Observation of correlated plasmons in low-valence nickelates

TL;DR

This study reports the first observation of dispersing plasmons in the low-valence nickelate Pr4Ni3O8 using O K-edge RIXS, and compares them to overdoped La2-xSrxCuO4 at similar doping. The plasmons in Pr4Ni3O8 have a lower energy scale, reduced dispersion, and stronger damping, with a notable softening at elevated temperatures, indicating a distinct charge-screening landscape from cuprates. The authors confirm these trends with RPA calculations on trilayer nickelate and single-layer cuprate models, attributing the differences to reduced hopping and smaller long-range Coulomb interactions in nickelates. Overall, the work provides quantitative constraints on nickelate charge dynamics, informing theories of unconventional superconductivity in these materials.

Abstract

The discovery of nickelate superconductors has opened a new arena for studying the behavior of correlated electron liquids that give rise to unconventional superconductivity. While critical information about a material's charge dynamics is encoded in its plasmons, collective modes of the electron gas, these excitations have not yet been observed in nickelate materials. Here, we use resonant inelastic x-ray scattering (RIXS) to detect plasmons in the metallic, low-valence nickelate Pr4Ni3O8. Although qualitatively similar to those in cuprates, the nickelate plasmons are more heavily damped and have a lower velocity than those in a cuprate at comparable doping, which we attribute to reduced electronic hopping and enhanced screening of the long-range Coulomb interactions. Furthermore, the plasmons in Pr4Ni3O8 soften with increasing temperature, in contrast to the cuprate, where plasmons remain at nearly fixed energy but become more strongly damped. Taken together, these results reveal a distinct charge-screening landscape in nickelates and place quantitative constraints on analogies to cuprates.

Figures (4)

• Figure 1: Dispersive plasmons in Pr$_{4}$Ni$_{3}$O$_{8}$. (a), Schematic of the crystal structures of Pr$_{4}$Ni$_{3}$O$_{8}$ (Pr438, red) and La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO, blue), along with the corresponding structural and electronic hopping parameters. For the single layer model, $d$ is the interlayer distance, and $t_z$ denotes the interlayer electronic hopping integral. For the trilayer model, $d$ and $d^\prime$ are the interlayer and intra-trilayer distances, respectively. $t_z$ denotes the intra-trilayer hopping, while inter-trilayer hopping is neglected, as it is expected to be much smaller than the intra-trilayer term. $t$ and $t^\prime$ are the nearest- and next-nearest-neighbor in-plane hopping for both Pr$_{4}$Ni$_{3}$O$_{8}$ and La$_{2-x}$Sr$_{x}$CuO$_{4}$. (b), Reciprocal space trajectories of the *RIXS measurements of the in-plane plasmon dispersions, which were performed at a fixed scattering angle $2\Theta$. The values presented are in r.l.u. (reciprocal lattice units). (c),(d), RIXS intensity maps of Pr$_{4}$Ni$_{3}$O$_{8}$ and La$_{2-x}$Sr$_{x}$CuO$_{4}$ ($\delta=0.35$), respectively, collected at 40 K, exhibiting dispersive plasmons with momenta primarily along the ($H$, 0) direction, following the $\bm{Q}$ trajectories presented in (b). The markers indicate plasmon peak positions extracted from fitting (See Ref. supp Sec. S2). (e),(f) Summary of the in-plane plasmon dispersions, showing the peak positions and *FWHM that we obtain from fitting. Data for La$_{2-x}$Sr$_{x}$CuO$_{4}$ ($\delta=0.16$) and Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+x}$ (Bi2223, $\delta=0.18$) are reproduced from Ref. Nag2020detection and Ref. Nakata2025Outofphasea, respectively. The shaded area in (f) represents the quasi-elastic regime, determined by the energy resolution of $\sim30$ meV. The full extent of the error bars in (c),(d) denote the *FWHM of plasmon peaks, whereas all other error bars are 1-$\sigma$ confidence intervals evaluated from the fitting.
• Figure 2: Out-of-plane plasmon dispersions. (a),(b), RIXS spectra of various $\bm{Q}$ along the out-of-plane $L$ direction for Pr$_{4}$Ni$_{3}$O$_{8}$ and La$_{2-x}$Sr$_{x}$CuO$_{4}$ ($\delta=0.35$), respectively. The shaded peak profiles represent the fitted plasmon contributions, and the solid black lines indicate the fitting results with all components accounted for, including the quasi-elastic line, low-energy phonons, and high-energy bimagnons combined with a continuous charge background. (c), Fitted plasmon peak positions of Pr$_{4}$Ni$_{3}$O$_{8}$. (d), Fitted plasmon peak positions of La$_{2-x}$Sr$_{x}$CuO$_{4}$. The solid lines are guides to the eye.
• Figure 3: Theoretical description of the dispersing plasmons. (a)--(c), Simulations of plasmons in Pr$_{4}$Ni$_{3}$O$_{8}$ based on RPA calculations of the dynamical susceptibility applied to a trilayer model, as described in the main text. (d)--(f), RPA simulations of plasmons in La$_{2-x}$Sr$_{x}$CuO$_{4}$ ($\delta=0.35$) applied to a single-layer model. The crosses represent plasmon peak positions derived from fitting the experimental data. In (a) and (d), the simulations followed the experimental $\bm{Q}$ trajectory as shown in Fig. \ref{['fig:Hdisp']}(b), while in other panels, the in-plane momentum transfer $H$ was fixed at the indicated values.
• Figure 4: Temperature dependence of the plasmons. (a),(b), Temperature dependent RIXS spectra for Pr$_{4}$Ni$_{3}$O$_{8}$ and La$_{2-x}$Sr$_{x}$CuO$_{4}$ ($\delta=0.35$), respectively, collected at the indicated $\bm{Q}$ vectors and temperatures. The shaded areas indicate the fitted plasmon profiles, and the black lines denote the summation of different contributions. (c),(d), Temperature dependence of the plasmon peak positions and FWHM, respectively, obtained from the fitting. The dashed lines in (a),(b) and bold lines in (c),(d) are guides to the eye.