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Among the iron-based pnictide superconductors the material KFe$_2$As$_2$ isunusual in that its Fermi surface does not consist of quasi-nested electron andhole pockets. Here we report measurements of the temperature dependent Londonpenetration depth of very clean crystals of this compound with residualresistivity ratio $>1200$. We show that the superfluid density at lowtemperatures exhibits a strong linear-in-temperature dependence which impliesthat there are line nodes in the energy gap on the large zone-centered holesheets. The results indicate that KFe$_2$As$_2$ is an unconventionalsuperconductor with strong electron correlations.

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