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

Different probes have found different superconducting pairing states in different iron-based high-temperature superconductors. Now transport measurements suggest that pressure drives the superconducting state in KFe 2 As 2 from d -wave to s -wave. Proximity to an antiferromagnetic phase suggests that pairing in iron-based superconductors is mediated by spin fluctuations 1 , 2 , 3 , 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 , 7 , 8 , 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 , 11 , 12 , 13 , 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 , 16 , 17 , 18 , 19 , we propose an s ± state above P c . A change from d -wave to s -wave would probably proceed through an intermediate s + i d state that breaks time-reversal symmetry 20 , 21 , 22 .