The in-plane thermal conductivity of iron-based superconductor ${\mathrm{RbFe}}_2{\mathrm{As}}_2$ single crystal $(T_c\approx$ 2.1 K) was measured down to 100 mK. In zero field, the observation of a significant residual linear term ${\kappa }_0/T$ = 0.65 mW ${\mathrm{K}}^{-2}$ ${\mathrm{cm}}^{-1}$ provides clear evidence for nodal superdonducting gap. The field dependence of ${\kappa }_0/T$ is similar to that of its sister compound ${\mathrm{CsFe}}_2{\mathrm{As}}_2$ with comparable residual resistivity ${\rho }_0$ and lies between the dirty and clean ${\mathrm{KFe}}_2{\mathrm{As}}_2$. These results suggest that the (K, Rb, Cs)${\mathrm{Fe}}_2{\mathrm{As}}_2$ serial superconductors have a common nodal gap structure.

• Received 5 December 2014
• Revised 21 December 2014

DOI:https://doi.org/10.1103/PhysRevB.91.024502