ELEMENTARY-SPIN-EXCITATION SPECTRUM OF UNDOPED AND DOPED SINGLE-BAND HUBBARD MODELS

We have calculated the elementary-spin-excitation spectrum of undoped and doped Hubbard models in one and two dimensions. We have obtained the gap of the spectrum at Q = (pi, pi) in the doped strong-coupling Hubbard model by a Lanczos diagonalization study of the square-root 10 X square-root 10 site cluster. The result is qualitatively in agreement with the experimental observation of the ''spin gap'' by Rossat-Mignod et al. We conclude that the ''spin gap'' can be accounted for with the doped strong-coupling Hubbard model without any symmetry reductions and is one of the intrinsic properties of the Hubbard model.