Gapless superconductivity from extremely dilute magnetic disorder in 2H-NbSe2-xSx

TL;DR

This study demonstrates that extremely dilute magnetic impurities in 2H-NbSe2-xSx induce gapless superconductivity, challenging traditional views on impurity effects and emphasizing the role of band structure modifications.

Contribution

It reveals that low concentrations of magnetic impurities can cause gapless superconductivity, highlighting the importance of band structure changes due to Se-S substitution.

Findings

Gapless superconductivity observed at low magnetic impurity levels.

Se-S substitution modifies band structure and influences in-gap scattering.

Material-specific band structure is crucial for understanding impurity effects.

Abstract

Most superconducting materials exhibit a vanishing density of states at the Fermi level and Anderson's theorem posits that the superconducting gap is robust against nonmagnetic disorder. Although dilute magnetic impurities lead to localized in-gap states, these states typically have no bearing on the material's bulk superconducting properties. However, numerous experiments reveal a finite density of states at the Fermi level in systems with an apparently negligible number of magnetic impurities. Here, using scanning tunneling microscopy and self-consistent Bogoliubov-de Gennes calculations, we find that gapless superconductivity emerges in 2H-NbSe2-xSx at remarkably low magnetic impurity concentrations. Furthermore, our density functional theory calculations and in-gap quasiparticle interference measurements demonstrate that the Se-S substitution significantly modifies the band structure. This modification favours nesting and dictates the in-gap scattering for x>0, in stark contrast to the dominant charge density wave interactions in pure 2H-NbSe2. Our findings reveal an unusual superconducting response to disorder and highlight the importance of incorporating material-specific band structures in the understanding of a superconductor's response to even very low concentrations of magnetic impurities.