Normal-State Conductivity of Cuprate Superconductors: One-Component and Two-Component Viewpoints

We discuss the evolution of our understanding of the normal state infrared conductivity of cuprate superconductors. The discussion begins with results from polycrystalline samples and continues up to the recent work on single domain (untwinned) crystals. The issue of whether the conductivity at low frequency (i.e. ω≲1000 cm−1) should be described with one (intraband) component, or with two (intraband + interband) components is central to the discussion.