[Paper Review] The electronic structure of LiFeAs and NaFeAs probed by resonant inelastic x-ray scattering spectra

E.Z. Kurmaev, John A. McLeod+6 more|arXiv (Cornell University)|Mar 27, 2009

X-ray Spectroscopy and Fluorescence Analysis6 citations

TL;DR

This study investigates the electronic structure of LiFeAs and NaFeAs using resonant inelastic x-ray scattering (RIXS) and density functional theory (DFT) calculations. It finds that Fe 3d states dominate near the Fermi level, and the I(L2)/I(L3) intensity ratio and Fe L3 peak width indicate weak to moderate electron correlation, suggesting these iron-based superconductors are not strongly correlated systems.

ABSTRACT

Results of resonant inelastic X-ray scattering (RIXS) measurements at Fe L-edges and electronic structure calculations of LiFeAs and NaFeAs are presented. Both experiment and theory show that in the vicinity of the Fermi energy, the density of states is dominated by contributions from Fe 3d-states. The comparison of Fe L2,3 non-resonant and resonant (excited at L2-threshold) X-ray emission spectra with spectra of LaOFeAs and CaFe2As2 show a great similarity in energy and I(L2)/I(L3) intensity ratio. The I(L2)/I(L3) intensity ratio of all FeAs-based superconductors is found to be more similar to metallic Fe than to correlated FeO. Basing on these measurements we conclude that iron-based superconductors are weakly or moderately correlated systems.

Motivation & Objective

• To determine the electronic structure of LiFeAs and NaFeAs using resonant inelastic x-ray scattering (RIXS) at the Fe L2,3 edges.
• To compare the measured XES spectra with DFT calculations to assess the nature of electronic correlations in these iron-based superconductors.
• To investigate whether LiFeAs and NaFeAs exhibit strong electron correlation, as seen in compounds like FeO, or are weakly correlated like metallic Fe.
• To resolve the puzzle of superconductivity in these materials despite low electron-phonon coupling and absence of spin-density wave order.
• To establish a comparative framework using related FeAs compounds (e.g., CaFe2As2, LaOFeAs) to identify general trends in Fe-based superconductor electronic structure.

Proposed method

• Performed resonant and non-resonant Fe L2,3 X-ray emission spectroscopy (XES) at beamline 8.0.1 of the Advanced Light Source.
• Used a Rowland circle geometry spectrometer with spherical gratings and an area-sensitive multichannel detector to achieve an instrumental resolving power of 10^3.
• Normalized spectra to incident photon current using a gold mesh to correct for beam intensity fluctuations.
• Conducted Fe 2p X-ray absorption spectroscopy (XAS) in total electron yield mode to determine optimal excitation energies for resonant XES.
• Performed full-potential linearized augmented plane wave (FP-LAPW) electronic structure calculations using the WIEN2k code with the GGA-PBE functional.
• Fitted XES spectra using pseudo-Voigt functions to extract peak positions, intensities, and widths, and aligned the Fermi level using the second derivative of the spectrum and DFT density of states.

Experimental results

Research questions

• RQ1 What is the contribution of Fe 3d states to the electronic density of states near the Fermi level in LiFeAs and NaFeAs?
• RQ2 How does the I(L2)/I(L3) intensity ratio in Fe L2,3 XES of LiFeAs and NaFeAs compare to that of metallic Fe and strongly correlated FeO?
• RQ3 To what extent are LiFeAs and NaFeAs weakly or moderately correlated systems based on their XES and DFT-derived electronic structure?
• RQ4 How do the Fe L3 peak width and spectral shape in XES reflect the degree of electron correlation in these iron-based superconductors?
• RQ5 What is the role of As 4p states in the valence band structure, and how do they compare to Fe 3d contributions?

Key findings

• The Fe 3d states dominate the electronic density of states near the Fermi level in both LiFeAs and NaFeAs.
• The I(L2)/I(L3) intensity ratio in LiFeAs and NaFeAs is closer to that of metallic Fe (0.65) than to strongly correlated FeO (0.55), indicating weak to moderate correlation.
• The Fe L3 peak full width at half maximum (FWHM) in XES is consistent with itinerant Fe 3d electrons, not localized states.
• Pseudo-Voigt fitting of resonant Fe L3 XES spectra yields four components for LiFeAs and four for resonant NaFeAs, with the main peak at ~705.5 eV closely aligned with the Fermi level.
• The Fermi level estimated from the second derivative of the XES spectrum (706.5–706.8 eV) is higher than the curve-fit alignment (705.9–706.3 eV), consistent with instrumental resolution effects.
• DFT calculations confirm that Fe 3d states are the primary contributors near the Fermi level, while As 4p states are located at the bottom of the valence band, consistent across all studied FeAs-based compounds.