Exotic Material Switches ‘on’ and ‘off’ Electron Behaviour for Future Devices

Researchers are increasingly focused on understanding the complex behaviour of correlated multi-orbital systems, and a new study details how geometric frustration impacts electronic properties in a specific nickelate material. Yidian Li, Mingxin Zhang from ShanghaiTech University, and Xian Du, alongside Cuiying Pei, Jieyi Liu, and Houke Chen et al., present compelling evidence of orbital-selective Mottness in Pr4Ni3O10, revealing a striking contrast with La4Ni3O10. Their work, utilising high-resolution angle-resolved photoemission spectroscopy and theoretical modelling, demonstrates that subtle changes in the interlayer Ni-O-Ni bonding angle can significantly modulate the coherence of electron orbitals. This finding is significant because it identifies a structural control parameter for tuning correlated electronic states in trilayer nickelates, potentially offering insights into the mechanisms behind high-pressure superconductivity.

Orbital-selective incoherence and coherence in trilayer nickelate electronic structure drive its unusual properties

Researchers have uncovered a striking interplay between electronic structure and material geometry in trilayer nickelates, potentially paving the way for enhanced superconductivity. This work focuses on La4Ni3O10 and Pr4Ni3O10, materials exhibiting complex correlated electron behaviour. Through high-resolution angle-resolved photoemission spectroscopy and theoretical calculations, scientists have revealed a pronounced difference in the behaviour of electrons within these compounds.
In La4Ni3O10, electrons in the d_(z^2) and d_(x^2-y^2) orbitals readily interact, demonstrating strong interorbital hybridization. However, in Pr4Ni3O10, a dramatic shift occurs, with the d_(z^2) band becoming markedly incoherent and losing spectral weight. Simultaneously, the d_(x^2-y^2) bands maintain their coherence in both materials, creating a distinct incoherence/coherence dichotomy.

This unexpected contrast is directly linked to an orbital-selective Mott phase, modulated by the angle of the Ni-O-Ni bonds between layers. The suppression of d_(z^2) orbital participation further weakens interorbital hybridization and significantly influences the density-wave transition observed in Pr4Ni3O10.
Furthermore, the energy gap associated with the density-wave is substantially reduced in Pr4Ni3O10, likely due to additional scattering caused by the local magnetic moments of the Pr3+ ions. These findings illuminate the intricate connections between a material’s lattice structure, orbital configuration, spin, and electronic properties.

The research demonstrates that structural modifications can effectively control the correlated electronic state in trilayer nickelates, offering a promising route towards understanding and ultimately enhancing superconductivity under high pressure. This structural control parameter provides a concrete framework for future materials design and exploration.

Valence band mapping of La4Ni3O10 and Pr4Ni3O10 trilayer nickelates via angle-resolved photoemission spectroscopy reveals complex electronic structures

High-resolution angle-resolved photoemission spectroscopy served as the primary experimental technique to investigate the electronic properties of trilayer nickelates. Researchers meticulously examined both La4Ni3O10 and Pr4Ni3O10, employing photon energies ranging to probe the valence band structure with exceptional detail.

The samples were cleaved in-situ under ultra-high vacuum conditions at a base pressure of 1×10^-10 Torr to ensure pristine surface sensitivity and minimise contamination. Measurements were conducted at a sample temperature of 20 Kelvin, stabilised using a closed-cycle helium cryostat. The emitted photoelectrons were analysed using a hemispherical analyser with an energy resolution of 15 meV, allowing for precise mapping of the electronic band structure.

Data acquisition involved systematically varying the angle of incidence of the incident photons and the analyser angle to construct momentum-resolved spectra. Theoretical calculations, utilising density functional theory, complemented the experimental work. These calculations provided a framework for interpreting the observed spectral features and understanding the underlying electronic structure of the nickelates.

Comparison between the experimental data and theoretical band structures facilitated the identification of key orbital characteristics and the assessment of interorbital hybridization. Specifically, the study focused on the d_(z^2 ) and d_(x^2-y^2 ) orbitals, revealing a striking incoherence/coherence dichotomy between the two compounds and linking it to the Ni-O-Ni bonding angle. The research further connected the observed orbital behaviour to the density-wave transition in Pr4Ni3O10, noting a substantial reduction in the density-wave gap likely due to scattering from Pr3+ local moments.

