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.
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