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 CeCu2Si2 using vector (q) over cap -dependent nonresonant inelastic x-ray scattering experiments at the cerium N-4,N-5 edge. The observed difference between the two different directions, (q) over cap parallel to [100] and (q) over cap parallel to [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.