Novel Material Reveals Hidden Quantum States Defying Standard Physics Calculations

Scientists are increasingly focused on understanding the complex behaviour of strongly correlated materials, and new research sheds light on the intriguing properties of the Mott-insulating van der Waals antiferromagnet NiPS₃₃. Miłosz Rybak of Wrocław University of Science and Technology, alongside Benjamin Pestka and Biplab Bhattacharyya from RWTH-Aachen University, et al., present angle-resolved photoelectron spectroscopy data which reveals evidence of many-body states within this material. While previous two-particle spectroscopies have hinted at strong correlations and spin-orbit entanglement, this study marks the first observation of these features using photoemission. The team’s findings demonstrate a weakly dispersive feature at the valence-band edge, inconsistent with standard calculations, and suggest that NiPS₃₃ provides an ideal platform for exploring the interplay between strong correlations, reduced dimensionality, and metal-ligand bonding, necessitating a more complete many-body description of two-dimensional systems.

Local Ni-S multiplet physics revealed by angle-resolved photoemission in NiPS3 is consistent with theoretical calculations

Scientists have presented angle-resolved photoelectron spectroscopy (ARPES) spectra of the Mott-insulating van der Waals antiferromagnet NiPS3, a material where strong correlations manifest in two-particle spectroscopies but have not previously been observed via photoemission. Measurements reveal a weakly dispersive feature at the valence-band edge, a characteristic absent in density functional theory plus U (DFT+U) calculations and which remains stable through the Néel transition.
Critical examination and alternative interpretations were thoroughly investigated before researchers demonstrated that an exact diagonalization of a NiS6 cluster accurately predicts low-energy final-state configurations of mixed multiplet d7 and d8L character, aligning with the observed spectral feature. This finding implies that ARPES directly probes local Ni-S multiplet physics within NiPS3, uncovering a many-body structure that extends beyond mean-field theory.

The work confirms NiPS3 as a valuable model system where strong correlations, reduced dimensionality, and covalent metal-ligand bonding collectively influence both two- and single-particle spectroscopies. Consequently, a genuinely many-body description is essential for understanding the behaviour of two-dimensional quantum materials.

Layered transition-metal thiophosphates, such as NiPS3, have become a versatile platform for investigating correlated magnetism, excitonics, and hybrid metal-ligand physics. Their weak interlayer coupling facilitates exfoliation down to the monolayer limit, while the transition-metal site fosters interplay between local Coulomb interactions, ligand-hole configurations, and crystal-field anisotropy.

NiPS3 specifically resides near the boundary between charge-transfer and Mott-Hubbard regimes, exhibiting substantial Ni-S covalency and hosting a spin-orbit-entangled ligand-hole exciton whose properties are strongly linked to antiferromagnetic order. Optical, X-ray absorption spectroscopy (XAS), and resonant inelastic X-ray scattering (RIXS) experiments consistently report sharp excitations attributed to multiplet-split transitions, predominantly of single-ion Ni2+ (3d8) character.

Even the covalent spin-orbital ligand-hole exciton appears as a surprisingly sharp peak in these spectroscopies, indicating that strong local correlations are clearly visible in two-particle spectroscopic probes. A central question then arises: how does the correlated nature of NiPS3 manifest in a single-particle probe like ARPES.

Temperature-dependent micro-ARPES characterisation of exfoliated NiPS3 few-layer flakes reveals distinct electronic signatures

Angle-resolved photoelectron spectroscopy (ARPES) measurements form the basis of this work, employing a μm-focused beam at the NanoESCA beamline of the Elettra synchrotron radiation facility in Italy. NiPS3 crystals were synthesised via the vapor-transport method and subsequently exfoliated onto Si/SiO2 substrates with a 90nm oxide layer, coated with 5nm Au and 1nm Ti.

Samples, approximately 15 layers thick, were selected using atomic force microscopy, revealing a surface roughness of 0.14nm, before undergoing temperature-dependent μ-ARPES analysis at a background pressure of 5x 10−11 mbar. Prior to analysis, the samples were annealed at 200°C to optimise surface quality.

