The Pseudogap Challenge: Understanding the ab-Plane AC Conductivity below Tc

Abstract

We establish that the cuprate pseudogap plays a crucial role in the $ab$-plane optical conductivity $σ(ω,T)$ for temperatures $T \le T_c$. The pseudogap signatures in $σ(ω,T)$ associated with competing proposals of a ``hidden order'' and ``superconducting'' origin for the $d_{x^2 - y^2}$ pseudogap are found to differ in their qualitative $ω,T$ dependences. For the latter case, as $T$ increases from 0, excess low $ω$ weight appears; moreover, a much wider range of $ω$ contributes to form the condensate. We discuss these theories in light of current experiments.