UCd_{11} is an antiferromagnetic uranium intermetallic compound (T_{N}=5.3K) with enhanced electron mass and uranium-uranium spacings nearly twice the Hill limit, suggesting a weakly hybridized 5f electronic character. Various x-ray spectroscopy techniques indicate that uranium in UCd_{11} adopts the formal U^{3+}5f^{3} configuration, while core-level photoemission spectroscopy (PES) data of UCd_{11} reveal only a weak satellite feature, typically interpreted as a signature of itinerancy. In this work, we present density functional theory (DFT) combined with dynamical mean-field theory (DMFT) calculations of UCd_{11}, using material-specific parameters tuned to reproduce valence-band PES spectra at different photon energies, thereby exploiting the energy dependence of photoionization cross sections. Our results demonstrate that UCd_{11} is a highly localized uranium 5f^{3} system. Furthermore, core-level spectra obtained from a DFT+DMFT Anderson impurity model reveal that, contrary to common assumptions, the presence or absence of satellite structures is not a reliable indicator of strong correlations or itinerant 5f behavior.

UCd11 is an antiferromagnetic uranium intermetallic compound (TN = 5.3 K) with enhanced electron mass and uranium-uranium spacings nearly twice the Hill limit, suggesting a weakly hybridized 5 f electronic character. Various x-ray spectroscopy techniques indicate that uranium in UCd11 adopts the formal U3+ 5 f3 configuration, while core-level photoemission spectroscopy (PES) data of UCd11 reveal only a weak satellite feature, typically interpreted as a signature of itinerancy. In this work, we present density functional theory (DFT) combined with dynamical mean-field theory (DMFT) calculations of UCd11, using material-specific parameters tuned to reproduce valence-band PES spectra at different photon energies, thereby exploiting the energy dependence of photoionization cross sections. Our results demonstrate that UCd11 is a highly localized uranium 5 f3 system. Furthermore, core-level spectra obtained from a DFT + DMFT Anderson impurity model reveal that, contrary to common assumptions, the presence or absence of satellite structures is not a reliable indicator of strong correlations or itinerant 5 f behavior.

The mechanism of a pressure-induced quantum critical point in the heavy fermion ferromagnet CeRh6Ge4 has attracted interest, as ferromagnetic quantum criticality in a clean itinerant Ce compound is typically avoided. The localized versus itinerant character of the 4 f electrons is a key aspect for understanding this behavior. We investigated the electronic structure of the 4 f shell in CeRh6Ge4 using core-level photoelectron and x-ray absorption spectroscopy, demonstrating the hybridization of Ce 4 f with the conduction electrons. Linearly polarized x-ray absorption reveals a temperature-dependent linear dichroism consistent with the crystalelectric-field sequence as inferred from the static susceptibility. This dichroism cannot be described by an ionic full-multiplet model alone, but is reproduced by including the Kondo effect within a single-impurity Anderson model in the noncrossing approximation. The Kondo effect mixes higher-lying crystal-field states into a resulting multiorbital ground state with 4 f occupancy, nf 0.9. Deviations at low temperatures between the measured linear dichroism and calculated dichroism suggest an orbital-dependent Kondo effect. A scenario in which there is a multiorbital ground state and orbital-dependent Kondo hybridization should be a starting point for a model of pressure-induced criticality in CeRh6Ge4.

Understanding the electronic structure of actinide materials is crucial for both fundamental research and nuclear applications. The partially filled 5f shells exhibit complex behavior due to strong correlations and ligand hybridization, requiring advanced spectroscopic techniques. Here, we report on the development and application of high-resolution valence band resonant inelastic x-ray spectroscopy (VB-RIXS) experiments at the uranium M_{4,5} edges (3551 and 3725 eV). We present data of UO_{2}, a well-established model actinide compound. VB-RIXS is particularly well suited for probing the 5f-shell electronic structure, as it probes, in contrast to core-to-core RIXS, excitations without leaving a high-energy core hole in the final state. In VB-RIXS, we achieve energy resolutions of 50 meV (M_{5}) and 90 meV (M_{4}), enabling the resolution of multiplet excitations and crystal-field effects, as well as charge-transfer and fluorescencelike features with unprecedented clarity. As such, high-resolution VB-RIXS offers direct insights into both low-energy, near-ground-state properties and high-energy hybridization and covalency effects. Our results demonstrate the power of VB-RIXS as a versatile and powerful tool for probing the strongly correlated electronic structure of actinide materials, providing essential input for quantitative modeling and the validation of theoretical concepts.

