X-ray magnetic circular dichroism provides a means to identify ferromagnetic, chiral, and altermagnetic orders via their time-reversal-symmetry (T) breaking. However, the symmetry properties that govern circular dichroism (CD) in resonant inelastic x-ray scattering (RIXS) remain poorly understood. We show that, due to the inherent irreversibility of the RIXS process, RIXS-CD does not require T breaking to be present, but reflects the change in unitary symmetries associated with magnetic ordering. Using the altermagnetic MnTe as a model system, we observe an azimuthal-angle dependent RIXS-CD signal in the magnon excitations. Our findings highlight the sensitivity of RIXS-CD to the relativistic symmetry in magnetic systems and its potential application as a probe of magnetic domains.

We present an experimental investigation of the magnetic ground state in Ba4NbIr3O12, a fractional valent trimer iridate. X-ray absorption and photoemission spectroscopy show that the Ir valence lies between 3+ and 4+ while Nb is pentavalent. Combined dc/ac magnetization, specific heat, and muon spin rotation/relaxation (mu SR) measurements reveal no magnetic phase transition down to 0.05 K. Despite a significant Weiss temperature (OW - -15 to -25 K) indicating antiferromagnetic correlations, a quantum spin-liquid (QSL) phase emerges and persists down to 0.1 K. This state likely arises from geometric frustration in the edge-sharing equilateral triangle Ir network. Our mu SR analysis reveals a two-component depolarization, arising from the coexistence of rapidly (90%) and slowly (10%) fluctuating Ir moments. Powder x-ray diffraction and Ir-L3edge x-ray absorption fine structure spectroscopy identify 8-10% Nb/Ir site-exchange, reducing frustration within part of the Ir network, and likely leading to the faster muon spin relaxation, while the structurally ordered Ir ions remain highly geometrically frustrated, giving rise to the rapidly spin-fluctuating QSL ground state. At low temperatures, the magnetic specific heat varies as gamma T + alpha T 2, indicating gapless spinon excitations, and possible Dirac QSL features with linear spinon dispersion, respectively.

The interplay of Kondo screening and magnetic ordering in strongly correlated materials containing local moments is a subtle problem. Usually the number of conduction electrons per unit cell matches or exceeds the number of moments, and a Kondo-screened heavy Fermi liquid develops at low temperatures. Changing the pressure, magnetic field, or chemical doping can displace this heavy Fermi liquid in favor of a magnetically ordered state. Alternatively, Kondo singlet formation can be suppressed when the number of conduction electrons is small compared to the number of magnetic moments, known as the Kondo exhaustion scenario. Here we report the discovery of such an "exhausted" Kondo lattice material, YbIr3Si7, where the bulk electrical conductivity tends to zero in the antiferromagnetic state below the Neel temperature TN = 4.1 K, as all the free carriers are consumed in the formation of Kondo singlets. By contrast, the surface is conducting, as the Yb3+ ions relax into larger nonmagnetic Yb2+ in the presence of reduced chemical pressure, which shifts the chemical potential.

The series of LiMO2 (M: transition metal) materials are highly relevant as cathode materials of Li-ion batteries. The stability of such systems remains an important factor for their usability in batteries, and depends strongly on the electronic configuration of the transition-metal ions. In particular, the promising class of multi-transition-metal systems exhibits complicated valence states due to intermetallic charge transfer and charge disproportionation. Here we perform a systematic study on the valence of the transition-metal ions using x-ray absorption spectroscopy on the M−L2,3 edges and O-K edges. In Li(Ni0.5Mn0.5)1−xCoxO2 we established that the valence is Co3+ and Ni0.52+Mn0.54+ throughout the whole series. Meanwhile, in LiNi1−xCoxO2 we found that the Ni displays a behavior consistent with a charge disproportionated negative charge transfer system, and that with increased concentration of Co3+, the disproportionation signal decreases. Since the number of O 2p holes also gets reduced, we infer that the material will also become more unstable. Published by the American Physical Society 2024

We investigated the electronic structure of BaEu2Nb5O15 and Eu3Nb5O15 by means of hard x-ray photoelectron spectroscopy. The Eu 3d core level spectra are primarily dominated by Eu2+ signals. The well -screened feature in the Nb 3d core level spectra of BaEu2Nb5O15 is suppressed in Eu3Nb5O15, consistent with the insulating behavior of Eu3Nb5O15 due to the atomic disorder and/or the rattling effect of the small Eu ions in the spacious A2 site. In the valence band spectra, the Nb 4d states at the Fermi level are close to the Eu 4 f states located around 2 eV below it. This suggests that the degree of localization of the Nb 4d electrons can be enhanced through the Eu 4 f-Nb 4d hybridization with the atomically and magnetically disordered Eu ions at the A2 site.

