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

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

Problem Statement
The perovskite oxide LaCoO3 exhibits a complex, thermally driven spin-state transition that has remained controversial for decades. While it is established that the material transitions from a non-magnetic low-spin (LS, $t_{2g}^6e_g^0$, $S=0$) insulator at low temperatures to a paramagnetic semiconductor and eventually a poor metal at higher temperatures, the nature of the intermediate and high-temperature spin states is debated. The central question is whether the Co$^{3+}$ ions transition to an intermediate-spin (IS, $S=1$) state, a high-spin (HS, $S=2$) state, or a mixed LS-HS configuration. Previous experimental and theoretical studies have yielded conflicting results due to the distinct sensitivities of different probes (e.g., neutron scattering, XAS, RIXS) and the indirect nature of the information they provide regarding occupied electronic states. A direct spectroscopic method capable of resolving the occupied electronic structure across a wide range of conditions is required to resolve these contradictions.

Methodology
This study employs a systematic photoemission spectroscopy (PES) approach, combining surface-sensitive soft X-ray PES (SXPS) and bulk-sensitive hard X-ray PES (HAXPES) to probe the electronic structure of polycrystalline LaCoO3.

• Experimental Setup: Measurements were conducted across a wide photon-energy window (Al K$\alpha$ at 1486.6 eV and 6 keV hard X-rays) and a temperature range from 100 K to 400 K (and up to 623 K for valence bands).
• Geometry Dependence: HAXPES measurements were performed in both parallel (P-polarization) and perpendicular (S-polarization) geometries to analyze orbital-selective responses and polarization dependencies.
• Theoretical Modeling: Co 2p core-level spectra were analyzed using full multiplet configuration-interaction (CI) cluster-model calculations (Quanty code). The system was modeled as a CoO$_6$ cluster in octahedral symmetry, incorporating parameters for on-site Coulomb interaction ($U_{dd}$), charge-transfer energy ($\Delta$), crystal-field splitting ($10Dq$), and $p$-$d$ hybridization. The ground state was expanded in terms of $3d^6$, $3d^7\underline{L}$, and $3d^8\underline{L}^2$ configurations.

Key Results

1. Valence Band (VB) Evolution:

   • Three primary features were identified in the VB spectra: Feature A (1.0 eV, Co 3d-derived) and Features B and C (3.0 and 5.0 eV, O 2p-dominated).
   • Photon Energy and Geometry: Comparisons between SXPS and HAXPES revealed that the relative intensity of the Co 3d-derived Feature A is suppressed in HAXPES compared to SXPS. This was attributed to the significant contribution of La 5p states in the HAXPES valence band, which are strongly suppressed in S-polarization, thereby enhancing the relative visibility of Co 3d features.
   • Temperature Dependence: Temperature-dependent measurements (both SXPS and HAXPES) showed a progressive suppression of Feature A (Co 3d) with increasing temperature. This indicates a reduction in spectral weight from the fully occupied $t_{2g}$ orbitals (characteristic of the LS state) and a redistribution toward $e_g$ states, consistent with a spin-state crossover.

2. Core-Level (Co 2p) Analysis:

   • The Co 2p core-level spectra exhibited distinct temperature-dependent changes. At low temperatures (100 K), a prominent feature appeared near 789 eV. As temperature increased to 400 K, this feature diminished, and a new feature emerged near 786 eV, accompanied by broadening and asymmetry in the main line.
   • Configuration-Interaction Modeling: The experimental spectra were successfully reproduced by a weighted linear combination of calculated LS and HS spectra. The IS configuration was tested but failed to reproduce the experimental line shapes or satellite positions, suggesting a negligible contribution from the IS state.
   • Quantitative Population: The modeling quantified the spin-state populations: at 100 K, the system is predominantly LS (94% LS, 6% HS). At 400 K, the population shifts to a mixed state (71% LS, 29% HS).

Key Contributions

• Direct Observation of Spin-State Crossover: The study provides direct spectroscopic evidence of the spin-state transition in LaCoO3 by tracking the redistribution of spectral weight between $t_{2g}$ and $e_g$ orbitals in the valence band and the evolution of core-level line shapes.
• Resolution of the IS vs. Mixed-State Debate: Through rigorous cluster-model analysis of Co 2p spectra, the authors demonstrate that the intermediate temperature range is best described by an inhomogeneous mixture of LS and HS states, with negligible contribution from an intermediate-spin state.
• Orbital-Selective Response: The work clarifies how the signature of the spin-state transition is linked to the orbital-selective response of $t_{2g}$ and $e_g$ states, particularly through the analysis of polarization-dependent photo-ionization cross-sections and geometry-dependent measurements.
• Quantitative Probe: The study establishes Co 2p photoemission, when analyzed with CI models, as a sensitive and quantitative probe for determining spin-state fractions in LaCoO3.

Significance
The paper claims that photoemission spectroscopy offers a powerful and complementary approach to traditional techniques for understanding the electronic behavior of LaCoO3. By combining bulk-sensitive HAXPES with surface-sensitive SXPS across varying temperatures and geometries, the authors provide a coherent picture of the spin-state evolution. The results confirm that LaCoO3 evolves from a predominantly low-spin ground state at low temperatures to a mixed low-spin/high-spin configuration at elevated temperatures (reaching ~30% HS at 400 K). This work underscores the importance of orbital-selective effects in the material's electronic structure and validates the use of photoemission spectroscopy as a definitive tool for resolving spin-state transitions in correlated oxides.