Evaluating covalency using RIXS spectral weights: Silver fluorides vs. cuprates

TL;DR

This work shows that silver fluorides, particularly AgF2, exhibit CuO$_2$-like electronic structure when probed at the Ag $L_3$ edge with XAS and RIXS, revealing both $dd$ and charge-transfer excitations akin to La$_2$CuO$_4$. By developing and applying a covalency metric based on the ratio of $dd$ to CT spectral weights, and mapping this ratio across geometries, the authors quantify the degree of hybridization in AgF2, AgFBF4, and cuprates, finding AgF2 to be moderately more covalent than cuprates. The results from RIXS at the Ag $L_3$ edge complement prior Cu/Ligand-edge studies and support AgF2 as a tangible cuprate analogue for exploring high-$T_c$ superconductivity and unusual magnetism in quasi-two-dimensional and quasi-one-dimensional silver fluorides. The work also demonstrates the feasibility of tender X-ray RIXS with TEXS and highlights core-hole lifetime effects as a limitation of current theoretical treatments, pointing to future high-resolution studies on single crystals and doped variants. Together, these findings establish a concrete electronic-structure bridge between silver fluorides and cuprates and open pathways to new superconducting and magnetic phases in related materials.

Abstract

We investigate the electronic structure of AgF2, AgFBF4, AgF and Ag2O using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Ag L3 edge. XAS results were compared with density functional theory computations of the spectra, allowing an identification of main features and an assessment of the theoretical approximations. Our RIXS measurements reveal that AgF2 exhibits charge transfer excitations and dd excitations, analogous to those observed in La2CuO4. We propose to use the ratio of dd to CT spectral weight as a measure of the covalence of the compounds and provide explicit equations for the weights as a function of the scattering geometry for crystals and powders. The measurements at the metal site L3 edge and previous measurements at the ligand K edge reveal a striking similarity between the fluorides and cuprates materials, with fluorides somewhat more covalent than cuprates. These findings support the hypothesis that silver fluorides are an excellent platform to mimic the physics of cuprates, providing a promising avenue for exploring high-Tc superconductivity and exotic magnetism in quasi-two-dimensional (AgF2) and quasi-one-dimensional (AgFBF4) materials.

Figures (14)

• Figure 1: Crystallographic structures of La2CuO4(a), AgF2(b) and AgFBF4(c).
• Figure 2: Absorption spectra of the investigated compounds and silver in the region of the Ag $L_3$ absorption edge. We show data from the present work (full colored lines) and Ref. Miyamoto2010 (dot-dashed lines). The curves were normalized to 1 at the highest energy measured and plotted with a vertical shift of 1.5 (arb. un.). We also show the SCH results (black). The intensity of the DFT data was adjusted to ease the comparison with the experiment. The theoretical spectra were shifted by 2 eV to higher energy, except for AgF2 (+4 eV shift) and Ag (-2 eV shift).
• Figure 3: Ag symmetry projected (orange, blue, red) DOS and total silver DOS (cyan shadowing) compared to the computation of the XAS in the SCH method shown in Fig. \ref{['fig:XANES']} (black, intensity in arbitrary units). To simulate the edge, we show only the unoccupied part of the DOS, and the Fermi energy has been shifted to match the main features of the SCH computation.
• Figure 4: Same as Fig. \ref{['fig:dosAg']} for $d^{10}$ compounds. The Ag2O data has been shifted vertically by 0.8 eV$^{-1}$.
• Figure 5: Same as Fig. \ref{['fig:dosAg']} for $d^{9}$ compounds. The AgF2 data has been shifted vertically by 0.8 eV$^{-1}$.