We investigate the nature of the 5f electrons in the unconventional odd-parity superconductor UTe_{2}, focusing on the degree of covalency, localization versus itinerancy, and dominant electronic configuration. This is achieved using density functional theory (DFT) in combination with dynamical mean-field theory (DMFT) calculations. A key aspect of our approach is the material-specific tuning of the double-counting correction parameter, μ_{dc}, within the DFT+DMFT part. This tuning is guided by the energy dependence of photoionization cross sections in valence band photoelectron spectroscopy. The reliability of the parameters is confirmed by the accurate reproduction of the angle-resolved valence-band photoemission spectra and the U4f core-level data. The DFT+DMFT model reveals that in UTe_{2} U5f^{n} configurations with n=1–4 contribute to the ground state, with the 5f^{2} configuration being most prevalent and an average 5f shell filling close to 2.5. The model further suggests that the 5f electrons form narrow bands and that charge fluctuations due to degeneracy play a role in addition to coherent valence dynamics arising from hybridization with the conduction bath. Additionally, the significance of the U6d states in UTe_{2} is discussed.

Understanding the electronic structure of actinide materials is crucial for both fundamental research and nuclear applications. The partially filled 5f shells exhibit complex behavior due to strong correlations and ligand hybridization, requiring advanced spectroscopic techniques. Here, we report on the development and application of high-resolution valence-band resonant inelastic x-ray spectroscopy (VB-RIXS) experiments at the uranium M_4,5 edges (3551 and 3725 eV). We present data of UO_2, a well-established model actinide compound. VB-RIXS is particularly well suited for probing the 5f-shell electronic structure, as it probes, in contrast to core-to-core RIXS, excitations without leaving a high-energy core hole in the final state. In VB-RIXS, we achieve energy resolutions of 50 meV (M_5) and 90 meV (M_4), enabling the resolution of multiplet excitations and crystal-field effects, as well as charge-transfer and fluorescence-like features with unprecedented clarity. As such, high resolution VB-RIXS offers direct insights into both low-energy, near ground-state properties and high-energy hybridization and covalency effects. Our results demonstrate the power of VB-RIXS as a versatile and powerful tool for probing the strongly correlated electronic structure of actinide materials, providing essential input for quantitative modeling and the validation of theoretical concepts.

The higher dimensionality in the crystal fields of the Ce2MIn8 (M=Rh,Ir) compounds and its interplay with hybridization and disorder are key ingredients to understand the complex phase diagrams by this family, which have been explored extensively by macroscopic techniques. Here, we present an investigation of the crystal-electric field schemes of Ce2Rh1−xIrxIn8 using x-ray absorption spectroscopy. Our full multiplet calculations for the 4f1 configuration of Ce3+ to describe the temperature-dependent linear dichroism in Ce2MIn8 are consistent with a Γ71=1−α2·|∓32〉−|α|·|±52〉 ground state containing a predominant |±3/2〉 contribution that increases further with x. This enhancement is believed to favor superconductivity in Ce-based heavy fermion materials, observed in previous results in the CeMIn5 family. Our recent observations shed light on the unexpected emergence of the ambient-pressure superconducting dome in the center of the composition phase diagram and its subsequent suppression on the Ir-rich side due to the early onset of fluctuations associated with the structurally more disordered state, inferred from previous neutron magnetic diffraction experiments. Published by the American Physical Society 2024

We investigate the topological superconductor candidate UTe$_2$ using high-resolution valence-band resonant inelastic x-ray scattering at the U $M_{4,5}$-edges. We observe atomic-like low-energy excitations that support the correlated nature of this unconventional superconductor. These excitations originate from the U $5f^2$ configuration, which is unexpected since the short Te2-Te2 distances exclude Te2 being 2-. By utilizing the photoionization cross-section dependence of the photoemission spectra in combination with band structure calculations, we infer that the stabilization of the U $5f^2$ configuration is due to the U $6d$ bonding states in the U-dimers acting as a charge reservoir. Our results emphasize that the description of the physical properties should commence with a $5f^2$ $ansatz$.

A procedure for quantifying the U 5 f electronic covalency and degree of localization in U intermetallic compounds is presented. To this end, bulk sensitive hard and soft x-ray photoelectron spectroscopy were utilized in combination with density-functional theory (DFT) plus dynamical mean-field theory (DMFT) calculations. The energy dependence of the photoionization cross sections allows the disentanglement of the U 5 f contribution to the valence band from the various other atomic subshells so the computational parameters in the DFT + DMFT can be reliably determined. Applying this method to UGa2 and UB2 as model compounds from opposite ends of the (de)localization range, we have achieved excellent simulations of the valence band and core-level spectra. The width in the distribution of atomic U 5 f configurations contributing to the ground state, as obtained from the calculations, quantifies the correlated nature and degree of localization of the U 5 f. The findings permit answering the longstanding question why different spectroscopic techniques give seemingly different numbers for the U 5 f valence in intermetallic U compounds.

