Superconductivity from repulsion: a variational view

This letter presents a new class of variational wavefunctions for Fermi sys tems in any dimension. These wavefunctions introduce correlations between C ooper pairs in different momentum states and the relevant correlations can be computed analytically. At half filling we have a ground state with criti cal superconducting correlations, that causes negligible increase of the ki netic energy. We find large enhancements in a Cooper-pair correlation funct ion caused purely by the interplay between the uncertainty principle, repul sion and the proximity of half filling. This is surprising since there is n o accompanying signature in usual charge and spin response functions, and t ypifies a novel kind of many-body cooperative behaviour.