Gapless vortex bound states in superconducting topological semimetals

TL;DR

This paper reveals that vortex bound states in superconducting topological semimetals are universally gapless due to their topological nature, across various exotic fermion types, and are protected by topological mechanisms.

Contribution

It demonstrates the universal presence of gapless vortex bound states in superconducting topological semimetals, linking them to topological protection and Andreev reflection phenomena.

Findings

Gapless vortex bound states are universal in topological semimetals.

These states are topologically protected.

The states are related to Andreev reflection and modes in SNS junctions.

Abstract

We find that the vortex bound states in superconducting topological semimetals are gapless owing to topological massless excitations in their normal states. We demonstrate this universal result in a variety of semimetals including Dirac and Weyl semimetals, threefold degenerate spin-1 fermions, spin-3/2 Rarita-Schwinger-Weyl fermion semimetals and other exotic fermion semimetals. The formation of these gapless bound states are closely related to their Andreev specular reflection and propagating Andreev modes in $\pi$ phase superconductor-normal metal-superconductor (SNS) junctions. We further demonstrate that these gapless states are topologically protected and can be derived from a topological pumping process.