Ground state oxygen holes and the metal-insulator transition in the negative charge transfer rare-earth nickelates

Abstract: The metal-insulator transitions and the intriguing physical properties of rare-earth perovskite nickelates have attracted considerable attention in recent years. Nonetheless, a complete understanding of these materials remains elusive. Here, taking a NdNiO3 thin film as a representative example, we utilize a combination of x-ray absorption and resonant inelastic x-ray scattering (RIXS) spectroscopies to resolve important aspects of the complex electronic structure of the rare-earth nickelates. The unusual coexistence of bound and continuum excitations observed in the RIXS spectra provides strong evidence for the abundance of oxygen 2p holes in the ground state of these materials. Using cluster calculations and Anderson impurity model interpretation, we show that these distinct spectral signatures arise from a Ni 3d8 configuration along with holes in the oxygen 2p valence band, confirming suggestions that these materials do not obey a conventional positive charge-transfer picture, but instead exhibit a negative charge-transfer energy, in line with recent models interpreting the metal to insulator transition in terms of bond disproportionation.

• The paper demonstrates that oxygen 2p holes drive the metal-insulator transition in NdNiO3 through advanced synchrotron techniques.
• It employs XAS, RIXS, and cluster calculations with the Anderson Impurity Model to confirm a 3d⁸L ground state as evidence of a negative charge-transfer scenario.
• The findings redefine the electronic structure of rare-earth nickelates, suggesting new pathways for tuning electrical properties via ligand-driven mechanisms.

Electronic Structure and Metal-Insulator Transition in Rare-Earth Nickelates

The study of rare-earth perovskite nickelates, particularly NdNiO3 thin films, presents significant advancements in understanding metal-insulator transitions (MIT) driven by complex electronic structures. This paper utilizes synchrotron-based techniques, specifically x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS). These methods elucidate the intricate balance between bound and continuum excitations, shedding light on the oxygen 2p hole phenomenology in the ground state of nickelates. By combining experimental data with theoretical models, the research posits a negative charge-transfer energy scenario that reinterprets [ReNiO3] electronic structures.

The experimental techniques employed, especially RIXS, have provided critical insights into these systems. The RIXS spectra exhibit the unique coexistence of Raman-like dd-excitations and continuum excitations, confirming the self-doping mechanism via oxygen 2p holes. Extensive cluster calculations and the application of the Anderson Impurity Model (AIM) further validate that the NdNiO3's electronic ground state is predominantly 3d^8L, delineating a negative charge-transfer system.

Major Findings

1. Charge-Transfer and Electronic Configuration:
   • The study emphasizes NdNiO3 as a negative charge-transfer compound with a high density of oxygen 2p holes. This departure from the conventional positive charge-transfer picture offers a new perspective on nickelates' electronic properties.
2. Spectral Evidence:
   • XAS and RIXS spectra collectively show bound excitations layered over a strong continuum, hinting at NiO-like multiplet effects but deviating distinctly towards oxygen 2p-dominated electronic structures. The data reveals that temperature-dependent spectral changes correlate with the metal-to-insulator phase transition.
3. RIXS Mapping and Analysis:
   • The differential behavior of RIXS signals, particularly the temperature-sensitive fluorescence-like continuum, reinforces the presence of energetic shifts linked to electronic transitions. The suppression of low-energy contributions at lower temperatures aligns with phase-dependent conductivity changes observed in nickelates.

Theoretical Implications

This investigation provides substantial empirical backing to the Mizokawa model, where bond disproportionation and the self-doping scenario dominate the framework explaining MITs in rare-earth nickelates. The research endorses a theoretical reevaluation of transition metal oxides, advocating an adjustment from primarily nickel-centric descriptions to those accentuating the role of ligands—here the oxygen subsystem.

Future Directions

This study opens pathways for exploring other compounds with possible negative charge-transfer properties and high formal oxidation states. With advanced techniques like RIXS and XAS, there's potential to classify and manipulate electronic structures in correlated electron systems like sulfides and selenides, which share similar charge-transfer complexities.

Practical Implications

Understanding the electronic ground states of nickelates can lead to better control of their intriguing electrical properties. Such knowledge could pioneer advancements in tuning functionalities through external stimuli, like strain, ultimately enhancing material design for electronic and spintronic applications.

This comprehensive analysis extends core insights into the physics of perovskite nickelates and continues to bridge gaps in comprehending the MIT mechanism. As experimental techniques progress, further research should aim at delineating specific mechanistic models, making substantial contributions to both fundamental science and material application realms.