[Paper Review] Probing electronic excitations in iridates with resonant inelastic x-ray scattering and emission spectroscopy techniques

Young‐June Kim, J. P. Clancy+7 more|arXiv (Cornell University)|May 9, 2018

Rare-earth and actinide compounds1 references3 citations

TL;DR

This study uses resonant inelastic x-ray scattering (RIXS) and resonant x-ray emission spectroscopy (RXES) to probe electronic excitations in iridates, overcoming core-hole lifetime broadening via high-resolution detection to reveal detailed unoccupied electronic structure. The key finding is that ultra-short core-hole lifetime simplifies resonance conditions for d-d transitions, enabling precise mapping of crystal-field and spin-orbit-split states across diverse iridate compounds with varying electronic bandwidths and oxidation states.

ABSTRACT

We report a comprehensive resonant inelastic x-ray scattering (RIXS) study of various iridate materials focusing on core-level excitations and transitions between crystal-field split d-levels. The 2p core hole created at the Ir $L_3$ absorption edge has a very short lifetime giving rise to a broad absorption width ($\sim 5$~eV). This absorption linewidth broadening can be overcome by studying the resonant x-ray emission spectroscopy (RXES) map, which is a two-dimensional intensity map of the Ir L$α_2$ emission obtained with high energy-resolution monochromator and analyzer. By limiting the emitted photon energy to a narrow range, one can obtain x-ray absorption spectra in the high energy-resolution fluorescence detection (HERFD) mode, while one can also simulate quasi-$M_4$-edge absorption spectra by integrating over incident photon energies. Both methods improve the absorption line width significantly, allowing detailed studies of unoccupied electronic structure in iridates and other $5d$ transition metal compounds. On the other hand, the short lifetime of the 2p core hole benefits the study of excitations of valence electrons. We show that the incident energy dependence of the RIXS spectra for $d-d$ transitions is simple to understand due to the short core-hole lifetime, which validates ultra-short core-hole lifetime approximation used widely in theoretical calculations. We compared $d-d$ excitations in various iridates and found that the excitations between the t$_{2g}$ and e$_g$ states share many similarities among different materials. However, the RIXS spectra due to the transitions between the spin-orbit-split t$_{2g}$ levels vary widely depending on the oxidation state and electronic bandwidths.

Motivation & Objective

• To overcome core-hole lifetime broadening in Ir L3-edge x-ray spectroscopy to access high-resolution electronic structure in iridates.
• To investigate how electronic excitations—particularly d-d transitions between t2g and eg orbitals, and spin-orbit-split t2g states—vary across different iridate materials.
• To validate the ultra-short core-hole lifetime approximation used in theoretical calculations by comparing experimental RIXS spectra with predictions.
• To establish RXES and RIXS as powerful tools for probing unoccupied states in 5d transition metal oxides, especially in systems where neutron scattering is impractical.
• To correlate peak widths and spectral features with electronic bandwidths and hybridization, distinguishing atomic-like from itinerant behavior in iridates.

Proposed method

• Employing resonant inelastic x-ray scattering (RIXS) with high-energy-resolution monochromator and analyzer to map Ir Lα2 emission as a function of incident photon energy.
• Using high-energy-resolution fluorescence detection (HERFD) mode to obtain narrow-line XAS spectra by limiting emitted photon energy to a narrow range.
• Simulating quasi-M4-edge absorption spectra by integrating RIXS spectra over incident photon energies, enabling high-resolution absorption-like spectra without core-hole broadening.
• Applying the ultra-short core-hole lifetime approximation to interpret RIXS spectra, validated by the simplicity of incident energy dependence for d-d transitions.
• Comparing RIXS spectra across iridates with varying oxidation states (Ir4+ vs. Ir5+) and structural environments (e.g., Sr2YIrO6 vs. IrO2) to isolate effects of bandwidth and hybridization.
• Using atomic models including spin-orbit coupling and Hund’s coupling to assign peak positions in RIXS spectra, particularly for spin-orbit-split t2g states.

Experimental results

Research questions

• RQ1 How can core-hole lifetime broadening in Ir L3-edge x-ray absorption be overcome to access high-resolution electronic structure?
• RQ2 To what extent does the ultra-short core-hole lifetime simplify the resonance condition for d-d excitations in iridates, and how does this validate theoretical approximations?
• RQ3 How do the energies and widths of d-d excitations between t2g and eg states vary across different iridate compounds with distinct oxidation states and electronic bandwidths?
• RQ4 What is the origin of the sharp vs. broad peak widths observed in RIXS spectra of Ir5+ and Ir4+ compounds, and how do they reflect hybridization and bandwidth?
• RQ5 How do spin-orbit-split t2g excitations differ in intensity and energy across iridates, and what do these differences reveal about electronic correlations and local symmetry?

Key findings

• HERFD-XAS mode using high-resolution detection reduces the effective absorption linewidth significantly, enabling detailed study of unoccupied electronic states despite the intrinsic ~5 eV core-hole width.
• Quasi-M4-edge absorption spectra can be simulated by integrating RIXS spectra over incident photon energy, providing a viable alternative for high-resolution absorption studies under extreme conditions (e.g., high pressure).
• The incident energy dependence of d-d RIXS transitions is simple and resonantly enhanced near the x-ray absorption maximum, validating the ultra-short core-hole lifetime approximation in theoretical modeling.
• Transitions between t2g and eg states show consistent energy scales (~10Dq) across iridates, but RIXS measures a larger splitting than XAS due to core-hole effects and the filled t2g initial state.
• In Sr2YIrO6 (Ir5+, 5d4), RIXS peaks are resolution-limited and extremely sharp, indicating isolated Ir ions with negligible hybridization and narrow d-bandwidths.
• In metallic IrO2, RIXS peaks are broad, reflecting wide metallic bandwidths and interband character, consistent with strong hybridization between Ir eg orbitals and O 2p states.