Subgap structures and pseudogap in cuprate superconductors: Role of density waves

Abstract

In scanning tunneling microscopy conductance curves, the superconducting gap of cuprates is sometimes accompanied by small subgap structures at very low energy. This was documented early on near vortex cores and later at zero magnetic field. Using mean-field toy models of coexisting $d$-wave superconductivity, $d$-form-factor density wave, and extended $s$-wave pair density wave (${s}^{\ensuremath{'}}\mathrm{PDW}$), we find agreement with this phenomenon, with ${s}^{\ensuremath{'}}\mathrm{PDW}$ playing a critical role. We explore the high variability of the gap structure with changes in band structure and density wave (DW) wave vector, thus explaining why subgap structures may not be a universal feature in cuprates. In the absence of nesting, nonsuperconducting results never show signs of pseudogap, even for large density wave magnitudes, therefore reinforcing the idea of a distinct origin for the pseudogap, beyond mean-field theory. Therefore, we also briefly consider the effect of DWs on a preexisting pseudogap.