Interplay of Magnetism and Superconductivity in CeCu$_{2}$Si$_{2}$ and RNi$_{2}$B$_{2}$C Probed by Neutron Scattering

We will discuss results of neutron scattering experiments performed on two systems that were investigated in close collaboration with other groups: the heavy fermion superconductor CeCu$_{2}$Si$_{2}$ and the borocarbides RNi$_{2}$B$_{2}$C (with R = rare earth). In both systems SC and magnetic ordering is observed. Due to minor changes in stoichiometry single crystals of A-, A/S and S-type CeCu$_{2}$Si$_{2}$ can be grown that show only long range magnetic order (LRO), both SC and LRO, and only SC. However, LRO is suppressed as soon as SC sets in (phase separation in A/S-type crystals). In S-type crystals only short range correlations are observed that develop an energy gap in the magnetic excitation spectrum below $T_{C}$ . In the borocarbides both LRO and SC coexist (e.g. for HoNi$_{2}$B$_{2}$C: $T_{C}$ = 8 K and $T_{N}$ = 6 K). However, there is no change in the diffuse neutron scattering (being a signature of the susceptibility of the conduction electrons) for temperatures above and below $T_{C}$ (but still above$ T_{N})$. The origin of SC in the borocarbides is manifested in a strong phonon softening intimately connected with the development of the SC gap. Conclusion: Magnetism and superconductivity in CeCu$_{2}$Si$_{2}$ is (phase) separated in real space while in the borocarbides both phenomena may be understood as being decoupled in momentum space.