COLLECTIVE EXCITATIONS AND SUM-RULES FOR THE HUBBARD-MODEL IN THE SPIN-DENSITY-WAVE REGIME

A variational estimate for the spin-wave velocity of the one-band Hubbard model on a square lattice in the spin-density-wave regime is studied. The estimate is given by the ratio of the f-sum rule to the static structure factor of the transverse-spin response function. The known results for the Heisenberg model are used to obtain results for those quantities in the large-U limit of the Hubbard model. The f-sum rule and static structure factor are calculated using the random-phase approximation (RPA). The spin-wave velocity calculated in the RPA, in the spin-density-wave ground state, violates the variational bound.