Investigation of the Electronic Structure and Spin-State Crossover in LaCoO3 Using Photoemission Spectroscopy

TL;DR

This work addresses the spin-state crossover in LaCoO$_3$ by combining excitation-energy-, temperature-, and geometry-dependent photoemission spectroscopy with a CoO$_6$ cluster CI model. The valence-band data show a temperature-driven redistribution of spectral weight from Co $t_{2g}$ toward $e_g$ character, while bulk Co 2$p$ PES reveals a progressive LS→HS mixture as temperature increases. CI analysis yields quantitative LS/HS populations (≈$94:6$ at 100 K to ≈$71:29$ at 400 K), confirming a predominantly LS ground state that evolves into a mixed LS/HS state at elevated temperatures. The results demonstrate PES as a direct, quantitative probe of spin-state transitions in LaCoO$_3$ and emphasize orbital-selective dynamics and strong $p$–$d$ covalency in its electronic structure.

Abstract

Photoemission spectroscopy is a powerful technique for studying electronic structure and spin-state transitions, as it reveals changes in the orbital configuration accompanying a spin-state crossover. In this report, we combine excitation-energy-, temperature-, and geometry-dependent photoemission measurements to probe the electronic structure of LaCoO3 across its thermally driven spin-state transition. By systematically comparing valence-band spectra across a wide photon-energy window - from surface-sensitive soft x-ray photoemission spectroscopy (SXPS) to bulk-sensitive hard x-ray photoemission spectroscopy (HAXPES) - we identify the Co 3d-derived feature (A) along with the O 2p-dominated features (B and C), and explain their relative evolution in terms of photon-energy-dependent photo-ionization cross-section ratios. The thermally induced spin-state crossover is demonstrated using temperature-dependent SXPS valence-band spectra, which show a progressive suppression of the feature A with heating. Geometry-dependent HAXPES measurements further clarify how the signature of the spin-state transition in LaCoO3 is intricately linked to the orbital-selective response of the t2g and eg states. Additionally, angular-dependent photo-ionization cross-section analysis provides a consistent description of the polarization dependence observed in HAXPES. Finally, configuration-interaction analysis of the Co 2p core-level spectra reveals that LaCoO3 evolves from a predominantly low-spin ground state at low temperature to a mixed low-spin/high-spin configuration at elevated temperatures, with the high-spin fraction reaching about 30 percent at 400 K. The temperature evolution of the core-level line shape thus establishes Co 2p photoemission as a sensitive quantitative probe of spin-state transitions in LaCoO3.

Figures (4)

• Figure 1: (a) Valence-band photoemission spectra of LaCoO$_3$ recorded with Al K$\alpha$ (SXPS, 1486.6 eV) and hard X-rays (HAXPES, 6000 eV) at room temperature. (b) Comparison of HAXPES valence-band spectra of LaCoO$_3$ recorded with S and P polarizations at room temperature.
• Figure 2: (a) Temperature-dependent HAXPES valence-band spectra of LaCoO$_3$ obtained with S polarization. (b) Temperature-dependent SXPS valence-band spectra of LaCoO$_3$ measured with an Al K$\alpha$ source.
• Figure 3: Co 2$p$ core-level HAXPES spectra of LaCoO$_3$ measured at 100 and 400 K. The emergence of new features around 786 eV (indicated by an arrow) at 400 K and around 789 eV (indicated by an arrow) at 100 K signifies the transition from the LS to the HS state. Spectra are vertically offset for clarity.
• Figure 4: Comparison between experimental and simulated Co 2$p$ core-level photoemission spectra of LaCoO$_3$. Panels (a) and (f) show the full-multiplet configuration-interaction cluster calculations for the HS and LS states, respectively. Panels (b) and (e) display the Co 2$p$ HAXPES experimental spectra measured at 400 K and 100 K. Panels (c) and (d) present the corresponding theoretical simulations at 400 K and 100 K, respectively, obtained as an incoherent weighted superposition of the calculated LS and HS spectra, with the LS/HS ratios indicated in each panel. The cluster-model simulations are performed for the LS ($t_{2g}^6$) configuration with $10Dq = 0.80$ eV and $\Delta = 0.80$ eV, and for the HS ($t_{2g}^4e_g^2$) configuration with $10Dq = 0.50$ eV and $\Delta = 0.50$ eV. All spectra are normalized to the total integrated intensity.