2-LOOP SCATTERING-AMPLITUDES FOR THE FINITE-CHARGE INFINITE-U HUBBARD-MODEL - EXISTENCE OF A SPIN-EXCHANGE COLLECTIVE MODE

We study the spin-exchange scattering amplitude at two-loop (1/N2) order for the infinite-U Hubbard model away from the metal-insulator filling, with finite electron charge, for which a well-defined 1/N expansion is possible, the mean-field level being equivalent to Gutzwiller enhanced quasiparticles. We find a positive spin-exchange Landau parameter which suggests that the spin-density fluctuations show a well-defined collective mode near the metal-insulator filling. This behavior is interpreted as paralleling the spin and charge separation in the Luttinger liquid.