High-temperature superconductivity in ${\text{Eu}}_{0.5}{\text{K}}_{0.5}{\text{Fe}}_2{\text{As}}_2$

${\text{EuFe}}_2{\text{As}}_2$ shows a spin-density wave (SDW) type transition at 190 K and antiferromagnetic (AF) order below 20 K. Here, we have studied the effect of K substitution on the SDW transition at high temperature and AF Eu order at low temperature. 50% K substitution suppresses the SDW transition and in turn gives rise to high-temperature superconductivity below $T_c=32\text{K}$, as observed in the electrical resistivity, AC susceptibility, as well as magnetization. A well defined anomaly in the specific heat provides additional evidence for bulk superconductivity. Below 10 K, short-range magnetic order of the Eu moments is suggested by a broad feature in the specific-heat data. Electronic structure calculations reveal very close similarity with the nonmagnetic superconductor ${\text{Sr}}_{0.5}{\text{K}}_{0.5}{\text{Fe}}_2{\text{As}}_2$, but yield localized $4f$ magnetic moments for the remaining Eu atoms.

Figure 1

Temperature dependence of the electrical resistivity of polycrystalline ${\text{EuFe}}_2{\text{As}}_2$ and ${\text{Eu}}_{0.5}{\text{K}}_{0.5}{\text{Fe}}_2{\text{As}}_2$. The arrows indicate anomalies at $T_{\text{SDW}}=190\text{K}$ and $T_c=30\text{K}$. The lower inset shows the electrical resistivity data in temperature range between 20 and 60 K and the line indicates the linear decrease between 60 and 32 K.

Figure 2

Temperature dependence of ${\text{Eu}}_{0.5}{\text{K}}_{0.5}{\text{Fe}}_2{\text{As}}_2$. (a) dc magnetic susceptibility for zero-field-cooled and field-cooled experiments in an applied field of 50 Oe and (b) the real part of the ac susceptibility measured (background subtracted) with oscillating frequency of 117 Hz.

Figure 3

Temperature dependence of the specific heat of ${\text{Eu}}_{0.5}{\text{K}}_{0.5}{\text{Fe}}_2{\text{As}}_2$ in the temperature range $4\text{K}\le T\le 40\text{K}$. The inset displays $C/T$ vs $T^2$.

Figure 4

(Color online) Comparison of the total DOS for ${\text{Eu}}_{0.5}{\text{K}}_{0.5}{\text{Fe}}_2{\text{As}}_2$ ($\text{LSDA}+U$, with nonmagnetic Fe sites) and ${\text{Sr}}_{0.5}{\text{K}}_{0.5}{\text{Fe}}_2{\text{As}}_2$ (LDA). The spin-up and spin-down DOS with nonmagnetic Fe sites for the ${\text{Eu}}_{0.5}{\text{K}}_{0.5}{\text{Fe}}_2{\text{As}}_2$ have been added together to make the comparison with the Sr analog easier. The large peak at $-2.4\text{eV}$ belongs to the spin-up $\text{Eu}4f$ states. The unfilled spin-down $\text{Eu}4f$ states are pushed to around 9.5 eV above the Fermi level.