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

TL;DR

This study compares spin excitations in octahedral and square-planar nickelates, revealing fundamental differences in their magnetic ground states and excitations related to superconductivity.

Contribution

It provides the first detailed RIXS analysis of both nickelate families, highlighting their contrasting magnetic excitations and ground states.

Findings

p-RP exhibits a spin-density-wave ground state with a specific wave vector.

r-RP shows dispersionless magnetic excitations associated with superconductivity.

Distinct magnetic excitation spectra characterize the two nickelate families.

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$\rightarrow\pi$ and $\pi \!\rightarrow\! \pi$ 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.