We leverage s-orbital non-resonant inelastic X-ray scattering (s-NIXS) to perform orbital imaging on three bulk rare-earth nickelates spanning a range of formal nickel valence (3d electron filling) from Ni^3+ (3d^7) to Ni^1+ (3d^9). Our results directly reveal the ground states of these compounds all with minimal theoretical input. In particular, we demonstrate the low-spin orbital configuration of trivalent LaNiO_3, the d_x^2-y^2 configuration of monovalent LaNiO_2, and resolve the effective e_g crystal field splitting in the distorted octahedral environment of divalent La_2NiO_4. This work illustrates the potential of s-NIXS to study the ground state and excited states of strongly correlated materials without needing complex theoretical analysis of spectroscopic data.

SrCu2(BO3)2 (SCBO) is a paradigmatic realization of the Shastry-Sutherland model, hosting geometrically frustrated spin dimers and a variety of quantum magnetic phases and phenomena. Although its magnetic properties have been extensively studied, the high-energy electronic excitations that determine the crystal-field environment and Cu-O hybridization have remained largely unexplored. Here we combine Cu L3-edge resonant inelastic x-ray scattering (RIXS), broadband optical spectroscopy, and electronic-structure calculations to determine the relevant local and interband excitation energy scales in SCBO. RIXS resolves a well-defined manifold of localized Cu2+ d-d excitations between 1.8 and 2.4 eV, whose energies and polarization dependence are well reproduced by multireference quantum-chemistry calculations. In contrast, optical spectroscopy identifies charge-transfer excitations with an absorption onset near 1.2-1.6 eV and a broader higher-energy structure around 4.5 eV, which are qualitatively captured by DFT+U calculations. Taken together, these results define the characteristic energy scales of d-d and CT excitations, offering quantitative benchmarks for computational frameworks and providing essential input for refining superexchange-based magnetic models of this prototypical frustrated quantum antiferromagnet.

Since the discovery of high-temperature superconductivity in nickelate superconductors, it is an open question how closely the superconducting state resembles that of cuprate superconductors. One salient feature of the phase diagram of the high-temperature cuprate superconductors is stripe order. Despite their prevalence, real-space imaging has been limited to the charge sector. Here we use spin-polarised scanning tunnelling microscopy to visualize the local magnetic and charge distribution emerging due to a stripe order in the trilayer nickelate La_4Ni_3O_10. The stripe order exhibits a four unit cell periodicity, closely resembling that seen in cuprates, and opens a near-complete ∼66meV gap at the Fermi level. Crucially, discrete phase slips can be triggered by tunneling electrons above a ∼ 20meV threshold, allowing imaging of stripe dynamics at the atomic scale. These results highlight the importance of correlation physics driving stripe-like orders in lanthanum nickelates with striking similarities to the cuprates.

In this study, we explore the potential of the RP-type bilayer manganite LaSr2Mn2O6.96 as an intercalation-based cathode material for all-solid-state fluoride ion batteries (FIBs). Structural changes of LaSr2Mn2O6.96 during fluoride intercalation and de-intercalation were analyzed via ex-situ X-ray diffraction, revealing that F- insertion induces the formation of three distinct tetragonal phases. To understand the complex behavior of these phases, we examined the changes in the Mn oxidation state and coordination environment using X-ray absorption spectroscopy and magnetic measurements. Under stack pressure (20 kN), electrochemical cycling of LaSr2Mn2O6.96 in the potential range of 1 V to -1 V exhibited a continuous increase in specific capacity from capacity of similar to 30 mAh g(-1) to similar to 68 mAh g(-1) over 200 cycles, with similar to 99% coulombic efficiency and no signs of capacity fading. This makes the bilayer manganite LaSr2Mn2O6.96 a promising candidate for a cycling stable cathode for all-solid-state FIBs, especially under the application of stack pressure.

Motivated by recent experiments showing pressure-induced suppression of magnetic order and the emergence of a dynamical ground state in the anisotropic kagome antiferromagnet Y-kapellasite Y3Cu9(OH)19Cl8, we perform ab initio density functional theory (DFT)calculations to investigate the evolution of magnetic exchange interactions under hydrostatic pressure. We show that pressure efficiently tunes the magnetic Hamiltonian by altering the CuOCu bond geometry, thereby driving the system towards a spin-liquid regime. This evolution is governed by a nonlinear dependence of the dominant exchange coupling on the CuOCu bond angle. We further examine the influence of hydrogen positions and find that both the OH bond length and the hydrogen out-of-plane angle strongly affect the magnetic interactions. Our results provide a microscopic explanation for the experimentally observed pressure-induced enhancement of frustration and highlight the key role of hydrogen geometry.

