The thermal conductivity κ 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 at T → 0, κ0/T , is observed for both current directions, confirming the presence of nodes in the superconducting gap. Our value of κ0/T in the plane is equal to that reported by Dong et al. [Phys. Rev. Lett. 104, 087005 (2010)] for a sample whose residual resistivity ρ0 was ten times larger. This independence of κ0/T 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 Γc for suppressing Tc to zero; the magnitude of κ0/T , and its dependence on current direction and on magnetic field; the temperature dependence of κ(T ).

The temperature and magnetic field dependence of the in-plane thermal conductivity κ of the ironarsenide superconductor Ba(Fe1−xCox)2As2 was measured down to T ≃ 50 mK and up to H = 15 T as a function of Co concentration x in the range 0.048 ≤ x ≤ 0.114. In zero magnetic field, a negligible residual linear term in κ/T as T → 0 at all x shows that there are no zero-energy quasiparticles and hence the superconducting gap has no nodes in the ab-plane anywhere in the phase diagram. However, the field dependence of κ reveals a systematic evolution of the superconducting gap with doping x, from large everywhere on the Fermi surface in the underdoped regime, as evidenced by a flat κ(H) at T → 0, to strongly k-dependent in the overdoped regime, where a small magnetic field can induce a large residual linear term, indicative of a deep minimum in the gap magnitude somewhere on the Fermi surface. This shows that the superconducting gap structure has a strongly k-dependent amplitude around the Fermi surface only outside the antiferromagnetic/orthorhombic phase.

Nodes in the gap structure of the iron-arsenide superconductor Ba(Fe 1−x Co x ) 2 As 2 from c-axis heat transport measurements The thermal conductivity κ of the iron-arsenide superconductor Ba(Fe1−xCox)2As2 was measured down to 50 mK for a heat current parallel (κc) and perpendicular (κa) to the tetragonal c axis, for seven Co concentrations from underdoped to overdoped regions of the phase diagram (0.038 ≤ x ≤ 0.127). A residual linear term κc0/T is observed in the T → 0 limit when the current is along the c axis, revealing the presence of nodes in the gap. Because the nodes appear as x moves away from the concentration of maximal Tc, they must be accidental, not imposed by symmetry, and are therefore compatible with an s± state, for example. The fact that the in-plane residual linear term κa0/T is negligible at all x implies that the nodes are located in regions of the Fermi surface that contribute strongly to c-axis conduction and very little to in-plane conduction. Application of a moderate magnetic field (e.g. Hc2/4) excites quasiparticles that conduct heat along the a axis just as well as the nodal quasiparticles conduct along the c axis. This shows that the gap must be very small (but non-zero) in regions of the Fermi surface which contribute significantly to in-plane conduction. These findings can be understood in terms of a strong k dependence of the gap ∆(k) which produces nodes on a Fermi surface sheet with pronounced c-axis dispersion and deep minima on the remaining, quasi-two-dimensional sheets.

Proximity to an antiferromagnetic phase suggests that pairing in iron-based superconductors is mediated by spin fluctuations 1-4 , but orbital fluctuations have also been invoked 5. The former typically favour a pairing state of extended s-wave symmetry with a gap that changes sign between electron and hole Fermi surfaces 6-9 (s ±), whereas the latter yield a standard s-wave state without sign change 5 (s ++). Here we show that applying pressure to KFe 2 As 2 induces a sudden change in the critical temperature T c , from an initial decrease with pressure to an increase above a critical pressure P c. The smooth evolution of the resistivity and Hall coefficient through P c rules out a change in the Fermi surface. We infer that there must be a change of pairing symmetry at P c. Below P c , there is compelling evidence for a d-wave state 10-14. Above P c , the high sensitivity to disorder rules out an s ++ state. Given the near degeneracy of d-wave and s ± states found theoretically 15-19 , we propose an s ± state above P c. A change from d-wave to s-wave would probably proceed through an intermediate s + id state that breaks time-reversal symmetry 20-22. KFe 2 As 2 is a stoichiometric iron arsenide with a superconducting critical temperature T c = 4 K. It is a member of the extensively studied 122 family of iron-based superconductors 23. Single crystals can be grown with very high purity, making it by far the cleanest of the iron-based superconductors. Its high hole concentration is such that its Fermi surface does not contain the usual electron pocket at the X point (of the unfolded Brillouin zone); it consists mainly of three hole-like cylinders: two located at the zone centre () and one at the corner (M; Fig. 1a). There is no antiferromagnetic order, but there are antiferromagnetic spin fluctuations, detected by inelastic neutron scattering 24. In iron-based superconductors, spin fluctuations generally favour the s ± pairing state in which the gap changes sign between hole and electron pockets 1-4 (Fig. 1b). In the absence of the electron pocket at X, this mechanism becomes much less effective, and functional-renormalization-group calculations find that a d-wave state (Fig. 1c) is the most stable state in KFe 2 As 2 (ref. 15). Other theoretical methods find that s ± and d-wave states are very close in energy 17,18. Experimentally, thermal conductivity studies in KFe 2 As 2 make a compelling case for d-wave symmetry 10-13 : line nodes are found to be vertical and present on all Fermi surfaces, and the thermal conductivity is independent of impurity scattering, as expected of symmetry-imposed line nodes 25. A d-wave state is also consistent with penetration depth data 14. However, in a recent angle-resolved photoemission spectroscopy (ARPES) study of KFe 2 As 2 , vertical line nodes in the gap were