Sudden reversal in the pressure dependence of T_c in the iron-based superconductor CsFe_2As_2: A possible link between inelastic scattering and pairing symmetry

摘要

We report a sudden reversal in the pressure dependence of T_c in the iron-based superconductor CsFe_2As_2, similar to that discovered recently in KFe_2As_2 [Tafti et al., Nat. Phys. 9, 349 (2013)]. As in KFe_2As_2, 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 ρ_0, we find that a sixfold increase in impurity scattering does not shift P_c. From a study of x-ray diffraction on KFe_2As_2 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 CsFe_2As_2 than in KFe_2As_2, but we find on the contrary that P_c is lower in CsFe_2As_2, 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.