Full-gap superconductivity in spin-polarized surface states of topological semimetal $\beta$-PdBi$_2$

TL;DR

This study demonstrates that the superconductor $eta$-PdBi$_2$ exhibits full-gap superconductivity in its spin-polarized topological surface states, combining experimental and theoretical methods to explore its electronic and spin textures.

Contribution

It provides the first detailed experimental and theoretical analysis of spin textures and superconductivity in the topological surface states of $eta$-PdBi$_2$, revealing full-gap superconductivity in these states.

Findings

Topological surface states exhibit spin polarization parallel to the surface.

Superconducting gap fully opens in all surface states.

Surface states show behavior consistent with a dominant spin-singlet pairing.

Abstract

A bulk superconductor possessing a topological surface state at the Fermi level is a promising system to realize long-sought topological superconductivity. Although several candidate materials have been proposed, experimental demonstrations concurrently exploring spin textures and superconductivity at the surface have remained elusive. Here we perform spectroscopic-imaging scanning tunnelling microscopy on the centrosymmetric superconductor $\beta$-PdBi$_2$ that hosts a topological surface state. By combining first-principles electronic-structure calculations and quasiparticle interference experiments, we determine the spin textures at the surface, and show not only the topological surface state but also all other surface bands exhibit spin polarizations parallel to the surface. We find that the superconducting gap fully opens in all the spin-polarized surface states. This behaviour is consistent with a possible spin-triplet order parameter expected for such in-plane spin textures, but the observed superconducting gap amplitude is comparable to that of the bulk, suggesting that the spin-singlet component is predominant in $\beta$-PdBi$_2$.