Linear-polarized soft-x-ray absorption (XAS) and inelastic neutron scattering (INS) experiments have been performed on $\text{Ce}M{\text{In}}_5$ with $M=\text{Rh}$, Ir, and Co to determine the crystal-field scheme and characteristic Kondo temperatures $T^{\ast }$ for the hybridization between $4f$ and conduction electrons. The ground-state wave functions are determined from the polarization-dependent soft-XAS data at the cerium $M_{4,5}$ edge and the crystal-field splittings from INS. The characteristic temperature $T^{\ast }$ has been determined from the line widths of the neutron scattering data. We find that the quasielastic linewidths of the superconducting compounds ${\text{CeCoIn}}_5$ and ${\text{CeIrIn}}_5$ are comparable with the low-energy crystal-field splitting.

Significance: The ground state of materials with strong electronic correlations depends on a delicate balance among competing interactions. The strongly correlated compounds Ce M In ₅ , with M = Co, Rh, and Ir, exhibit superconducting and magnetic ground states as well as Fermi surface changes upon substituting one M element for another and become even higher temperature superconductors when Ce is substituted by Pu. They are therefore recognized as important model systems in which a search for parameters correlating with the occurrence of these ground states could be successful. The present X-ray absorption study of CeRh _{1− x} Ir _x In ₅ reveals that anisotropy of the Ce 4 f -wave function is a significant parameter that is highly sensitive to the ground-state formation and should be taken into account when modeling these systems.

Soft x-ray absorption experiments on the transition-metal $L_{2,3}$ edge of multiferroic ${\text{MnWO}}_4$ and antiferromagnetic ${\text{CoWO}}_4$ are presented. The observed linear polarization dependence, analyzed by full-multiplet calculations, is used to determine the ground-state wave function of the magnetic ${\text{Mn}}^{2+}$ and ${\text{Co}}^{2+}$ ions. The impact of the local structure and the spin-orbit coupling on the orbital moment is discussed in terms of the single-ion anisotropy. It is shown that the orbital moment in ${\text{CoWO}}_4$ is responsible for the collinear antiferromagnetism, while the small size of spin-orbit coupling effects make spiral magnetic order in ${\text{MnWO}}_4$ possible, enabling the material to be multiferroic.

We have succeeded in establishing the crystal-field ground state of CeRu${}_2$Al${}_{10}$, an orthorhombic intermetallic compound recently identified as a Kondo insulator. Using polarization-dependent soft x-ray absorption spectroscopy at the Ce $M_{4,5}$ edges, together with input from inelastic neutron and magnetic susceptibility experiments, we were able to determine unambiguously the orbital occupation of the $4f$ shell and to explain quantitatively both the measured magnetic moment along the easy $a$ axis and the small ordered moment along the $c$ axis. The results provide not only a platform for a realistic modeling of the spin and charge gap of CeRu${}_2$Al${}_{10}$, but demonstrate also the potential for soft x-ray absorption spectroscopy to obtain information not easily accessible by neutron techniques for the study of Kondo insulators in general.

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.