Impact of Electron Correlations on Infinite-Layer Cuprates and Nickelates

TL;DR

This work directly compares electron-correlation strength in infinite-layer nickelates and cuprates by combining X-ray absorption spectroscopy and resonant inelastic X-ray scattering on isostructural PrNiO$_2$ and SrCuO$_2$. An extended single-band Hubbard analysis, incorporating higher-order exchange and magnon renormalization, yields $U$, $t$, and the correlation ratio $U/t$, revealing that PrNiO$_2$ has a ~20% stronger $U/t$ despite a 25–30% lower $U$ than SrCuO$_2$. The findings show larger zone-boundary magnon and $d_{xy}$ dispersions in SrCuO$_2$, while PrNiO$_2$ exhibits stronger localization and a reduced orbital exchange. The results suggest that moderating $U/t$ toward an optimal range could enhance superconductivity in nickelates, and provide a quantitative framework for cross-family comparison of correlation effects in unconventional superconductors.

Abstract

Optimization of unconventional superconductivity involves a balance of interaction strengths. Precise determination of correlation strength across different material families is therefore important. Here, we present a combined X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) study of infinite-layer PrNiO$_2$ and SrCuO$_2$ that enables fair comparison of their interaction strengths. For both compounds, we study the orbital and magnetic excitations and extract their dispersions along high-symmetry directions. Using a single-band Hubbard model and including higher-order exchange interactions, we derive the correlation factor $U/t$ for both compounds. A key finding is that despite a smaller Coulomb repulsion $U$, PrNiO$_2$ exhibits a correlation strength that is 20% stronger than that of its isostructural cuprate counterpart SrCuO$_2$. This indicates that a moderation of the correlation strength may further optimize superconductivity in nickelates.

Figures (3)

• Figure 1: X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) on SrCuO$_2$ and PrNiO$_2$ films. (a) Schematic illustration of the RIXS scattering geometry --- with $\theta$ describing the incident angle and $\phi$ the sample rotation. $\sigma$ and $\pi$ indicate respectively linear horizontal and vertical incident light polarizations. (b) RIXS energy map recorded on PrNiO$_2$. The purple rectangle inset marks the resonant magnon intensity. (c,d) Cu $L$-resonance of SrCuO$_2$ and Ni $L$-resonance of PrNiO$_2$ probed with XAS (e,f) RIXS spectra recorded on SrCuO$_2$ and PrNiO$_2$ at respectively the Cu and Ni $L$-edges and at in-plane momentum transfers as indicated. Small down-pointing arrows mark the shift (marginal-shift) of $d_{xy}$ excitations in SrCuO$_2$ (PrNiO$_2$). Modeling of the crystal field excitations are indicated with shaded Gaussian profiles. The insets illustrate the equivalent crystal structure of SrCuO$_2$ and PrNiO$_2$.
• Figure 2: RIXS spectroscopy of infinite layer PrNiO$_2$. (a,b) Representative RIXS spectra of PrNiO$_2$ with polarization as indicated. Vertical lines mark the phonon and magnon modes. (c,d) RIXS spectra recorded with $\pi$ polarization on PrNiO$_2$ at two high-symmetry zone boundary points as depicted in the insets. The modeling fits are described in the text. Grey component covers elastic scattering and the two blue components are interpreted as phonon excitations. Red shaded areas models the magnetic excitations. Vertical dashed lines indicate the obtained poles. (e,f) RIXS spectra of PrNiO$_2$ along three high-symmetry directions. Red arrows in (e) indicate the dispersion of damped magnon energy $\omega_1$ (see Supplementary Information) of the fits. Vertical line in (f) is a guide to the eye.
• Figure 3: Dispersion of magnons in PrNiO$_2$ and SrCuO$_2$. (a-c) Renormalization factor $Z_c$ for PrNiO$_2$ (red) and SrCuO$_2$ (blue) along the momentum trajectories indicated in the corresponding insets of (d-f). (d-f) Dispersions of magnons for PrNiO$_2$ (right y-axis, red) and SrCuO$_2$ (left y-axis, blue) along the same trajectories. Symbols represent measured data with error bars, and solid lines are fits. Insets in (d-f) show the momentum path in the Brillouin zone for each case. In (b) and (e), the two samples follow different trajectories, as highlighted in the inset. Therefore, we use azimuthal angles $\phi$ to label the momentum transferred. (g,i) RIXS spectra --- recorded along the $(h,0)$ and $(h,h)$ directions for SrCuO$_2$ and PrNiO$_2$ --- concentrating on the $dd$ excitations. Vertical gray lines are visual guides to the eye. (h,j) Same data as in (g,i) but represented in a false color-map format. Dots are center of the Gaussian fits of the $d_{xy}$ excitations. Vertical bars indicate the experimental energy resolutions.