A Pre-Formed Pair Approach to the Dynamical (THz) Complex Conductivity in the Cuprates

In this talk we present a theory for $\sigma(\omega)=\sigma_1(\omega)+i\sigma_2(\omega)$ in the underdoped cuprates. Our work presumes that the pseudogap is associated with preformed pairs. We demonstrate how the puzzling extended range of finite $\sigma_2(\omega)$ above $T_c$ (implying a ``dynamical superfluid density'') arises from a new form of pair breaking contribution to $\sigma_2(\omega)$. This is only present in these moderately clean superconductors because of stronger than BCS attraction (BCS-BEC). Sum rule compatibility and good semiquantitative agreement with experiment is found.