Density Waves Cause Sub-Gap Structures but no Pseudogap in Superconducting Cuprates

TL;DR

This study uses mean-field models to explain sub-gap structures in cuprate superconductors, showing they are caused by density waves and not pseudogaps, highlighting the non-universal nature of these features.

Contribution

The paper introduces a mean-field modeling approach demonstrating how density waves create sub-gap structures without pseudogaps in cuprates.

Findings

Density waves cause sub-gap structures in STM conductance curves.

Sub-gap features vary with band structure and density wave wave vector.

Pseudogaps are not induced by density waves in the absence of nesting.

Abstract

In scanning tunneling microscopy (STM) conductance curves, the superconducting gap of cuprates is sometimes accompanied by small sub-gap 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 ($d$SC), \emph{d}-form factor density wave ($d$FF-DW), and extended s-wave pair density wave ($s'$PDW), we find agreement with this phenomenon, with $s'$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 sub-gap structures may not be a universal feature in cuprates. In the absence of nesting, non-superconducting results never show signs of pseudogap, even for large density waves 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 pre-existing pseudogap.