The kagome lattice of spin-1/2 copper atoms in herbertsmithite is conjectured to sustain a quantum spin liquid state with spinon quasiparticles. Ideally, the kagome crystal planes are each separated by a plane of spinless zinc atoms. However, in real crystals, some spin-1/2 copper atoms substitute randomly onto these inter-kagome zinc sites. Here we reconceptualize such 'impurity' atoms as quantum witness spins whose dynamics is designed to probe the spin liquid state. We then introduce spin noise spectroscopy to measure the frequency and temperature dependence of witness spin dynamics, demonstrating that their phenomenology is consistent with extensive interactions between witness spins mediated by propagation of spinons through a quantum spin liquid. Ultimately, a sharp transition occurs at around 260 mK, below which the properties of both spin noise and magnetic susceptibility suggest that the witness spins form a spin glass phase. Among the theoretical models considered, we demonstrate that our observations are only consistent with spinon-mediated interactions between witness spins by either a Z2 or U(1) quantum spin liquid, with the former model more closely matching the data. Our work demonstrates that quantum mechanical witness spins may now conceivably be used as a widely applicable probe of quantum spin liquid physics.

Ruddlesden-Popper nickelates exhibit high-temperature superconductivity closely intertwined with charge and spin density waves. However, fundamental questions persist regarding the interplay between the associated density wave (DW) fluctuations and superconductivity, as well as the orbital character and symmetry underlying the DW instabilities. Here we utilize polarized Raman scattering to investigate the phononic and electronic Raman responses of the trilayer nickelate La_4Ni_3O_10 across its concomitant charge and spin density wave transitions. In addition to distinct phonon anomalies occurring below the transition temperature, we observe a depletion of continuum spectral weight up to 114 meV and a pronounced peak centered at this energy. By combining momentum-selective information from polarized electronic Raman scattering with Raman-response model calculations based on a multiorbital Raman vertex in a reconstructed two-orbital DW state involving both Ni-3d_x^2 - y^2 and Ni-3d_z^2 orbitals, we identify 114 meV as the energy scale 2Δ_DW of the DW gap, characterized by incoherent opening and non-mean-field behavior. Furthermore, the model calculations reveal that the corresponding 2Δ_DW peak has a multiorbital origin, requiring both orbital contributions and their mixing beyond single-orbital projections, thus shedding light on the nature of the DW instabilities in La_4Ni_3O_10.

Y-kapellasite [Y_{3}Cu_{9}(OH)_{19}Cl_{8}], which hosts an original anisotropic kagome sublattice, is a promising candidate for studying elusive and complex correlated physics. It exhibits a theoretically predicted in-plane (1/3,1/3) magnetic order [Hering et al., npj Comput. Mater. 8, 1 (2022)2057-396010.1038/s41524-021-00689-0], but its magnetic interaction values place it close to a phase boundary to a spin liquid state [Chatterjee et al., Phys. Rev. B 107, 125156 (2023)PRBMDO2469-995010.1103/PhysRevB.107.125156]. Our μSR measurements under hydrostatic pressure demonstrate the complete suppression of static magnetism in favor of a fully dynamical ground state at 2.3 GPa. Complementary high-pressure x-ray and optical phonon measurements reveal a gradual reduction of the kagome anisotropy, enhancing magnetic frustration without structural transitions. Our results establish Y-kapellasite as a rare clean kagome model in which long-range order is suppressed by pressure-tuned frustration, the first fingerprint for the realization of a quantum spin liquid without strong disorder.

Fractional magnetization plateaus provide a sensitive probe of many-body spin states in frustrated quantum magnets, yet their microscopic origin in kagome antiferromagnets remains unresolved. This is particularly true of the mysterious 1/9 plateau, which is predicted by theory but infrequently observed in experiment. Here, we investigate this problem in the S = 1/2 anisotropic kagome antiferromagnet Y-kapellasite, Y_3Cu_9(OH)_19Cl_8, using pulsed-field magnetization measurements on single crystals and high-field ^35Cl NMR. We identify a hierarchy of field-induced fractional features, including 1/3 and 1/9 plateaus, as well as a weaker low-field feature. Analysis of the NMR spectra and the magnetic susceptibility across the 1/9 plateau demonstrate that it is accompanied by an ordered local spin configuration, a strong suppression of low-energy spin fluctuations and activated behavior, consistent with a gapped fractional state. These features differ from those in the only other material YCu_3(OH)_6Br_2[Br_1-y(OH)_y] in which this plateau is observed, implying a surprising robustness of the 1/9 state to the details of the underlying magnetism.

