d-wave pairing in lightly doped Mott insulators

We define a suitable quantity Z(c) that measures the pairing strength of two electrons added to the ground-state wave function by means of the anomalous part of the one-particle Green's function. Z(c) discriminates between systems described by one-electron states, like ordinary metals and band insulators, for which Z(c)=0, and systems where the single particle picture does not hold, like superconductors and resonating valence bond insulators, for which Z(c)not equal 0. By using a quantum Monte Carlo projection technique for the Hubbard model at U/t=4, a finite Z(c), with d-wave symmetry, is found at half filling and in the lightly doped regime, thus emphasizing a qualitatively new feature coming from electronic correlation.