Crystal-field investigations in rare earth compounds using linearpolarized soft X-ray absorption spectroscopy.

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We propose to continue with the investigation of crystal-electric field (CEF) ground states wave functions in rare earth Heavy Fermion compounds where the hybridization of 4f and conduction electrons leads to a manifold of different ground state properties (e.g. magnetic order, itinerant 4f electron behavior, quantum criticality, unconventional superconductivity). The wave functions of the 4f electrons are highly anisotropic and their knowledge is crucial for understanding why structurally and chemically similar compounds have such different ground states properties. For example some theories based on DMFT calculations predict the anisotropic crystal-field split Hunds rule ground state may favour certain ground states due to k dependent hybridization with the conduction electrons. The conventional methods of determining the crystal-field potential, i.e. inelastic neutron scattering, specific heat and magnetic susceptibility, often yield conflicting results, reflecting the difficulties in determining correctly the wave function of the rare earth ion. We exploit the dipole selection rules of linear polarized soft X-ray spectroscopy (soft-XAS) to obtain the CEF wave function with an unprecedented accuracy. The first three years of this project we established soft-XAS as a tool in the field of heavy fermion physics. We were able to determine wave functions in orthorhombic Kondo insulators/semimetals and at the example of CeRh1-xIrxIn5 we were able to show for the first time, thanks to the high precision, that the wave function correlates with a heavy fermion phase diagram. This is particularly exciting for phase diagrams, which cover all the aspects of quantum criticality, non-Fermi liquid behavior, unconventional superconductivity and magnetic order. We know want to consolidate our work by measuring systematically wave functions of substitution series and we also want to continue with our CEF work on orthorhombic Kondo insulators.