We present magnetization and dilatometry measurements on the honeycomb cobaltate Na3Co2SbO6 and map out its detailed field-temperature phase diagram down to sub-Kelvin temperatures. Our data for in-plane magnetic fields show a strongly anisotropic c*-axis lattice response, which is dominated by the variation of Co-O-Co bond angles according to ab initio calculations. At T = 0.4 K, the magnetization M(B) exhibits steplike features that are also highly anisotropic. In the case of B H b, a small hysteresis observed around the second field-induced magnetic transition (Bc2) indicates its first-order character, whereas the apparent divergence of the magnetic Gr & uuml;neisen parameter at Bc2 is suppressed upon cooling and signals the absence of quantum critical behavior upon entering the field-polarized state. None of our thermodynamic measurements provide evidence for a field-induced quantum spin-liquid state near or above Bc2.

Topochemical fluorination offers a low–temperature route for modifying the anion chemistry and electronic ground states of layered transition-metal oxides, providing access to metastable phases and functionalities that are not able to be achieved through conventional solid–state synthesis. Despite extensive work on polycrystalline samples and thin films, topochemical fluorination of bulk single crystals has not been studied, limiting insights into intrinsic structure property relationships. Here, we investigate the topochemical fluorination of optical float zone grown (OFZ) La_2NiO_4+δ single crystals using polymer-based PTFE, PVDF and inorganic CuF_2 fluorination agents and compare it to our topochemical pathways of reduction of LaNiO_3-x. By systematically investigating direct and indirect contact reaction pathways, we can understand fluorination mechanisms, quantify the degree of fluorine incorporation, and evaluate the resulting structural and magnetic modifications in a detail that was not possible in powder and thin films. Powder and single–crystal X-ray diffraction reveal that fluorination proceeds without destroying the Ruddlesden–Popper framework, while inducing lattice parameter changes consistent with anion intercalation in the bulk and ion exchange on the surface. This even induces a clear superstructure, which was not reported before and extends the understanding of anion insertion reactions beyond what is known on stage ordering in nickelates. Energy-dispersive X–ray spectroscopy confirms strong fluorine incorporation on the surface and reduced homogeneity in the bulk. Magnetic susceptibility measurements demonstrate a change in antiferromagnetic ordering upon fluorination.

Describing lithium-based battery positive electrodes based on different transition metal or oxygen-redox regimes can cause confusion in understanding metal-ligand hybridization, oxygen dimerization and degradation processes. Therefore, it is urgent to investigate the electronic structure of these materials and identify the role each cation and anion has in charge compensation at the subnanoscale. Here, using X-ray resonance photoemission spectroscopy, single-impurity Anderson models, spectral simulations and theoretical calculations, we examine redox mechanisms in positive electrodes during lithium-based battery operation. This approach reconciles the redox description of two positive electrode active materials-LiMn0.6Fe0.4PO4 and LiNiO2-in terms of varying degrees of charge transfer using the Zaanen-Sawatzky-Allen framework. In LiMn0.6Fe0.4PO4, the lack of strong hybridization indicates that the capacity results from the depopulation of metal 3d states, that is, conventional metal redox. However, in cells with LiNiO2-based positive electrodes, negative charge transfer dominates, and redox occurs through the formation and elimination of ligand-hole states. These results clarify the role of oxygen in Ni-rich systems and provide a framework to explain how the charge/discharge capacities are linked to oxygen-dominated states in highly covalent systems, without the need to consider oxygen dimerization.

