A new analysis of optical excitations in the Hubbard model

A new theoretical analysis of the conductivity tensor is developed by use o f the composite operator method for the two-dimensional Hubbard model. With the aid of the Ward-Takahashi identities which phrase the charge conservat ion law, the calculation of the current-current propagator is reduced hiera rchically to the current-charge and charge-charge ones. Besides, the fermio nic character of the propagating charges is preserved because of a constrai ning equation with the content of the Pauli principle which uniquely fixes the single-particle dynamics. (C) 1999 Elsevier Science B.V. All rights res erved.