  The thermal conductivity of the iron-arsenide superconductor KFe2As2 was
measured down to 50 mK for a heat current parallel and perpendicular to the
tetragonal c-axis. A residual linear term (RLT) at T=0 is observed for both
current directions, confirming the presence of nodes in the superconducting
gap. Our value of the RLT in the plane is equal to that reported by Dong et al.
[Phys. Rev. Lett. 104, 087005 (2010)] for a sample whose residual resistivity
was ten times larger. This independence of the RLT on impurity scattering is
the signature of universal heat transport, a property of superconducting states
with symmetry-imposed line nodes. This argues against an s-wave state with
accidental nodes. It favors instead a d-wave state, an assignment consistent
with five additional properties: the magnitude of the critical scattering rate
for suppressing Tc to zero; the magnitude of the RLT, and its dependence on
current direction and on magnetic field; the temperature dependence of the
thermal conductivity.
