Synthesis and properties of 12442-family superconductor

We report a synthesis of two members of recently discovered high-temperature superconductors of 12442 family, with formula MCa$_2$Fe$_4$As$_4$F$_2$ (M = Rb, K) and transition temperatures of 32.7 and 34.6 K, respectively. Quality of the samples was assessed using X-ray powder diffraction, superconducting transitions were identified through transport and magnetic experiments. The temperature dependence of the upper critical field and vortex activation energy was investigated under magnetic fields up to 19 T. Two distinct thermally activated flux flow regimes were observed in both systems. Field dependences of activation energy $U_0(H)$ indicate a change in the properties of vortex matter in these regimes and distinctly different dissipation mechanisms, reminiscent of cuprate HTSC.