Evidence for superconducting gap nodes in the zone-centered hole bands of KFe2As2 from magnetic penetration-depth measurements

TL;DR

This study reveals that KFe2As2 has line nodes in its superconducting gap, indicating unconventional superconductivity with strong electron correlations, based on precise magnetic penetration-depth measurements.

Contribution

First direct evidence of line nodes in the superconducting gap of KFe2As2 from high-quality penetration depth measurements.

Findings

Linear temperature dependence of superfluid density at low temperatures

Presence of line nodes in the energy gap on zone-centered hole sheets

KFe2As2 is an unconventional superconductor with strong electron correlations

Abstract

Among the iron-based pnictide superconductors the material KFe$_2$As$_2$ is unusual in that its Fermi surface does not consist of quasi-nested electron and hole pockets. Here we report measurements of the temperature dependent London penetration depth of very clean crystals of this compound with residual resistivity ratio $>1200$. We show that the superfluid density at low temperatures exhibits a strong linear-in-temperature dependence which implies that there are line nodes in the energy gap on the large zone-centered hole sheets. The results indicate that KFe$_2$As$_2$ is an unconventional superconductor with strong electron correlations.