Universal V-shaped phase diagram in the iron-based superconductors XFe$_2$As$_2$ (X = K, Rb, Cs)

Following the discovery of a sharp reversal in the dependence of T$_{c}$ on pressure in KFe$_{2}$As$_{2}$ [1] and CsFe$_{2}$As$_{2}$ [2], we report a similar behavior in RbFe$_{2}$As$_{2}$. The application of hydrostatic pressure initially decreases T$_{c}$ until a critical pressure P$_{c}$ = 11 kbar where T$_{c}$ suddenly starts to increase. For the three materials, we find a universal V-shaped temperature-pressure phase diagram, with identical slopes $\frac{dT_{c}}{dP}$ below and above P$_{c}$. Upon crossing P$_{c}$, the upper critical field H$_{c2}$ is observed to jump, by a similar factor for KFe$_{2}$As$_{2}$ and RbFe$_{2}$As$_{2}$. We interpret these universal features in terms of a pressure-induced change in the pairing state of these superconductors. [1] F. F. Tafti et al., Nature Physics 9, 349 (2013). [2] F. F. Tafti et al., Physical Review B 89, 134502 (2014).