CeRh2As2 is a new multiphase superconductor with strong suggestions for an additional itinerant multipolar ordered phase. The modeling of the low -temperature properties of this heavy-fermion compound requires a quartet Ce3 thorn crystal -field ground state. Here, we provide the evidence for the formation of such a quartet state using x-ray spectroscopy. Core -level photoelectron and x-ray absorption spectroscopy confirm the presence of Kondo hybridization in CeRh2As2. The temperature dependence of the linear dichroism unambiguously reveals the impact of Kondo physics for coupling the Kramer's doublets into an effective quasiquartet. Nonresonant inelastic x-ray scattering data find that the j Gamma-7i state with its lobes along the 110 direction of the tetragonal structure (xy orientation) contributes most to the multiorbital ground state of CeRh2As2.

The application of pressure as well as the successive substitution of Ru with Fe in the hidden order (HO) compound URu2Si2 leads to the formation of the large-moment antiferromagnetic phase. Here, we investigate the substitution series URu2-xFexSi2 from x = 0.0 to 2.0 by U 4 f core-level photoelectron spectroscopy and observe nonmonotonic changes in the spectra. The initial increase and subsequent decrease in the spectral weight of the 4 f core-level satellite with increasing x stands for a nonmonotonic 5 f filling across the substitution series. The competition of chemical pressure and increase in the density of states at the Fermi energy, both due to substitution of Ru with Fe, can explain such behavior. An extended Doniach phase diagram including the x dependence of the density of states is proposed. Also in URu2-xFexSi2 the ground state is a singlet or quasidoublet state consisting of two singlets. Hence, the formation of magnetic order in the URu2-xFexSi2 substitution series must be explained within a singlet magnetism model.

CeRh_2As_2 is a new multiphase superconductor with strong suggestions for an additional itinerant multipolar ordered phase. The modeling of the low temperature properties of this heavy fermion compound requires a quartet Ce^3+ crystal-field ground state. Here we provide the evidence for the formation of such a quartet state using x-ray spectroscopy. Core-level photoelectron and x-ray absorption spectroscopy confirm the presence of Kondo hybridization in CeRh_2As_2. The temperature dependence of the linear dichroism unambiguously reveils the impact of Kondo physics for coupling the Kramer's doublets into an effective quasi-quartet. Non-resonant inelastic x-ray scattering data find that the |Γ_7^- ⟩ state with its lobes along the 110 direction of the tetragonal structure (xy orientation) contributes most to the multi-orbital ground state of CeRh_2As_2.

We investigated the electronic structure of the enigmatic CeRh_3B_2 using resonant inelastic scattering and x-ray absorption spectroscopy in combination with ab initio density functional calculations. We find that the Rh 4d states are irrelevant for the high-temperature ferromagnetism and the Kondo effect. We also find that the Ce 4f crystal-field strength is too small to explain the strong reduction of the Ce magnetic moment. The data reveal instead the presence of two different active Ce 4f orbitals, with each coupling selectively to different bands in CeRh_3B_2. The inter-site hybridization of the |J=5/2,Jz=+/-1/2> crystal-field state and Ce 5d band combined with the intra-site Ce 4f-5d exchange creates the strong ferromagnetism, while hybridization between the |J=5/2,Jz=+/-5/2> and the B sp in the ab-plane contributes to the Kondo interaction which causes the moment reduction. This orbital selective coupling explains the unique and seemingly contradictory properties of CeRh_3B_2.

Using high resolution tender-x-ray resonant inelastic scattering and hard-x-ray non-resonant inelastic scattering beyond the dipole limit we were able to detect electronic excitations in intermetallic UGa$_2$ that are highly atomic in nature. Analysis of the spectral lineshape reveals that the local $5f^2$ configuration characterizes the correlated nature of this ferromagnet. The orientation and directional dependence of the spectra indicate that the ground state is made of the $\Gamma_1$ singlet and/or $\Gamma_6$ doublet symmetry. With the ordered moment in the $ab$ plane, we infer that the magnetism originates from the higher lying $\Gamma_6$ doublet being mixed with the $\Gamma_1$ singlet due to inter-site exchange, qualifying UGa$_2$ to be a true quantum magnet. The ability to observe atomic excitations is crucial to resolve the on-going debate about the degree of localization versus itineracy in U intermetallics.

