The electronic structure of the reentrant superconductor $\mathrm{Eu}({\mathrm{Fe}}_{0.86}{\mathrm{Ir}}_{0.14}$)${}_2{\mathrm{As}}_2$ ($T_c=22$ K) with coexisting ferromagnetic order ($T_M=18$ K) is investigated using angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy. We study the in-plane and out-of-plane band dispersions and Fermi surface (FS) of $\mathrm{Eu}{\left({\mathrm{Fe}}_{0.86}{\mathrm{Ir}}_{0.14}\right)}_2{\mathrm{As}}_2$. The near-$E_F$ Fe-$3d$-derived band dispersions near the $\mathrm{\Gamma }$ and $X$ high-symmetry points show changes due to Ir substitution, but the FS topology is preserved. From momentum-dependent measurements of the superconducting gap measured at $T=5$ K, we estimate an essentially isotropic $s$-wave gap ($\mathrm{\Delta }\sim 5.25\pm 0.25$ meV), indicative of strong-coupling superconductivity with $2\mathrm{\Delta }/k_B{T}_c\simeq 5.8$. The gap gets closed at temperatures $T\ge 10$ K, and this is attributed to the resistive phase which sets in at $T_M=18$ K due to the ${\mathrm{Eu}}^{2+}$-derived magnetic order. The modification of the FS with Ir substitution clearly indicates an effective hole doping with respect to the parent compound.

• Received 28 July 2017

DOI:https://doi.org/10.1103/PhysRevB.96.174513