We have successfully determined the hitherto unknown sign of the $B_4^4$ Stevens crystal-field parameter of the tetragonal heavy-fermion compound ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ using vector $\widehat{\mathbf{q}}$-dependent nonresonant inelastic x-ray scattering experiments at the cerium $N_{4,5}$ edge. The observed difference between the two different directions, $\widehat{\mathbf{q}}\parallel [100]$ and $\widehat{\mathbf{q}}\parallel [110]$, 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.

• Received 3 May 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.046401