Heavy-fermion superconductivity in the Kondo-lattice system CeCu2Si2

The formation of “heavy fermions” (HFs), i.e., charge carriers with largely enhanced effective mass, in Ce-based Kondo lattice (KL) systems is recalled. In contrast to the disastrous pair-breaking effect of magnetic Ce3+ impurities in certain superconducting Kondo alloys, a periodic dense array of Ce3+ ions is a prerequisite for the generation of HF superconductivity (SC) below Tc = 0.6 K in the KL system CeCu2Si2. Subsequent to this discovery in 1979, more than 35 other Ce-, Pr-, Yb-, U-, Np- and Pu-based HF superconductors (with maximum Tc = 18.5 K) have been found. These materials exhibit a variety of fascinating properties, e.g., even- or odd-parity Cooper pairs, multi-phase diagrams or the coexistence with magnetic order. In particular, the interplay between HF SC and quantum criticality is of high timely interest. CeCu2Si2 has emerged as a prototype system showing a three-dimensional (3D) spindensity-wave (SDW) quantum-critical point. Recent results of inelastic-neutron-scattering experiments on this compound are briefly discussed. They lend strong support to the perception of Cooper pairs mediated by nearly quantum-critical 3D-SDW fluctuations covering a wide frequency range.