For the development of new high-temperature superconductors a basic understanding of the mechanism driving the superconducting pairing, is indispensable. However, The origin of high-temperature superconductivity as well as heavy-fermion superconductivity is still discussed controversially. These superconductors are not phonon-mediated, simple BCS s-wave superconductors, but are unconventional in the respect that additional symmetries are broken in the superconducting state or that the superconducting pairing is not mediated by phonons. Instead of lattice vibrations magnetic fluctuations are thought to play a dominant role in the formation of Cooper pairs. Using inelastic neutron scattering we observed for the first time a clear spin excitation gap in the superconducting state of the prototypical heavy-fermion compound CeCu2Si2, at an incommensurate wave vector determined by the nesting properties of the Fermi surface. Our results demonstrate that the spin excitations are highly relevant for the superconducting pairing in CeCu2Si2. In comparison to high-Tc and other heavy-fermion superconductors, the observation of the spin excitation gap in three-dimensional CeCu2Si2 suggests that their appearance is a more general manifestation of unconventional superconductivity.
O. Stockert, J. Arndt, E. Faulhaber, K. Schmalzl, W. Schmidt, A. Schneidewind, H. S. Jeevan, C. Geibel, M. Loewenhaupt and F. Steglich. Paramagnon-mediated superconductivity in CeCu2Si2? Max-Planck-Institut für Chemscihe Physik fester Stoffe, Scientific report 2006 – 2008, p. 104 – 107 (2009).