Orbital-selective incoherence and Fermi surface evolution in lanthanum and praseodymium nickelates reveal complex electronic behavior

High-resolution angle-resolved photoemission spectroscopy, combined with theoretical calculations, reveals pronounced interorbital hybridization in La4Ni3O10, while Pr4Ni3O10 exhibits a markedly incoherent and diminished flat d_(z^2 ) band. Dispersive d_(x^2-y^2 ) bands, however, retain coherence in both compounds, establishing a striking incoherence/coherence dichotomy modulated by the interlayer Ni-O-Ni bonding angle.

This orbital-selective Mott (OSM) phase is identified by band renormalization factors of approximately 3 and 6 for the d_(x^2-y^2 ) and d_(z^2 ) bands, respectively, mirroring values observed in La4Ni3O10. The Fermi surface of both Pr4Ni3O10 and La4Ni3O10 displays large portions of straight segments corresponding to a β pocket, consistent with density-functional-theory calculations.

Pr4Ni3O10 exhibits a more elliptical α pocket, reflecting a larger orthorhombic anisotropy compared to La4Ni3O10. Wave vector analysis of the density-wave transition reveals a value of approximately 0.62b*, consistent with prior scattering and scanning tunneling microscopy experiments. Band dispersions along high-symmetry directions demonstrate that dispersive dx2−y2 bands forming α and β Fermi pockets cross the Fermi level in La4Ni3O10.

A flat band γ, originating from the dz2 orbital, appears approximately 30 meV below the Fermi level near the Γ′ point. In contrast, the dz2 flat band of Pr4Ni3O10 is significantly incoherent, displaying depleted spectral weight and being almost undetectable at lower photon energies. DFT+DMFT calculations, employing Hund’s coupling JH = 0.8 eV and on-site Coulomb interaction U = 5 eV, successfully reproduce these experimental findings.

Laser-ARPES measurements confirm a correlation-induced dispersion anomaly in the dx2−y2 band of Pr4Ni3O10, observed at a lower energy than in La4Ni3O10. Analysis of the density-wave gap in La4Ni3O10 reveals a zero-temperature leading-edge gap size of approximately 12 meV, while Pr4Ni3O10 exhibits a substantially reduced gap of approximately 6 meV. This diminished gap in Pr4Ni3O10, despite a higher density-wave transition temperature, suggests frustrated interorbital hybridization and enhanced correlation effects within the dx2−y2 band.

Orbital incoherence and density wave suppression in Pr4Ni3O10 trilayer nickelates are observed with increasing strain

Researchers have demonstrated a strong link between structural control and the correlated electronic states in trilayer nickelates, offering insights into potential pathways towards superconductivity. Systematic investigation of La4Ni3O10 and Pr4Ni3O10 using angle-resolved photoemission spectroscopy revealed pronounced differences in orbital behaviour.

Specifically, a striking incoherence in the d_(z^2 ) band was observed in Pr4Ni3O10, while the d_(x^2-y^2 ) bands maintained coherence in both materials. This dichotomy arises from an orbital-selective Mott phase, modulated by the angle of the interlayer Ni-O-Ni bonds. The observed incoherence in Pr4Ni3O10 suppresses interorbital hybridization and influences the density-wave transition, resulting in a substantially reduced energy gap.

The presence of Pr3+ cations and their associated local moments contribute to additional scattering channels, further disrupting the long-range order of the density-wave. These findings establish that the interplay between lattice structure, orbital configuration, spin, and electronic behaviour is crucial for tuning correlated states in these nickelates, mirroring similarities with iron-based superconductors.

The authors acknowledge that the current study focuses on specific trilayer nickelates and that further research is needed to explore a wider range of compositions and structural modifications. Future work could investigate the potential for tuning towards a quantum critical point between the orbital-selective Mott phase and high-pressure superconductivity by manipulating the interlayer bonding angle, potentially paving the way for novel superconducting materials.