Resonant inelastic X-ray scattering investigation of Hund's and spin-orbit coupling in $5d^2$ double perovskites

TL;DR

This study uses Re $L_2$/$L_3$-edge RIXS and exact diagonalization to probe Hund's coupling and spin-orbit interactions in 5d^2 double perovskites Ba_2YReO_6 and Sr_2CrReO_6. It finds similar energy scales $\zeta$ and $J_H$ in both materials, with $J_H/\zeta \approx 1.3$, but markedly different spectra due to itinerancy: Ba_2YReO_6 shows sharp atomic-like features while Sr_2CrReO_6 displays a broad electron-hole continuum, indicating a small or indirect band gap. An edge-dependent low-energy feature in Ba_2YReO_6 is attributed to potential dynamic Jahn–Teller effects, and the RIXS data necessitate revising the low-lying excited-state ordering relative to previous models, yielding $\zeta=0.290(5)$ eV and $J_H=0.38(2)$ eV. Together, these results highlight how B-site substitution in 5d^2 double perovskites controls the interplay of correlation, SOC, and itinerancy, with ED calculations providing quantitative support for the extracted energy scales and spectral assignments.

Abstract

B site ordered $5d^2$ double perovskites ($\mathrm{A_2BB'O_6,\ B'}=5d^2)$ display a remarkable range of physical properties upon variation of the chosen B and $\mathrm{B'}$ site ions. This sensitivity to chemical substitution reflects the delicate balance and profound impact of strong electronic correlation and spin-orbit coupling in such systems. We present rhenium $L_2$ and $L_3$ resonant inelastic X-ray scattering (RIXS) measurements of two such physically dissimilar materials, Mott-insulating $\mathrm{Ba_2YReO_6}$ and semiconducting $\mathrm{Sr_2CrReO_6}$. Despite these differences, our RIXS results reveal similar energy scales of Hund's ($J_H$) and spin-orbit coupling ($ζ$) in the two materials, with both systems firmly in the intermediate Hund's coupling regime where $J_H/ζ\sim 1$. However, there are clear differences in their RIXS spectra. The conductive character of $\mathrm{Sr_2CrReO_6}$ broadens and obfuscates the atomic transitions within an electron-hole continuum, while the insulating character of $\mathrm{Ba_2YReO_6}$ results in sharp atomic excitations. This contrast in their RIXS spectra despite their similar energy scales reflects a difference in the itinerancy-promoting hopping integral and illustrates the impact of the local crystal environment in double perovskites. Finally, $L_2$ and $L_3$ edge analyses of the atomic excitations in $\mathrm{Ba_2YReO_6}$ reveal that the ordering of the low lying excited states is inverted compared to previous reports, such that the appropriate energy scales of Hund's and spin-orbit coupling are significantly modified. We present exact diagonalization calculations of the RIXS spectra at both edges which show good agreement with our results for new energy scales of $ζ=0.290(5)$ eV and $J_H=0.38(2)$ eV ($J_H/ζ=1.30(5)$).

Figures (10)

• Figure 1: a) Ideal cubic crystal structure of the B site ordered double perovskites. b) Schematic of $5d^2$ excitations from the single-particle perspective. c) $5d^2$ excited state energy level diagram as a function of $J_H$ for $\zeta =0.29$ eV, calculated using exact diagonalization (see $\S$\ref{['sec:BYRO model']} for details). The non-interacting limit is represented at $J_H/\zeta=0$. The highlighted region corresponds to Yuan et al.'s chosen parameters for $\mathrm{Ba_2YReO_6}$BYRO_RIXS_Yuan2017.
• Figure 2: Top (bottom): $\mathrm{Ba_2YReO_6}$ Re $L_3$ ($L_2$) edge RIXS map with $\mathrm{E_t}$ up to 2.5 eV collected at $\mathrm{T=18}$ K.
• Figure 3: Re $L_3$ edge RIXS map of $\mathrm{Ba_2YReO_6}$ with $\mathrm{E_t}$ up to 0.3 eV obtained at $\mathrm{T=18}$ K.
• Figure 4: $\mathrm{Ba_2YReO_6}$ RIXS spectra at the Re $L_2$($L_3$) edge resonances up to 2.5 eV in $\mathrm{E_t}$ collected at $\mathrm{T=18}$ K and $\mathrm{E_i}=11.961$(10.533) keV. The RIXS spectra are normalized by the maximum intensity of the 0.5 eV feature. The right inset schematically shows the allowed and forbidden single-particle RIXS transitions at the $L_2$ and $L_3$ edges.
• Figure 5: Top (bottom): EDRIXS calculated Re $L_3(L_2)$ edge RIXS heatmap of a $5d^2$ system as a function of $J_H/\zeta$ and $\mathrm{E_t}$. The heatmaps are calculated for fixed $\zeta=0.29$ eV at a temperature T = 18 K.