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.

The structure of the superconducting gap provides important clues on the symmetry of the order parameter and the pairing mechanism. The presence of nodes in the gap function imposed by symmetry implies an unconventional order parameter, other than s-wave. Here we show how measurements of the thermal conductivity at very low temperature can be used to determine whether such nodes are present in a particular superconductor, and shed light on their nature and location. We focus on the residual linear term at T → 0. A finite value in zero magnetic field is strong evidence for symmetry-imposed nodes, and the dependence on impurity scattering can distinguish between a line of nodes or point nodes. Application of a magnetic field probes the low-energy quasiparticle excitations, whether associated with nodes or with a small value of the gap on some part of the Fermi surface, as in a multi-band superconductor. We frame our discussion around archetypal materials: Nb for s-wave, Tl 2 Ba 2 CuO 6+δ for d-wave, Sr 2 RuO 4 for p-wave, and NbSe 2 for multi-band superconductivity. In that framework, we discuss three heavy-fermion superconductors: CeIrIn 5 , CeCoIn 5 and UPt 3 .

The thermal conductivity of the iron-arsenide superconductor Ba 1−x K x Fe 2 As 2 ͑T c Ӎ 30 K͒ was measured in single crystals at temperatures down to T Ӎ 50 mK͑ӍT c / 600͒ and in magnetic fields up to H =15 T͑ӍH c2 / 4͒. A negligible residual linear term in / T as T → 0 shows that there are no zero-energy quasiparticles in the superconducting state. This rules out the existence of line and in-plane point nodes in the superconducting gap, imposing strong constraints on the symmetry of the order parameter. It excludes d-wave symmetry, drawing a clear distinction between these superconductors and the high-T c cuprates. However, the fact that a magnetic field much smaller than H c2 can induce a residual linear term indicates that the gap must be very small on part of the Fermi surface, whether from strong anisotropy or band dependence, or both.

Abstract. The nature of the pairing state in iron-based superconductors is the subject of much debate. Here we argue that in one material, the stoichiometric iron pnictide KFe 2 As 2 , there is overwhelming evidence for a d-wave pairing state, characterized by symmetry-imposed vertical line nodes in the superconducting gap. This evidence is reviewed, with a focus on thermal conductivity and the strong impact of impurity scattering on the critical temperature T c . We then compare KFe 2 As 2 to Ba 0.6 K 0.4 Fe 2 As 2 , obtained by Ba substitution, where the pairing symmetry is s-wave and the T c is ten times higher. The transition from d-wave to s-wave within the same crystal structure provides a rare opportunity to investigate the connection between band structure and pairing mechanism. We also compare KFe 2 As 2 to the nodal ironbased superconductor LaFePO, for which the pairing symmetry is probably not d-wave, but more likely s-wave with accidental line nodes.