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 successfully grew the high-quality single crystal of Rb0.78Fe2Se1.78, which shows sharp superconducting transition in magnetic susceptibility and electrical resistivity. Resistivity measurements show the onset superconducting transition (Tc) at 32.1 K and zero resistivity at 30 K. From the low-temperature iso-magnetic-field magnetoresistance, large upper critical field Hc2(0) has been estimated as high as 180 T for in-plane field and 59 T for out-of-plane field. The anisotropy H ab c2 (0)/H c c2 (0) is around 3.0, right lying between those observed in KxFe2Se2 and CsxFe2Se2.

The nature of the parent compound of a high-temperature superconductor (HTS) often plays a pivotal role in determining its superconductivity. The parent compounds of the cuprate HTSs are antiferromagnetically ordered Mott insulators, while those of the iron-pnictide HTSs are metals with spin-density-wave order. Here we report the electronic identification of two insulating parental phases and one semiconducting parental phase of the newly discovered family of K x Fe 2Ày Se 2 superconductors. The two insulating phases exhibit Mott-insulator-like signatures, and one of the insulating phases is even present in the superconducting and semiconducting K x Fe 2Ày Se 2 compounds. However, it is mesoscopically phaseseparated from the superconducting or semiconducting phase. Moreover, we find that both the superconducting and semiconducting phases are free of the magnetic and vacancy orders present in the insulating phases, and that the electronic structure of the superconducting phase could be developed by doping the semiconducting phase with electrons. The rich electronic properties discovered in these parental phases of the K x Fe 2Ày Se 2 superconductors provide the foundation for studying the anomalous behavior in this new class of iron-based superconductors.