Sudden reversal in the pressure dependence of Tc in the iron-based superconductor CsFe2As2: A possible link between inelastic scattering and pairing symmetry

We report a sudden reversal in the pressure dependence of T-c in the iron-based superconductor CsFe2As2, similar to that discovered recently in KFe2As2 [Tafti et al., Nat. Phys. 9, 349 (2013)]. As in KFe2As2, we observe no change in the Hall coefficient at T -> 0, again ruling out a Lifshitz transition across the critical pressure P-c. We interpret the T-c reversal in the two materials as a phase transition from one pairing state to another, tuned by pressure, and we investigate which parameters control this transition. Comparing samples of different residual resistivity rho(0), we find that a sixfold increase in impurity scattering does not shift P-c. From a study of x-ray diffraction on KFe2As2 under pressure, we report the pressure dependence of lattice constants and As-Fe-As bond angle. The pressure dependence of the various lattice parameters suggests that P-c should be significantly higher in CsFe2As2 than in KFe2As2, but we find on the contrary that P-c is lower in CsFe2As2, indicating that other factors control T-c. Resistivity measurements under pressure reveal a change of regime across P-c, suggesting a possible link between inelastic scattering and pairing symmetry.