The magnetic properties of iron-based superconductors $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($A=\mathrm{K}$, Cs, and Rb), which are characterized by the V-shaped dependence of the critical temperature ($T_{\mathrm{c}}$) on pressure ($P$), were studied by means of the muon spin rotation/relaxation technique. In all three systems studied the magnetism was found to appear for pressures slightly below the critical one ($P_{\mathrm{c}}$), i.e., at pressure where $T_{\mathrm{c}}\left(P\right)$ changes the slope. Rather than competing, magnetism and superconductivity in $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ are coexisting at the $P\gtrsim P_{\mathrm{c}}$ pressure region. Our results support the scenario of a transition from one pairing state to another, with different symmetries on either side of $P_{\mathrm{c}}$.

• Received 6 May 2020
• Accepted 18 September 2020

DOI:https://doi.org/10.1103/PhysRevB.102.140502