Gapless Superconductivity From Extremely Dilute Magnetic Disorder in 2H‐NbSe _{2‐ <i>x</i>} S _<i>x</i>

Abstract

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‐ 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 , in stark contrast to the dominant charge density wave interactions in pure 2H‐. 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.