High-energy magnetic excitations from heavy quasiparticles in CeCu2Si2

Magnetic fluctuations is the leading candidate for pairing in cuprate, iron-based, and heavy fermion superconductors. This view is challenged by the recent discovery of nodeless superconductivity in ${CeCu}_2{Si}_2$, and calls for a detailed understanding of the corresponding magnetic fluctuations. Here, we mapped out the magnetic excitations in superconducting (S-type) ${CeCu}_2{Si}_2$ using inelastic neutron scattering, finding a strongly asymmetric dispersion for $E\lesssim 1.5meV$, which at higher energies evolves into broad columnar magnetic excitations that extend to $E\gtrsim 5meV$. While low-energy magnetic excitations exhibit marked three-dimensional characteristics, the high-energy magnetic excitations in ${CeCu}_2{Si}_2$ are almost two-dimensional, reminiscent of paramagnons found in cuprate and iron-based superconductors. By comparing our experimental findings with calculations in the random-phase approximation,we find that the magnetic excitations in ${CeCu}_2{Si}_2$ arise from quasiparticles associated with its heavy electron band, which are also responsible for superconductivity. Our results provide a basis for understanding magnetism and superconductivity in ${CeCu}_2{Si}_2$, and demonstrate the utility of neutron scattering in probing band renormalization in heavy fermion metals.