Unique phase diagram with narrow superconducting dome in EuFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ due to Eu${}^{2+}$ local magnetic moments

The interplay between superconductivity and Eu${}^{2+}$ magnetic moments in EuFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ is studied with electrical resistivity measurements under hydrostatic pressure on $x=0.13$ and $x=0.18$ single crystals. We can map hydrostatic pressure to chemical pressure $x$ and show that superconductivity is confined to a very narrow range $0.18\le x\le 0.23$ in the phase diagram, beyond which ferromagnetic (FM) Eu ordering suppresses superconductivity. The change from antiferro- to FM Eu ordering at the latter concentration coincides with a Lifshitz transition and the complete depression of iron magnetic order.

Figure 1

Electrical resistivity normalized by its value at 300 K, $\rho /{\rho }_{300\mathrm{K}}$, for EuFe${}_2$(As${}_{0.87}$P${}_{0.13}$)${}_2$ under hydrostatic pressure. The inset displays the temperature derivative of the resistivity $d\rho /dT$ vs $T$. Arrows indicate $T_s$ and $T_{\mathrm{SDW}}$ which are determined from the inflection points and minima, respectively.

Figure 2

Electrical resistivity normalized by its value at 250 K, $\rho /{\rho }_{250\mathrm{K}}$, for EuFe${}_2$(As${}_{0.82}$P${}_{0.18}$)${}_2$ under hydrostatic pressure. The two insets enlarge the low-temperature region.

Figure 3

Phase diagram of EuFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$, including data points from the previous ambient-pressure work (black symbols) and this study (red symbols). The effective P concentration $x$ for the data points from this study has been obtained by using the bulk modulus $B=82.9\pm 1.4$ GPa (Ref. 25) and the lattice constants of EuFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ (Ref. 11). Lines are guides to the eye. The inset shows the exponent $n$ of the temperature dependence of the electrical resistivity $\rho ={\rho }_0+A{T}^n$ as a function of the calculated effective P concentration for EuFe${}_2$(As${}_{0.82}$P${}_{0.18}$)${}_2$ under hydrostatic pressure. It shares the horizontal axis for $x$ of the main panel. The exponent $n$ is obtained by fitting the data between 25 and 100 K.