Sudden reversal in the pressure dependence of Tc in the iron-based superconductor KFe2As2

Proximity to an antiferromagnetic phase suggests that pairing in iron-based superconductors is mediated by spin fluctuations1–4, but orbital fluctuations have also been invoked5. The former typically favour a pairing state of ex-tended s-wave symmetry with a gap that changes sign between electron and hole Fermi surfaces6–9 (s±), whereas the latter yield a standard s-wave state without sign change5 (s++). Here we show that applying pressure to KFe2As2 induces a sudden change in the critical temperature Tc, from an initial decrease with pressure to an increase above a critical pressure Pc. The smooth evolution of the resistivity and Hall coefficient through Pc rules out a change in the Fermi surface. We infer that there must be a change of pairing symmetry at Pc. Below Pc, there is compelling evidence for a d-wave state10–14. Above Pc, the high sensitivity to disorder rules out an s++ state. Given the nea