An emerging concept for identification of different types of spin liquids [C. Broholm et al. , Science 367 , eaay0668 (2020)] is through the use of spontaneous spin noise [S. Chatterjee, J. F. Rodriguez-Nieva, E. Demler, Phys. Rev. B 99 , 104425 (2019)]. Here, we develop spin noise spectroscopy for spin liquid studies by considering Ca 10 Cr 7 O 28 , a material hypothesized to be either a quantum or a spiral spin liquid (SSL). By enhancing techniques introduced for magnetic monopole noise studies [R. Dusad et al. , Nature 571 , 234–239 (2019)], we measure the time and temperature dependence of spontaneous flux Φ ( t , T ) and thus magnetization M ( t , T ) of Ca 10 Cr 7 O 28 samples. The resulting power spectral density of magnetization noise S M ω , T reveals intense spin fluctuations with S M ω , T ∝ ω - α ( T ) and 0.84 < α T < 1.04 . Both the variance σ M 2 T and the correlation function C M t , T of this spin noise undergo crossovers at a temperature T ∗ ≈ 450 mK . While predictions for quantum spin liquids are inconsistent with this phenomenology, those from Monte–Carlo simulations of a two-dimensional (2D) SSL state in Ca 10 Cr 7 O 28 yield overall quantitative correspondence with the measured frequency and temperature dependences of S M ω , T , C M t , T , and σ M 2 T , thus indicating that Ca 10 Cr 7 O 28 is an SSL.

Describing Li-ion battery positive electrodes in terms of distinct transition metal or oxygen redox regimes can lead to confusion in understanding metal-ligand hybridisation, oxygen dimerisation, and degradation. There is a pressing need to study the electronic structure of these materials and determine the role each cation and anion plays in charge compensation. Here, we employ transition metal L-edge X-ray Resonance Photoemission Spectroscopy in conjunction with Single Impurity Anderson models, Self-consistent Real Space Multiple Scattering spectral simulations, and Dynamical Mean-Field theory calculations to directly evaluate the redox mechanisms in (de-)lithiated battery electrodes. This approach reconciles the redox description of two canonical cathodes – LiMn_0.6Fe_0.4PO_4 and LiNiO_2 – in terms of varying degrees of charge transfer using the established Zaanen-Sawatzky-Allen framework, common to condensed matter physics. In LiMn_0.6Fe_0.4PO_4, the absence of charge transfer means capacity arises due to the depopulation of metal 3d states, i.e. conventional metal redox. Whereas, in LiNiO_2, charge transfer dominates and redox occurs through the formation and elimination of ligand hole states. This work clarifies the role of oxygen in Ni-rich system and provides a framework to explain how capacity can be extracted from oxygen-dominated states in highly covalent systems without needing to invoke dimerisation.

The anomalous Hall effect (AHE) has emerged as a key indicator of time-reversal symmetry breaking (TRSB) and topological features in electronic band structures. Absent of a magnetic field, the AHE requires spontaneous TRSB but has proven hard to probe due to averaging over domains. The anomalous component of the Hall effect is thus frequently derived from extrapolating the magnetic field dependence of the Hall response. We show that discerning whether the AHE is an intrinsic property of the field free system becomes intricate in the presence of strong magnetic fluctuations. As a study case, we use the Weyl semimetal PrAlGe, where TRSB can be toggled via a ferromagnetic transition, providing a transparent view of the AHE's topological origin. Through a combination of thermodynamic, transport and muon spin relaxation measurements, we contrast the behaviour below the ferromagnetic transition temperature to that of strong magnetic fluctuations above. Our results on PrAlGe provide general insights into the interpretation of anomalous Hall signals in systems where TRSB is debated, such as families of Kagome metals or certain transition metal dichalcogenides.

By single crystal diffraction we characterize the chemostructural disorder introduced by Zn-Cu site mixing in the kagome spin S=12 systems herbertsmithite ZnCu3(OH)6Cl2 and YCu3(OH)6Br2[Brx(OH)1−x]. For an untwinned single crystal of herbertsmithite of composition Zn0.95(1)Cu2.99(3)O5.9(1)H5.8(1)Cl2 we find substitution by Cu of the Zn atoms in the layers separating the kagome layers as well as substantial Zn substitution for Cu in the kagome layers. In YCu3(OH)6Br2[Brx(OH)1−x] site mixing disorder is present for intermediate x. Analogous to the Cl homologous system in crystals with x=1/3 disorder is absent and a low-temperature structural transition emerges driven by strong magnetophonon coupling as a release of frustration. Apart from this structural anomaly we find the physical properties of these crystals unchanged compared to intermediate x and closely resembling the Cl homologue where long-range magnetic order was observed. Published by the American Physical Society 2025

The anomalous Hall effect (AHE) has emerged as a key indicator of time-reversal symmetry breaking (TRSB) and topological features in electronic band structures. Absent of a magnetic field, the AHE requires spontaneous TRSB but has proven hard to probe due to averaging over domains. The anomalous component of the Hall effect is thus frequently derived from extrapolating the magnetic field dependence of the Hall response. We show that discerning whether the AHE is an intrinsic property of the field-free system becomes intricate in the presence of strong magnetic fluctuations. As a study case, we use the Weyl semimetal PrAlGe, where TRSB can be toggled via a ferromagnetic transition, providing a transparent view of the AHE's topological origin. Through a combination of thermodynamic, transport, and muon spin relaxation measurements, we contrast the behavior below the ferromagnetic transition temperature to that of strong magnetic fluctuations above. Our results on PrAlGe provide general insights into the interpretation of anomalous Hall signals in systems where TRSB is debated, such as families of kagome metals or certain transition metal dichalcogenides.

