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-Tc materials. Then we consider the real part of the finite-frequency conductivity, σ₁({ω}). In the attractive-U case, we find an unexpected feature in σ₁({ω}) associated with an unpaired electron moving through a background of bound pairs.
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Fye et al. (1992) studied this question.
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