Gapless Superconductivity From Extremely Dilute Magnetic Disorder in 2H-NbSe_2-xS_x.

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- <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics> <mrow><msub><mi>NbSe</mi> <mrow><mn>2</mn> <mo>-</mo> <mi>x</mi></mrow> </msub> <mi>S</mi></mrow> <annotation>${\rm NbSe}_{2-x}{\rm S}$</annotation></semantics> </math> <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msub><mrow></mrow> <mi>x</mi></msub> <annotation>$_x$</annotation></semantics> </math> 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 <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><mrow><mi>x</mi> <mo>></mo> <mn>0</mn></mrow> <annotation>$x>0$</annotation></semantics> </math> , in stark contrast to the dominant charge density wave interactions in pure 2H- <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msub><mi>NbSe</mi> <mn>2</mn></msub> <annotation>${\rm NbSe}_2$</annotation></semantics> </math> . 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.