Phase diagram as a function of doping level and pressure in Eu$_{1-x}$La$_x$Fe$_2$As$_2$ system

Abstract

We establish the phase diagram of Eu$_{1-x}$La$_x$Fe$_2$As$_2$ system as a function of doping level x and the pressure by measuring the resistivity and magnetic susceptibility. The pressure can suppress the spin density wave (SDW) and structural transition very efficiently, while enhance the antiferromagnetic (AFM) transition temperature T$_N$ of Eu$^{2+}$. The superconductivity coexists with SDW order at the low pressure, while always coexists with the Eu$^{2+}$ AFM order. The results suggests that Eu$^{2+}$ spin dynamics is disentangeld with superconducting (SC) pairing taken place in the two-dimensional \emph{Fe-As} plane, but it can strongly affect superconducting coherence along c-axis.