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 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 KFe2As2 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).