FREQUENCY-DEPENDENT CONDUCTIVITY OF THE ONE-DIMENSIONAL HUBBARD-MODELAT STRONG-COUPLING

The one-dimensional Hubbard model is characterized by charge-spin sepa ration, but nevertheless in the doped system sigma(omega > 0) does not vanish due to a small remaining coupling between charge and spin degr ees of freedom. We derive an effective Hamiltonian and give in leading order (approximately J2) an analytic expression for sigma(omega) for the case of the Mott insulator doped with one hole. The complete frequ ency dependence in the strong-coupling limit of the Hubbard model and for the t-J model agrees with results from exact diagonalization. The limit sigma(omega --> 0) at T = 0 is found to be very different for th e two models, i.e., approximately omega3/2 in the strong-coupling limi t and approximately omega-1/2 for the t-J model.