Characterization of an Electronic Excitation on a Peierls–Hubbard Hamiltonian for a Small Cluster

Abstract

We have studied the behavior of the first electronic-like excitation in a three-site Peierls–Hubbard Hamiltonian as a function of the electron-lattice coupling. This excitation corresponds to a pure electronic excitation in the absence of electron-lattice coupling. For values of electronic and phononic parameters typical of copper-oxide superconductors, this excitation is far above the first phononic excitation energies. As the electron-lattice coupling increases this first electronic-like excitation shows a decrease in energy with respect to the ground state. In contrast to the first excited state of this model, which corresponds to a polaronic excitation, the energy of this excitation does not converge to zero for strong coupling values. Based on this behavior and the projection of the corresponding state into a basis with defined charge occupation numbers, we have interpreted this result as evidence of partial charge localization within the three-site cluster. Unlike a ferroelectric state, in this model we do not observe complete charge localization even when the lattice distortion, associated with the polaronic excitation, becomes static.