  The question of whether one should speak of a "pairing glue" in the Hubbard
and t-J models is basically a question about the dynamics of the pairing
interaction. If the dynamics of the pairing interaction arises from virtual
states, whose energies correspond to the Mott gap, and give rise to the
exchange coupling J, the interaction is instantaneous on the relative time
scales of interest. In this case, while one might speak of an "instantaneous
glue", this interaction differs from the traditional picture of a retarded
pairing interaction. However, if the energies correspond to the spectrum seen
in the dynamic spin susceptibility, then the interaction is retarded and one
speaks of a spin-fluctuation glue which mediates the d-wave pairing. Here we
present results from numerical studies which provide insight into this
question.
