Quasiparticle interference in multiband superconductors with strong coupling

TL;DR

This paper develops a strong-coupling Eliashberg theory to analyze quasiparticle interference in multiband superconductors, identifying universal features that distinguish between different superconducting order parameters.

Contribution

It introduces a comprehensive strong-coupling framework for QPI in multiband superconductors and highlights a universal signature near the smallest gap for identifying order parameter symmetry.

Findings

Three singularities in the QPI response near interband momentum transfer.

The smallest gap singularity can distinguish between s++ and s± pairing.

The response peak near the smaller gap is robust across parameter variations.

Abstract

We develop a theory of the quasiparticle interference (QPI) in multiband superconductors based on strong-coupling Eliashberg approach within the Born approximation. In the framework of this theory, we study dependencies of the QPI response function in the multiband superconductors with nodeless s-wave superconductive order parameter. We pay a special attention to the difference of the quasiparticle scattering between the bands having the same and opposite signs of the order parameter. We show that, at the momentum values close to the momentum transfer between two bands, the energy dependence of the quasiparticle interference response function has three singularities. Two of these correspond to the values of the gap functions and the third one depends on both the gaps and the transfer momentum. We argue that only the singularity near the smallest band gap may be used as an universal tool to distinguish between $s_{++}$ and $s_{\pm}$ order parameters. The robustness of the sign of the response function peak near the smaller gap value, irrespective of the change in parameters, in both the symmetry cases is a promising feature that can be harnessed experimentally.