We report a sudden reversal in the pressure dependence of $T_{\mathrm{c}}$ in the iron-based superconductor ${\text{RbFe}}_2{\text{As}}_2$, at a critical pressure $P_{\mathrm{c}}$ $=11$ kbar. Combined with our prior results on ${\text{KFe}}_2{\text{As}}_2$ and ${\text{CsFe}}_2{\text{As}}_2$, we find a universal V-shaped phase diagram for $T_{\mathrm{c}}$ versus $P$ in these fully hole-doped 122 materials, when measured relative to the critical point $(P_{\mathrm{c}},T_{\mathrm{c}})$. From measurements of the upper critical field $H_{\mathrm{c}2}\left(T\right)$ under pressure in ${\text{KFe}}_2{\text{As}}_2$ and ${\text{RbFe}}_2{\text{As}}_2$, we observe the same twofold jump in $(1/T_{\mathrm{c}}){\left(-\partial H_{\text{c}2}/\partial T\right)}_{T_{\mathrm{c}}}$ across $P_{\mathrm{c}}$, compelling evidence for a sudden change in the structure of the superconducting gap. We argue that this change is due to a transition from one pairing state to another, with different symmetries on either side of $P_{\mathrm{c}}$. We discuss a possible link between scattering and pairing, and a scenario where a $d$-wave state favored by high-$Q$ scattering at low pressure changes to a state with $s_{\pm }$ symmetry favored by low-$Q$ scattering at high pressure.

• Received 18 December 2014
• Revised 23 January 2015

DOI:https://doi.org/10.1103/PhysRevB.91.054511