CeRh3Si2 has been reported to exhibit metamagnetic transitions below 5 K, a giant crystal field splitting, and anisotropic magnetic properties from single crystal magnetization and heat capacity measurements. Here we report results of neutron and x-ray scattering studies of the magnetic structure and crystal-field excitations to further understand the magnetism of this compound. Inelastic neutron scattering and resonant inelastic x-ray scattering reveal a Jz = 1/2 ground state for Ce when considering the crystallographic a direction as quantization axis, thus explaining the anisotropy of the static susceptibility. Furthermore, we find a total splitting of 78 meV for the J = 5/2 multiplet. The neutron diffraction study in zero field reveals that, on cooling from the paramagnetic state, the system first orders at TN1 = 4.7 K in a longitudinal spin density wave with ordered Ce moments along the b axis (i.e., the [0 1 0] crystal direction) and an incommensurate propagation vector k = (0, 0.43, 0). Below the lower-temperature transition TN2 = 4.48 K, the propagation vector locks to the commensurate value k = (0, 0.5, 0), with a so-called lock-in transition. Our neutron diffraction study in applied magnetic field H II b axis shows a change in the commensurate propagation vector and development of a ferromagnetic component at H = 3 kOe, followed by a series of transitions before the fully field-induced ferromagnetic phase is reached at H = 7 kOe. This explains the nature of the steps previously reported in field-dependent magnetization measurements. A very similar behavior is also observed for the H II [0 1 1] crystal direction.

CeIrSn with a quasikagome Ce lattice in the hexagonal basal plane is a strongly valence fluctuating compound, as we confirm by hard x-ray photoelectron spectroscopy and inelastic neutron scattering, with a high Kondo temperature of T_{K}∼480  K. We report a negative in-plane thermal expansion α/T below 2 K, which passes through a broad minimum near 0.75 K. Volume and a-axis magnetostriction for B∥a are markedly negative at low fields and change sign before a sharp metamagnetic anomaly at 6 T. These behaviors are unexpected for Ce-based intermediate valence systems, which should feature positive expansivity. Rather they point towards antiferromagnetic correlations at very low temperatures. This is supported by muon spin relaxation measurements down to 0.1 K, which provide microscopic evidence for a broad distribution of internal magnetic fields. Comparison with isostructural CeRhSn suggests that these antiferromagnetic correlations emerging at T≪T_{K} result from geometrical frustration.

Herein we show that nonresonant inelastic x-ray scattering involving an s core level is a powerful spectroscopic method to characterize the excited states of transition metal compounds. The spherical charge distribution of the s core hole allows the orientational dependence of the intensities of the various spectral features to produce a spatial charge image of the associated multiplet states in a straightforward manner, thereby facilitating the identification of their orbital character. In addition, the s core hole does not add an extra orbital angular momentum component to the multiplet structure so that the well-established Sugano-Tanabe-Kamimura diagrams can be used for the analysis of the spectra. For α-MnS we observe the spherical charge density corresponding to its high-spin 3d^{5} (^{6}A_{1}) ground state configuration and we were able to selectively image its excited states and identify them as t_{2g} (^{5}T_{2}) and e_{g} (^{5}E) with an energy splitting 10Dq of 0.78 eV.

CeRhSn with the Ce atoms forming a quasikagome lattice in the hexagonal plane has recently been discussed in the context of quantum criticality driven by magnetic frustration. Furthermore, it has been reported that the successive substitution of Rh by Pd leads to magnetic order. Here we have investigated the change of the 4 f occupation in the substitution series CeRh1-xPdxSn for for x = 0, 0.1, 0.3, 0.5, 0.75 by means of photoelectron spectroscopy with hard x rays (HAXPES). The quantitative analysis of the core level spectra with a combined full multiplet and configuration interaction analysis shows a smooth decrease of the 4 f 0 contribution with rising x due to an increase of the effective 4f binding energy epsilon(4f) and the reduction of the effective hybridization V-eff. We further compare valence band data with the calculated partial density of states and find that the Pd 4d states are about 1 eV further away from the Ce 4 f states at the Fermi energy than the Rh 4d states. In fact, the effective binding energy epsilon(4f) of the 4 f states in the configuration interaction analysis of the core level spectra decreases by the same amount.

The application of pressure as well as the successive substitution of Ru with Fe in the hidden order (HO) compound URu$_2$Si$_2$ leads to the formation of the large moment antiferromagnetic phase (LMAFM). Here we have investigated the substitution series URu$_{2-x}$Fe$_x$Si$_2$ with $x$ = 0.2 and 0.3 with non-resonant inelastic x-ray scattering (NIXS) and 4$f$ core-level photoelectron spectroscopy with hard x-rays (HAXPES). NIXS shows that the substitution of Fe has no impact on the symmetry of the ground-state wave function. In HAXPES we find no shift of spectral weight that would be indicative for a change of the 5$f$-electron count. Consequently, changes in the exchange interaction $\cal{J}$ due to substitution must be minor so that the conjecture of chemical pressure seems unlikely. An alternative scenario is discussed, namely the formation of long range magnetic order due the substitution induced local enhancement of the magnetization in the vicinity of the $f$-electron ions while the overall electronic structure remains unchanged.