Orbital selective coupling in CeRh3_3B2_2: co-existence of high Curie and high Kondo temperature

We investigated the electronic structure of the enigmatic CeRh$_3$B$_2$ using resonant inelastic scattering and x-ray absorption spectroscopy in combination with $ab$ $initio$ density functional calculations. We find that the Rh 4$d$ states are irrelevant for the high-temperature ferromagnetism and the Kondo effect. We also find that the Ce 4$f$ 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 4$f$ orbitals, with each coupling selectively to different bands in CeRh$_3$B$_2$. The inter-site hybridization of the |J=5/2,Jz=+/-1/2> crystal-field state and Ce 5$d$ band combined with the intra-site Ce 4$f$-5$d$ 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$_3$B$_2$.Comment: 15 pages, 14 figure

Singlet magnetism in intermetallic UGa2_2 unveiled by inelastic x-ray scattering

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.Comment: 9 pages, 7 figure

Spectroscopic evidence of Kondo-induced quasi-quartet in CeRh2_2As2_2

CeRh$_2$As$_2$ 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$_2$As$_2$. 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 $|\Gamma_7^- \rangle$ state with its lobes along the 110 direction of the tetragonal structure ($xy$ orientation) contributes most to the multi-orbital ground state of CeRh$_2$As$_2$.Comment: 8 pages, 7 figure

CeRu4Sn6: A strongly correlated material with nontrivial topology

Topological insulators form a novel state of matter that provides new opportunities to create unique quantum phenomena. While the materials used so far are based on semiconductors, recent theoretical studies predict that also strongly correlated systems can show non-trivial topological properties, thereby allowing even the emergence of surface phenomena that are not possible with topological band insulators. From a practical point of view, it is also expected that strong correlations will reduce the disturbing impact of defects or impurities, and at the same increase the Fermi velocities of the topological surface states. The challenge is now to discover such correlated materials. Here, using advanced x-ray spectroscopies in combination with band structure calculations, we infer that CeRu4Sn6 is a strongly correlated material with non-trivial topology

Quantitative investigation of the 4f occupation in the quasikagome Kondo lattice CeRh1-xPdxSn

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

Direct imaging of orbitals in quantum materials

The electronic states of quantum materials based on transition-metal, rare-earth and actinide elements are dominated by electrons in the d and f orbitals intertwined with the strong band formation of the solid. Until now, to estimate which specific orbitals contribute to the ground state and thereby determine their physical properties we have had to rely on theoretical calculations combined with spectroscopy. Here, we show that s-core-level non-resonant inelastic X-ray scattering can directly image the active orbital in real space, without the necessity for any modelling. The power and accuracy of this new technique is shown using the textbook example, x(2) - y(2)/3z(2) - r(2) orbital of the Ni2+ ion in NiO single crystal

Origin of Ising magnetism in Ca3_3Co2_2O6_6 unveiled by orbital imaging

The one-dimensional cobaltate Ca$_3$Co$_2$O$_6$ is an intriguing material having an unconventional magnetic structure, displaying quantum tunneling phenomena in its magnetization. Using a newly developed experimental method, $s$-core-level non-resonant inelastic x-ray scattering ($s$-NIXS), we were able to image the atomic Co 3$d$ orbital that is responsible for the Ising magnetism in this system. We can directly observe that corrections to the commonly accepted ideal prismatic trigonal crystal field scheme occur in Ca$_3$Co$_2$O$_6$, and it is the complex $d_2$ orbital occupied by the sixth electron at the high-spin Co$^{3+}_\text{trig}$ ($d^6$) sites that generates the Ising-like behavior. The ability to directly relate the orbital occupation with the local crystal structure is essential to model the magnetic properties of this system

Origin of Ising magnetism in Ca3Co2O6 unveiled by orbital imaging

The one-dimensional cobaltate Ca3Co2O6 is an intriguing material having an unconventional magnetic structure, displaying quantum tunneling phenomena in its magnetization. Using a newly developed experimental method, s-core-level non-resonant inelastic x-ray scattering (s-NIXS), we were able to image the atomic Co 3d orbital that is responsible for the lsing magnetism in this system. We can directly observe that corrections to the commonly accepted ideal prismatic trigonal crystal field scheme occur in Ca3Co2O6, and it is the complex d(2) orbital occupied by the sixth electron at the high-spin Co-trig(3+)(d(6)) sites that generates the Ising-like behavior. The ability to directly relate the orbital occupation with the local crystal structure is essential to model the magnetic properties of this system

Tuning the hybridization and magnetic ground state of electron and hole doped CeOs2Al10: An x-ray spectroscopy study

Here we present linear and circular polarized soft x-ray absorption spectroscopy (XAS) data at the Ce M-4,M-5 edges of the electron (Ir) and hole-doped (Re) Kondo semiconductor CeOs2Al10. Both substitutions have a strong impact on the unusual high Neel temperature T-N = 28.5 K, and also the direction of the ordered moment in case of Ir. The substitution dependence of the linear dichroism is weak thus validating the crystal-field description of CeOs2Al10 being representative for the Re and Ir substituted compounds. The impact of electron and hole doping on the hybridization between conduction and 4 f electrons is related to the amount of f(0) in the ground state and reduction of x-ray magnetic circular dichroism. A relationship of c f-hybridization strength and enhanced T-N is discussed. The direction and doping dependence of the circular dichroism strongly supports the idea of strong Kondo screening along the crystallographic a direction