Neutron study of crystal field excitations in single crystal CeCu$_{2}$Ge$_{2}$

CeCu$_{2}$Ge$_{2}$ is the counterpart of the heavy-fermion superconductor CeCu$_{2}$Si$_{2}$. CeCu$_{2}$Ge$_{2}$ is a magnetically ordering (T$_{N}$= 4.1 K) Kondo lattice with a moderate Sommerfeld coefficient of 140 mJ/molK$^{2}$ [1]. Inelastic neutron measurements on a polycrystalline sample revealed a doublet ground state and a quasi-quartet at 16.5 meV [1] though a splitting of the 4f$^{1}$ (J=5/2) ground state multiplet into 3 doublets is expected from the point symmetry of the Ce$^{3+}$ ions. We performed detailed inelastic neutron scattering experiments on the thermal triple-axis spectrometer PUMA at FRM II at temperatures between 10 K and 300 K and for different crystallographic directions from low to high momentum transfers. In this way we obtained a reliable separation of magnetic and phonon contributions. From our results we infer that the quasi-quartet consists in fact of two doublets at 17 and 18 meV which exhibit a strong directional dependence of their transition matrix elements to the ground state doublet. Finally we will present a new set of crystal field parameters and their implications on other magnetic properties. [1] G. Knopp et al., Z. Physik B 77 (1989) 95