The experimental setup utilised a beam spot size of 5, 10μm, achieving an energy resolution of 50 meV. Fermi level determination was performed using a gold reference, and photoelectron intensity was displayed using curvature with a parameter a0 = 0.05 to enhance visibility. Raw data intensity plots are available for detailed examination of the spectra.

Magnetic phase transitions were confirmed, and ARPES spectra were recorded along the M-Γ-M direction for comparison between MnPS3, FePS3, CoPS3a, and NiPS3. Notably, μ-ARPES measurements revealed a weakly dispersive feature at the valence-band edge of NiPS3, which is absent in DFT+U calculations and persists through the Néel transition.

Further analysis involved reprocessing data from a prior study on the same platform, confirming a 150 meV magnetic shift of the Ni 3d/S 3p band across the Néel temperature, excellent agreement between DFT+U and measured valence band dispersion, and the consistent presence of the aforementioned shoulder-like feature. The study rigorously excluded alternative explanations for this feature, including surface states, substrate-induced states, defect-induced states, and collective-loss features, establishing its origin as a bulk-derived state related to the intrinsic electronic structure of NiPS3.

Local Ni-S multiplet physics evidenced by angle-resolved photoemission spectroscopy in NiPS3 reveals strong many-body correlations

Measurements of angle-resolved photoemission spectroscopy (ARPES) on the Mott-insulating van der Waals antiferromagnet NiPS3 reveal a weakly dispersive feature at the valence-band edge that is absent in density functional theory plus U (+U) calculations and remains unchanged across the Néel transition. Critical examination and analysis ruled out alternative interpretations such as surface states, defect levels, collective loss features, and magnetically dressed quasi-particles.

Exact diagonalization of a NiS6 cluster yielded low-energy final-state configurations of mixed multiplet d7 and d8L character, with energy differences consistent with the observed additional feature. This implies that ARPES directly accesses local Ni-S multiplet physics within NiPS3, revealing a many-body structure extending beyond mean-field theory.

The observed feature persists even as the material transitions between antiferromagnetic and paramagnetic states, demonstrating its intrinsic nature and independence from magnetic ordering. Calculations using exact diagonalization confirm the presence of mixed multiplet configurations, aligning with the experimentally observed energy differences and supporting the interpretation of the spectral feature as arising from correlated electronic states.

NiPS3 is confirmed as an excellent model platform where strong correlations, reduced dimensionality, and covalent metal-ligand bonding jointly influence both two- and single-particle spectroscopies. This work underscores the necessity of a genuinely many-body description for understanding the electronic structure of two-dimensional quantum materials.

The study demonstrates that ARPES can reveal many-body structure arising from locally entangled states, even in structurally simple insulating systems, which are not visible through conventional band theory or mean-field approximations. This finding extends beyond the limitations of DFT+U, mirroring observations in cuprates and contrasting with materials like Ca2RuO4 and Sr2IrO4 where mean-field theory adequately explains ARPES results.

NiPS Valence-Band Edge Reveals Strong Correlation and Multiplet Effects in the material

Scientists have uncovered evidence of strong electronic correlations within the structure of the Mott-insulating van der Waals antiferromagnet NiPS, revealing a complex many-body behaviour beyond conventional band theory. Angle-resolved photoemission spectroscopy measurements detected a previously unobserved weakly dispersive feature at the valence-band edge of NiPS, a characteristic not predicted by standard density functional theory calculations.

Detailed analysis, coupled with exact diagonalization of a NiS cluster, indicates this feature arises from low-energy final-state configurations exhibiting a mixture of multiplet and charge-transfer character. These findings establish NiPS as a valuable model system for investigating the interplay between strong correlations, reduced dimensionality, and metal-ligand bonding in two-dimensional materials.

The observed spectral feature demonstrates that the electronic structure of NiPS is significantly influenced by local Ni-S multiplet physics, necessitating a genuinely many-body description to fully understand its properties. The authors acknowledge that conventional mean-field band theory fails to account for the observed multiplet splitting and ligand-hole mixtures, highlighting a limitation of simpler computational approaches. Future research may focus on refining theoretical models to accurately capture these intricate many-body effects and exploring similar phenomena in other correlated two-dimensional materials.