Probing the pairing symmetry in the over-doped Fe-based superconductor Ba0.35Rb0.65Fe2As2 as a function of hydrostatic pressure

Abstract

We report muon spin rotation experiments on the magnetic penetration depth λ and the temperature dependence of λ-2 in the over-doped Fe-based high-temperature superconductor (Fe-HTS) Ba1-xRbxFe2As2 (x = 0.65) studied at ambient and under hydrostatic pressures up to p=2.3 GPa. We find that in this system λ-2(T) is best described by d-wave scenario. This is in contrast to the case of the optimally doped x=0.35 system which is known to be a nodeless s+ - wave superconductor. This suggests that the doping induces the change of the pairing symmetry from s+- to d wave in Ba1-xRbxFe2As2. In addition, we find that the d-wave order parameter is robust against pressure, suggesting that d is the common and dominant pairing symmetry in over-doped Ba1-xRbxFe2As2. Application of pressure of p=2.3 GPa causes a decrease of λ(0) by less than 5%, while at optimal doping x=0.35 a significant decrease of λ(0) was reported. The superconducting transition temperature Tc as well as the gap to Tc ratio 2Δ/kBTc show only a modest decrease with pressure. By combining the present data with those previously obtained for optimally doped system x=0.35 and for the end member x = 1, we conclude that the SC gap symmetry as well as the pressure effects on the SC quantities strongly depend on the Rb doping level. These results are discussed in the light of the putative Lifshitz transition, i.e., a disappearance of the electron pockets in the Fermi surface of Ba1-xRbxFe2As2 upon hole doping.