Field dependence of thermal conductivity in \textit{X}Fe$_2$As$_2$ (\textit{X} = K, Rb, Cs)

There is ongoing debate over the pairing symmetry in the hole-overdoped iron-based superconductor KFe$_2$As$_2$. While thermal conductivity [1] and penetration depth [2] data have been taken as evidence of a d-wave pairing state, heat capacity [3] and ARPES [4] have been interpreted within an s-wave state with accidental nodes on some parts of the Fermi surface. Here we report a complete study of the magnetic field dependence of thermal conductivity in the T = 0 limit for the isostructural materials XFe$_2$As$_2$ with X = K, Rb and Cs. Extending our previous study of KFe$_2$As$_2$ to RbFe$_2$As$_2$ and CsFe$_2$As$_2$ reveals a universal behaviour, implying that all three materials must have a very similar nodal structure. All data are found to be in excellent agreement with calculations for a d-wave superconductor. A similar nodal quasiparticle behaviour across different materials is natural within a d-wave state, a common nodal structure being automatically imposed by symmetry. By contrast, such similarity would be highly coincidental if nodes are accidental, as in an s-wave state. \\[4pt] [1] J.-Ph. Reid et al., PRL 109, 087001 (2012).\\[0pt] [2] H. Kim et al., PRB 89, 174519 (2014).\\[0pt] [3] F. Hardy et al., JPSJ 83, 014711 (2014).\\[0pt] [4] K. Okazaki et al., Science 337, 1314 (2012).