Bulk electronic structure of pyrite-type NiS2 and NiS2-xSex (x = 0.03) has been investigated by means of hard x-ray photoemission spectroscopy (HAXPES) and theoretical calculations. The Mott gap at the Fermi level is confirmed by the HAXPES experiment. The S 3p-Ni 3d charge-transfer energy is estimated to be negative from the cluster-model analysis of the Ni 2p spectra. The observed S 3s/3p spectral distribution including the bonding-antibonding splitting of the S-S dimer is well described by density functional theory for a paramagnetic metallic state. The Mott gap is not affected by the sparse Se substitution for S although the bonding-antibonding splitting of S 3s is slightly reduced. The negative charge-transfer energy and the S 3s/3p spectral distribution by the S-S dimerization are important ingredients of the bulk insulating state of NiS2.

We report hard x-ray photoelectron spectroscopy on SrFeO3 which is one of the classical conducting transition-metal oxides with a noncollinear magnetic structure. The obtained spectra show a detailed charge-transfer (CT) satellite structure, the Fe 2p3/2 main peak exhibits multiplet splitting, and the deterioration signs present in previous reports are absent here, allowing for a better determination of its intrinsic electronic structure. The results are well described by a FeO6 cluster model with a charge-transfer energy of about −1.0 eV, confirming the values obtained in the previous works. The negative CT energy indicates that the electronic configuration of the tetravalent Fe is d5L rather than d4 where L represents an O 2p hole. The small spectral weight observed at the Fermi level indicates the correlated metallic state with localized Fe 3d electrons and mobile O 2p holes which are governed by a large d−d Coulomb interaction and negative CT energy. Published by the American Physical Society 2024

We report on our hard x-ray photoemission study of Nb3Cl8, wherein the breathing Kagome lattice of Nb provides interesting magnetic properties with spin 1/2 per Nb trimer. The Nb3d(5/2) and 3d(3/2) core level peaks are accompanied by satellite peaks, suggesting the importance of Nb4d charge degrees of freedom. The Cl1s and 2p core level spectra have high binding energy peaks which can be assigned to the Cl site with Cl3p ligand holes near the center of the Nb trimer. The core level spectra indicate that the Nb-Nb and Nb-Cl charge fluctuations play crucial roles in the magnetic interaction between the trimers.

The bulk electronic structure of AV3Sb5 (A = K, Cs) has been investigated by means of hard x-ray photoemission spectroscopy (HAXPES). The asymmetric shape of V and Sb core level peaks indicates that the V 3d and Sb 5p electrons are involved in the conduction band. The absence of a satellite structure in the V 2p HAXPES spectra shows a weak electronic correlation in the V 3d states. Splitting of the V 2p peak is not observed in the density wave phase indicating the charge disproportionation between the V sites is undetectably small, consistent with the weakness of the on-site electronic correlation and the possibility of bond order. The Sb 4d5/2 binding energy agrees with that of the Cs-terminated surface, indicating that the electronic structure of the V3Sb5 layer just below the Cs surface is close to the bulk.

We have investigated the electronic structure of La3Ni2O7-8 (8 approximate to 0.07) by means of hard x-ray photoemission spectroscopy (HAXPES). Although the nominal Ni valence is close to +2.5, the Ni 2p HAXPES spectra show an absence of Ni2+/Ni3+ charge disproportionation. The Ni 2p spectral shape including the main peak and the charge-transfer satellite indicate that oxygen 2p holes are heavily involved in the transport properties. The spectral weight suppression at the Fermi level indicates that the carriers of O 2p character (mixed with Ni 3d) are affected by electronic correlation which would be associated with the density wave transition and the superconductivity controlled by pressure.

