Thermal conductivity study of KFe${}_2$As${}_2$ single crystal: clear evidence for unconventional superconducting gap with nodes

The in-plane resistivity $\rho$ and thermal conductivity $\kappa$ of extremely overdoped KFe${}_2$As${}_2$ ($T_c$ = 3.0 K) single crystal were studied. It is found that $\rho \sim T^{1.5}$ at low temperature, a typical non-Fermi liquid behavior of electrons scattered by antiferromagnetic spin fluctuations. In zero field, we observed a large residual linear term ${\kappa }_0\mathrm{/}T$, about one third of the normal-state value. In low magnetic fields, ${\kappa }_0\mathrm{/}T(H)$ increases very fast. Such a behavior of ${\kappa }_0\mathrm{/}T$ mimics the d-wave cuprate superconductors, therefore provides clear evidence for nodes in the superconducting gap of KFe${}_2$As${}_2$. Based on the Fermi surface topology of KFe${}_2$As${}_2$, it is believed that the dominant intraband pairing via antiferromagnetic spin fluctuations results in the unconventional superconducting gap with nodes.