In the quest to increase the critical temperature T c of cuprate superconductors, it is essential to identify the factors that limit the strength of superconductivity. The upper critical field H c2 is a fundamental measure of that strength, yet there is no agreement on its magnitude and doping dependence in cuprate superconductors. Here we show that the thermal conductivity can be used to directly detect H c2 in the cuprates YBa 2 Cu 3 O y , YBa 2 Cu 4 O 8 and Tl 2 Ba 2 CuO 6 þ d , allowing us to map out H c2 across the doping phase diagram. It exhibits two peaks, each located at a critical point where the Fermi surface of YBa 2 Cu 3 O y is known to undergo a transformation. Below the higher critical point, the condensation energy, obtained directly from H c2 , suffers a sudden 20-fold collapse. This reveals that phase competitionassociated with Fermi-surface reconstruction and charge-density-wave order-is a key limiting factor in the superconductivity of cuprates.

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 ).

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.

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.