Sudden reversal in the pressure dependence of T c in the iron-based superconductor KFe 2 As 2

Abstract

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. 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. 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, 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 . © 2013 Macmillan Publishers Limited. All rights reserved.