OPTICAL-CONDUCTIVITY PROPERTIES OF ONE-DIMENSIONAL HUBBARD RINGS - REPULSIVE-U AND ATTRACTIVE-U CASES

We compare the optical-conductivity properties of one-dimensional repulsive-U and attractive-U Hubbard rings. First we determine the Drude weight, or "charge stiffness," D as a function of band filling and the ratio U/t, where t is the overlap "hopping" integral. We note similarities in the repulsive-U case to experimental data both from hole-doped and from electron-doped high-T(c) materials. Then we consider the real part of the finite-frequency conductivity, sigma(1)(omega). In the attractive-U case, we find an unexpected feature in sigma(1)(omega) associated with an unpaired electron moving through a background of bound pairs.