Uncertainty principle enhanced pairing correlations in projected fermi systems near half filling

Abstract

We point out the curious phenomenon of order by projection in a class of lattice Fermi systems near half filling. Enhanced pairing correlations of extended s-wave Cooper pairs result from the process of projecting out s-wave Cooper pairs, with negligible effect on the ground-state energy. The Hubbard model is a particularly nice example of the above phenomenon, which is revealed with the use of rigorous inequalities including the uncertainty principle inequality. In addition, we present numerical evidence that at half filling a related but simplified model shows off-diagonal long range order ODLRO of extended s-wave Cooper pairs.