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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

We report superconductivity in the ternary half-Heusler compound LuPtBi, with Tc = 1.0 K and Hc2 = 1.6 T. The crystal structure of LuPtBi lacks inversion symmetry, hence the material is a noncentrosymmetric superconductor. Magnetotransport data show semimetallic behavior in the normal state, which is evidence for the importance of spin-orbit interaction. Theoretical calculations indicate that the strong spin-orbit interaction in LuPtBi should cause strong band inversion, making this material a promising candidate for 3D topological superconductivity. 74.25.fc, 71.20.E, 71.30.+h I.

We present the first copper iridium binary metal oxide with the chemical formula CuIrO. The material is synthesized from the parent compound NaIrO by a topotactic reaction where sodium is exchanged with copper under mild conditions. CuIrO has the same monoclinic space group (C2/c) as NaIrO with a layered honeycomb structure. The parent compound NaIrO is proposed to be relevant to the Kitaev spin liquid on the basis of having Ir with an effective spin of 1/2 on a honeycomb lattice. Remarkably, whereas NaIrO shows a long-range magnetic order at 15 K and fails to become a true spin liquid, CuIrO remains disordered until 2.7 K, at which point a short-range order develops. Rietveld analysis shows less distortions in the honeycomb structure of CuIrO with bond angles closer to 120° compared to NaIrO. Thus, the weak short-range magnetism combined with the nearly ideal honeycomb structure places CuIrO closer to a Kitaev spin liquid than its predecessors.