LOW-LYING EXCITATIONS IN THE HALF-FILLED AND DOPED HUBBARD-MODEL IN ONE DIMENSION

Dispersions and wave functions of all the low-lying excitations are studied numerically along with the Bethe-ansatz analysis for the finite-U one-dimensional Hubbard model for U = 0 --> U = infinity. We find the following. (i) In addition to conventional charge or spin excitations such as des Cloizeaux-Pearson modes, there are classes of excitations involving both spin and charge degrees of freedom. These include the heavy-mass modes arising from the Doucot-Wen gauge potentials. (ii) As U is increased, some level crossings take place at a crossover region of U approximately 10t in which low-lying spin- and charge-excitation bands become separated and the antiferromagnetic spin correlation becomes significant. (iii) For systems doped with more than one hole, the heavy-mass modes split into branches.