Plasmons in high-temperature cuprate superconductors above Tc.

Abstract

In an attempt to understand the density-density response of the cuprate superconductors, we study plasmons in a layered strange metal using the Gubser-Rocha model. The latter is a well-known bottom-up holographic model for a strange metal that is used here to describe the strongly repulsive on-site interactions between the electrons in each copper-oxide layer, whereas the long-range Coulomb interactions are incorporated by a so-called double-trace deformation. To be able to model the bilayer cuprates more realistically, we consider in particular the case of two closely-spaced copper-oxide layers per unit cell. In the response we then obtain for vanishing out-of-plane momentum both an optical and an acoustic plasmon, whereas for nonvanishing out-of-plane momentum there are two acoustic plasmon modes. We present the full density-density spectral functions with parameters typical for cuprates and discuss both the dispersion and the lifetime of these plasmon excitations. We also compute the conductivity after introducing disorder into the system and introduce the loss function to facilitate a comparison with experimental results. Finally, we also treat the effects of Umklapp scattering and inter-layer hopping on the density-density response.