F. Strigari

Crystal-field ground state of the orthorhombic Kondo insulator CeRu2Al10

We have succeeded in establishing the crystal-field ground state of CeRu2Al10, an orthorhombic intermetallic compound recently identified as a Kondo insulator. Using polarization dependent soft x-ray absorption spectroscopy at the Ce M4,5 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 CeRu2Al10, but demonstrate also the potential of soft x-ray absorption spectroscopy to obtain information not easily accessible by neutron techniques for the study of Kondo insulators in general.

Correlation between ground state and orbital anisotropy in heavy fermion materials

Significance The ground state of materials with strong electronic correlations depends on a delicate balance among competing interactions. The strongly correlated compounds CeMIn5, 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 CeRh1−xIrxIn5 reveals that anisotropy of the Ce 4f-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. The interplay of structural, orbital, charge, and spin degrees of freedom is at the heart of many emergent phenomena, including superconductivity. Unraveling the underlying forces of such novel phases is a great challenge because it not only requires understanding each of these degrees of freedom, it also involves accounting for the interplay between them. Cerium-based heavy fermion compounds are an ideal playground for investigating these interdependencies, and we present evidence for a correlation between orbital anisotropy and the ground states in a representative family of materials. We have measured the 4f crystal-electric field ground-state wave functions of the strongly correlated materials CeRh1−xIrxIn5 with great accuracy using linear polarization-dependent soft X-ray absorption spectroscopy. These measurements show that these wave functions correlate with the ground-state properties of the substitution series, which covers long-range antiferromagnetic order, unconventional superconductivity, and coexistence of these two states.

Determining the In-Plane Orientation of the Ground-State Orbital of CeCu2Si2

We have successfully determined the hitherto unknown sign of the B(4)(4) Stevens crystal-field parameter of the tetragonal heavy-fermion compound CeCu(2)Si(2) using vector q-dependent nonresonant inelastic x-ray scattering experiments at the cerium N(4,5) edge. The observed difference between the two different directions, q∥[100] and q∥[110], is due to the anisotropy of the crystal-field ground state in the (001) plane and is observable only because of the utilization of higher than dipole transitions possible in nonresonant inelastic x-ray scattering. This approach allows us to go beyond the specific limitations of dc magnetic susceptibility, inelastic neutron scattering, and soft x-ray spectroscopy, and provides us with a reliable information about the orbital state of the 4f electrons relevant for the quantitative modeling of the quasiparticles and their interactions in heavy-fermion systems.

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.

Evolution of ground-state wave function in CeCoIn5 upon Cd or Sn doping

We present linear polarization-dependent soft x-ray absorption spectroscopy data at the Ce

Crystal field ground state of the orthorhombic Kondo semiconductors CeOs2Al10 and CeFe2Al10

M_{4,5}

Absence of orbital rotation in superconducting CeCu2Ge2

edges of Cd and Sn doped CeCoIn

Correlation between the valence state of cerium and the magnetic transition in Ce (Ru 1 - X Fe x) 2 Al 10 studied by resonant x-ray emission spectroscopy

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Exchange field effect in the crystal-field ground state of CeMAl4Si2

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Magnetic Field Induced Orbital Polarization in Cubic YbInNi4: Determining the Quartet Ground State Using X-Ray Linear Dichroism

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