f-electron charge densities probed using core level non-resonant inelastic X-ray scattering

Strongly correlated materials are characterized by the presence of electron-electron interactions in their electronic structure. They often have remarkable properties and transitions between competing phases of very different electronic and magnetic order. This thesis focuses on strongly correlated f-electron compounds containing Ce, Sm, and U. These materials exhibit a so-called heavy-fermion or Kondo-lattice behavior. They can become insulating due to hybridization effects (Kondo-insulator) or develop multipolar (hidden) order. Kondo insulators have recently been discussed in the context of strongly correlated topological insulators. This new aspect caused an enormous activity in the field of Kondo insulators, theoretically as well as experimentally. Multipolar order as well as the formation of a Kondo insulating state strongly depend on the symmetry of the f states involved. Also the character of the surface states in a topological insulator is determined by the properties of the bulk states. Therefore the scope of this thesis has been to unveil the underlying symmetries of the bulk f states. Here the compounds CeB${}_6$, UO${}_2$, and URu${}_2$Si${}_2$, which exhibit multipolar order, as well as the Kondo insulators (semimetals) SmB${}_6$ and CeRu${}_4$Sn${}_6$ have been studied. Non-resonant inelastic X-ray scattering (NIXS) has been established as a bulk sensitive technique for determining ground-state wave functions. NIXS at large momentum transfer measures a higher than dipole scattering signal, so that anisotropies of cubic point symmetries are accessible, and it is the only technique that shows multiplet structures in metallic uranium compounds. CeB${}_6$ (Ch. 5) and UO${}_2$ (Ch. 8) were measured because they can serve as benchmark compounds; their ground states were known from other studies, so that the validity of the NIXS experiment and its analysis could be tested. For SmB${}_6$ (Ch. 6), CeRu${}_4$Sn${}_6$ (Ch. 7), and URu${}_2$Si${}_2$ (Ch. 9) the ground states were determined. For SmB${}_6$ NIXS was needed because SmB${}_6$ is cubic, for CeRu${}_4$Sn${}_6$ because NIXS is bulk sensitive, and for URu${}_2$Si${}_2$ because the multipolar NIXS signal is more excitonic. Finally, the linear independent directions of the multipole expansion of the transition operator in NIXS have been investigated (Ch. 3.2.2) and a tensor notation has been developed (Ch. 3.4). This is useful for a better understanding of the directional dependence of the momentum transfer of the single crystal NIXS spectra.