, we use Ni $L_3$-edge resonant inelastic x-ray scattering (RIXS) to probe the low-energy excitations in a representative compound of both families: the parent octahedral $n=8$ RP phase Nd$_9$Ni$_8$O$_{25}$ (p-RP), which is non-superconducting, and its reduced square-planar counterpart Nd$_9$Ni$_8$O$_{18}$ (r-RP), which exhibits superconducting correlations with a $T_c \approx 5$ K. The $n=8$ p-RP develops a spin-density-wave (SDW) ground state with ordering wave vector $q_{\mathrm{SDW}} = (1/4,\, 1/4)$, analogous to the bilayer RP, while the $n=8$ r-RP shows an elastic peak at $q^\star = (1/3,\, 0)$. Polarimetric RIXS shows that the p-RP exhibits low-energy spectra dominated by weakly dispersive paramagnons along the 0$\rightarrowπ$ and $π\!\rightarrow\! π$ directions, whereas the r-RP with superconducting correlations displays dispersionless magnetic excitations. Our results comprehensively map out the spin excitations and reveal fundamental differences in the ground state between these two distinct structural families.

Figures (4)

• Figure 1: (a) Qualitative overview of the structural and electronic properties of the $n=8$ parent (p-RP) and reduced Ruddlesden-Popper (r-RP) nickelates, where the reduced phase is obtained via topotactic reduction of the parent phases. (b)The schematic phase diagram as a function of $3d$ electron count denotes where the $n^{\mathrm{th}}$ line phase lies relative to well characterized electron fillings $d^{7}$, $d^{8}$, and $d^{9}$. A schematic of the hole-doped cuprate phase diagram denoting antiferromagnetic (AF) and superconducting (SC) phases is provided as reference. The 3d electron count is defined as $d^{7+1/n}$ for p-RP and $d^{9-1/n}$ for r-RP, where n is the number of Ni-O layers.
• Figure 2: DFT electronic structure and Fermi surface in the $k_{z}=0$ plane of the $n=8$ p-RP Nd$_{9}$Ni$_{8}$NiO$_{25}$ (a-c) and the r-RP Nd$_{9}$Ni$_{8}$O$_{18}$ (d-f). Eight layer structural unit (left) and atom and orbital-resolved density of states: Nd($5d$) (green), Ni($3d$) (blue), and O($2p$) (orange) (right). DFT band structure along high-symmetry lines with Ni-$e_{g}$ character overlaid: Ni-$d_{x^{2}-y^{2}}$ (blue), Ni-$d_{z^{2}}$ (pink), and purple denoting the admixture of the two. Constant energy contour at $\varepsilon_{\mathrm{F}}$ in the $k_{z}=0$ plane with the Ni-$e_{g}$ orbital character overlaid using the same color scheme as the band structure.
• Figure 3: (a-b) Low energy-loss RIXS maps along (H, 0) and (H, H) directions of the BZ of the p-RP and (c-d) of the r-RP. (e) Momentum profile of the elastic line at E$_\mathrm{loss}$=0 meV of the p- and r-RP along the (H, 0) direction. (f) Momentum dependence of the E$_\mathrm{loss}$=0 RIXS intensity of the p- and r-RPs along the (H, H) direction, highlighting the SDW at q=($\frac{1}{4}$, $\frac{1}{4}$). (g) RIXS spectra of the p- and r-RP, covering the 0-5 eV E$_\mathrm{loss}$ range. The orbital excitations are labeled in the r-RP together with signatures of the Nd-Ni hybridization.
• Figure 4: (a) pol-RIXS data at q=(0.25,0) r.l.u. in the r-RP sample, showing the non-cross polarized channel ($\pi-\pi^\prime$) in red and the cross polarized channel ($\pi-\sigma^\prime$) in yellow. (b) Example fitting at q=(0.45,0) r.l.u. in the r-RP sample. An example fitting of a p-RP spectra, which is the same model aside from the background, can be found in Supplementary Figure S2. (c-d) Momentum dependence of the spin excitations of p- and r-RP along the (c) (H, 0) and (d) (H,H) directions. (e-f) Damping of p- and r-RP along the (e) (H, 0) and (f) (H,H) directions. The 95% confidence intervals are plotting for each data point.