Mechanism for Nodal Topological Superconductivity on PtBi$_2$ Surface

TL;DR

This paper proposes a microscopic mechanism involving anisotropic electron-phonon coupling and Coulomb interactions that explains nodal topological superconductivity observed on the surface of PtBi$_2$, a Weyl semimetal.

Contribution

It introduces a detailed theoretical model for nodal pairing in topological surface states, predicting conditions for nodeless gaps and higher critical temperatures.

Findings

Anisotropic electron-phonon coupling can produce nodal gaps in PtBi$_2$ surface states.

Screening Coulomb interactions can suppress nodal gaps, leading to nodeless superconductivity.

Enhanced Coulomb screening may increase the critical temperature of surface superconductivity.

Abstract

Experiments show that the Weyl semimetal PtBi$_2$ hosts unconventional superconductivity in its topological surface states. Hence, the material is a candidate for intrinsic topological superconductivity. Measurements indicate nodal gaps in the center of the Fermi arcs. We derive that anisotropic electron-phonon coupling on Weyl semimetal surfaces, combined with statically screened Coulomb repulsion, is a microscopic mechanism for this nodal pairing. The dominant solution of the linearized gap equation shows nodal gaps when the surface state bandwidth is comparable to the maximum phonon energy, as is the case in PtBi$_2$. We further predict that if the screening of Coulomb interaction on the surface is enhanced by Coulomb engineering, the superconducting gap becomes nodeless, and the critical temperature increases.