The bulk electronic structure of ${A\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ $(A=\text{K}, \mathrm{Cs})$ has been investigated by means of hard x-ray photoemission spectroscopy (HAXPES). The asymmetric shape of V and Sb core level peaks indicates that the V $3d$ and Sb $5p$ electrons are involved in the conduction band. The absence of a satellite structure in the V $2p$ HAXPES spectra shows a weak electronic correlation in the V $3d$ states. Splitting of the V $2p$ peak is not observed in the density wave phase indicating the charge disproportionation between the V sites is undetectably small, consistent with the weakness of the on-site electronic correlation and the possibility of bond order. The Sb $4{d}_{5/2}$ binding energy agrees with that of the Cs-terminated surface, indicating that the electronic structure of the ${\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ layer just below the Cs surface is close to the bulk.

Frustrated magnets offer a plethora of exotic magnetic ground states, including quantum spin liquids (QSLs), in which enhanced quantum fluctuations prevent a long-range magnetic ordering of the strongly correlated spins down to lowest temperature. Here we have investigated the trimer based mixed valence hexagonal rhodate Ba4NbRh3O12 using a combination of dc and ac magnetization, electrical resistivity, specific heat, and muon spin rotation/relaxation (μSR) measurements. Despite the substantial antiferromagnetic exchange interactions, as evident from the Weiss temperature (θW∼−35 to −45K), among the Rh-local moments, neither long-range magnetic ordering nor spin freezing is observed down to at least 50 mK, in ac-susceptibility, specific heat, and zero-field μSR measurements (down to 0.26 K). We ascribe the absence of any magnetic transition to enhanced quantum fluctuations as a result of geometrical frustration arising out of the edge-sharing equilateral Rh-triangular network in the structure. Our longitudinal-field μSR result evidences persistent spin fluctuations down to 0.26 K, thus stabilizing a dynamic QSL ground state in Ba4NbRh3O12. Furthermore, the magnetic specific heat data at low T reveal a significant T-linear contribution plus a quadratic T dependence, which may indicate the gapless Dirac QSL phenomenology of the spinon excitations with a linear dispersion. Published by the American Physical Society 2024

We investigated the electronic structure of ${\mathrm{BaEu}}_{2}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ and ${\mathrm{Eu}}_{3}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ by means of hard x-ray photoelectron spectroscopy. The Eu $3d$ core level spectra are primarily dominated by ${\mathrm{Eu}}^{2+}$ signals. The well-screened feature in the Nb $3d$ core level spectra of ${\mathrm{BaEu}}_{2}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ is suppressed in ${\mathrm{Eu}}_{3}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$, consistent with the insulating behavior of ${\mathrm{Eu}}_{3}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ due to the atomic disorder and/or the rattling effect of the small Eu ions in the spacious $A2$ site. In the valence band spectra, the Nb $4d$ states at the Fermi level are close to the Eu $4f$ states located around 2 eV below it. This suggests that the degree of localization of the Nb $4d$ electrons can be enhanced through the Eu $4f$--Nb $4d$ hybridization with the atomically and magnetically disordered Eu ions at the $A2$ site.

We investigate the electronic structure of LaCoO3 across the gradual spin-state and insulator-to-metal transitions using bulk-sensitive hard x-ray photoelectron and soft x-ray absorption spectroscopies. The spectra exhibit strong variations with temperature. The energy gap is reduced by about 0.6 eV in going from 80 to 650 K but the near Fermi level intensity remains small, classifying LaCoO3 as a bad metal even in the metallic phase. We are able to explain the spectra in terms of incoherent sums of low-spin and high-spin Co3+ spectra. We also find that the energy parameters for the two Co sites are very different, revealing that paramagnetic LaCoO3 is a highly inhomogeneous system with local lattice relaxations that are spin-state-specific. This, in turn, provides a natural explanation for the much-debated temperature dependence of the activation energy for the transitions.

