Gap structure of the iron-based superconductor KFe$_2$As$_2$ via thermal conductivity

The thermal conductivity of the iron-based superconductor KFe$_2$As$_2$ was measured at temperatures down to 50 mK in magnetic fields up to 15 T, as a way to probe the superconducting gap structure. A large residual linear term in the $T=0$ limit is observed in zero field, showing that the gap structure contains nodes, consistent with a previous report [J.K. Dong et al., PRL 104, 087005 (2010)]. We discuss the possible interpretations for the nature of these nodes, in light of three different theoretical proposals: accidental line nodes in an extended $s$-wave state which are either horizontal [K. Suzuki et al., arXiv:1108.0657] or vertical [S. Maiti et al., arXiv:1111.0306], or symmetry-imposed vertical line nodes in a $d$-wave state [R. Thomale et al., PRL 107, 117001 (2011)].