The continuous development of innovative optical materials with lanthanoid ions as activators has emerged as a modern sector of materials chemistry. The experience with the fabrication of single crystals with the optical float zone has motivated one to investigate the luminescence of Nd3+ and Ho3+ ions in the garnets (Gd3-xREx)In2Ga3O12 (RE = Nd and Ho, x = 0; 0.15-0.30). Upon usage of an Ar/O-2 (80:20 ratio) atmosphere and application of an auxiliary pressure (6 bar) to suppress In2O3 evaporation, single-crystalline domain sizes in the order of approximate to 6 x 6 x 1 mm(3) are obtained. Structural analysis confirms the formation of a cubic garnet phase with space group Ia (3) over bard, with the substituents incorporated in accordance with Vegard's law. Backscattered electron imaging and energy-dispersive X-ray spectroscopy are conducted, demonstrating a homogeneous elemental distribution within the crystals. Photoluminescence studies are carried out, revealing the characteristic narrow-line 4f (n) -> 4f (n) transitions of Nd3+ and Ho3+, with decay times in the submillisecond range, suggesting non-negligible cross-relaxation effects are present. Despite this, the large nearest-neighbor Gd-Gd distance (3.88 angstrom) in Gd3In2Ga3O12 and the low phonon cutoff energy (approximate to 700 cm(-1)) are found to limit cross-relaxation pathways, preserving significant photoluminescence brightness. These results highlight the potential of Gd3In2Ga3O12:RE3+ single crystals as promising candidates for advanced optical applications.

Ruddlesden-Popper (RP) nickelates have attracted intense interest following the discovery of superconductivity in several members of the series, including bilayer (BL) La_3Ni_2O_7, trilayer (TL) La_4Ni_3O_10, and structural polymorphs composed of monolayer-bilayer or monolayer-trilayer (ML-TL) units. However, an inherent propensity of the RP series to form intergrown phases during single-crystal synthesis, together with spatial variations in oxygen stoichiometry, has complicated the determination of their intrinsic material properties. As a consequence, conflicting reports have emerged on both their electronic phase transitions and lattice dynamics. In this work, we perform a comparative study of the phononic and electronic Raman responses of high-quality ML-TL single crystals and contrast them with those of other RP nickelates, using samples with optimized oxygen content. We establish several Raman spectral features that enable unambiguous phase identification across the series. Moreover, we uncover characteristics in the phononic and electronic Raman response of ML-TL that are not reflected in the pure ML and TL compounds. We attribute these differences to a distinctive electronic structure arising from self-doping and confinement effects induced by the ML unit within the ML-TL lattice architecture.

Among the orthochromites comprising a magnetic rare-earth constituent, CeCrO 3 exhibits the highest Néel temperature of ∼ 260 K . Using high-resolution neutron powder diffraction, we redetermined the low-temperature antiferromagnetic Cr and Ce magnetic moment configuration in CeCrO 3 . The Cr moments order with a weakly canted G -type antiferromagnetic structure ( Γ 2 ) where the moments essentially align along the b axis ( P n m a setting of the space group No. 62). Very weak Ce magnetic coherent scattering appears below ∼ 22 K due to a C x -type ordering of the Ce moments ( μ Ce ≈ 0.06 μ B ). From the temperature dependence of the lattice parameters, we analyze the volume magnetoelastic effects which cause a contraction of the cell volume on ordering of the Cr moments. Below ∼ 150 K a subtle but continuous adjustment of the a and c lattice parameters takes place which is attributed to a spin reorientation of the Cr magnetic moments. The temperature dependence of the Cr magnetic order parameter indicates a gap in the magnon spectrum. We reanalyze the specific heat capacity of CeCrO 3 which at low temperatures exhibits additional contributions due to thermal excitation within the crystal-field split levels of the Ce 3 + F 5 / 2 2 Hund's rule ground state. A model calculation of the magnetization including magnetic polarization of the Ce moments is consistent with a spin reorientation of the Cr magnetic moments.