We have studied the Ti3+/Ti4+ mixed valence state in Mg-doped Ti2O3 using hard x-ray photoemission spectroscopy. The Ti 2p spectrum for the corundum-type Ti2O3 revealed the Ti3+ configuration with strong electronic coupling in the c-axis Ti-Ti pairs whereas the data for the ilmenite-type MgTiO3 confirmed the Mg2+-Ti4+ charge state in the c-axis cation pairs. In Mg0.29Ti1.71O3, the Ti 2p spectrum hardly showed the Ti4+ peak, which was in MgTiO3 indicating that the c-axis pairing of the Mg is with a Ti3+ ion rather than a Ti4+ and that the Ti3+/Ti4+ mixed valence state is materialized within the Ti-Ti c-axis pairs. In Mg0.63Ti1.37O3, we detected the presence of Mg2+-Ti4+ pairs. The results indicate the important role of hybridization within the c-axis pairs not only in the Ti3+-Ti3+ configuration, but also above all in the Ti3+/Ti4+ mixed valence state, superseding the Madelung energy gain of the Mg2+-Ti4+ c-axis pair formation.

Corundum oxide Ti2O3 shows the metal-insulator transition around 400-600 K accompanying the nearest Ti3+-Ti3+ bond (a1ga1g singlet state) formation along the c axis. In order to clarify the hole-doping effect for the a1ga1g singlet bond in Ti2O3, we investigated Ti 3d orbital anisotropy between corundum-type Ti2O3 and ilmenite-type MgTiO3 using linear dichroism of soft x-ray absorption spectroscopy of the Ti L2,3 edge. From the linear dichroic spectral weight in MgyTi2-yO3, we confirmed that the a1ga1g state is dominant not only in y = 0.01 (almost Ti2O3), but also in y = 0.29, indicating that the Ti-Ti bond survives against a certain level of hole doping. In y = 0.63 corresponding to 46% hole doping per Ti, the 3d orbital symmetry changes from a1g to e pi g.

The electronic structure of FeWO4 is studied by photoelectron spectroscopy at x-ray and hard x-ray photon energies on high-quality single crystals. Photoionization cross-section effects together with full atomic multiplet configuration interaction and band structure calculations allow us to identify the contributions of iron and tungsten to the valence band. The analysis shows that the correlations in FeWO4 necessitate theoretical approaches beyond standard band structure models even for the description of the tungsten with a formal 5d(0) configuration.

We present a comprehensive study of CaCu$_3$Ru$_4$O$_{12}$ using bulk sensitive hard and soft x-ray spectroscopy combined with local-density approximation (LDA) + dynamical mean-field theory (DMFT) calculations. Correlation effects on both the Cu and Ru ions can be observed. From the Cu $2p$ core level spectra we deduce the presence of magnetic Cu$^{2+}$ ions hybridized with a reservoir of itinerant electrons. The strong photon energy dependence of the valence band allows us to disentangle the Ru, Cu, and O contributions and thus to optimize the DMFT calculations. The calculated spin and charge susceptibilities show that the transition metal oxide CaCu$_3$Ru$_4$O$_{12}$ must be classified as a Kondo system and that the Kondo temperature is in the range of 500-1000 K.

We have studied the electronic structure of CsW2O6 across its metal-insulator transition by means of hard/soft x-ray photoelectron spectroscopy. In the high-temperature metallic phase, the W 5d band exhibits a clear Fermi edge. The W 4 f7/2 and 4 f5/2 core-level peaks are accompanied by shoulders on their lower binding energy side. The shoulder and main peaks, respectively, are attributed to the well-and poorly screened final states where the screening is due to the W 5d electronic states in the vicinity of the chemical potential. In going from 300 to 180 K (across the metal-insulator transition), a band gap of about 0.2 eV is created at the Fermi level. The origin of the band gap can be assigned to the trimerization of the W sites. The W 4 f and 5d spectra exclude the possibility of charge disproportionation and suggest moderate electronic correlation effects.

ReO${}_{3}$ is a remarkable transition metal oxide in that it has the highest conductivity of all oxides, comparable even with that of silver. Using state of the art bulk-sensitive angle-resolved photoelectron spectroscopy, the authors are able to observe clear dispersions of the Re 5$d$ and O 2$p$ derived bands as well as the momentum splitting of the Fermi surface due to the Re 5$d$ spin-orbit interaction. The experimental results are compared quantitatively to density functional theory band structure calculations, thereby providing a deeper understanding of the material class of the 5$d$ oxides.