  In heavy-fermion superconductors, it is widely believed that the
superconducting gap function has sign-reversal due to the strong electron
correlation. However, recently discovered fully-gapped s-wave superconductivity
in CeCu2Si2 has clarified that strong attractive pairing interaction can appear
even in heavy-fermion systems. To understand the origin of attractive force, we
develop the multipole fluctuation theory by focusing on the inter-multipole
many-body interaction called the vertex corrections. By analyzing the periodic
Anderson model for CeCu2Si2, we find that hexadecapole fluctuations mediate
strong attractive pairing interaction. Therefore, fully-gapped s-wave
superconductivity is driven by pure on-site Coulomb repulsion, without
introducing electron-phonon interactions. The present theory of
superconductivity will be useful to understand rich variety of the
superconducting states in heavy fermion systems.
