  The structures, the phase diagrams, and the appearance of a neutron resonance
in the superconducting state provide phenomenological evidence which relate the
heavy fermion, cuprate and Fe superconductors. Single- and multi-band Hubbard
models have been found to describe a number of the observed properties of these
materials so that it is reasonable to examine the origin of the pairing
interaction in these models. Here based on the experimental phenomenology and
studies of the momentum and frequency dependence of the pairing interaction for
Hubbard-like models, we suggest that spin-fluctuation mediated pairing is the
common thread linking this broad class of superconducting materials.
