Phase diagram of pressure-induced superconductivity in EuFe${}_2$As${}_2$ probed by high-pressure resistivity up to 3.2 GPa

We have constructed a pressure-temperature ($P$-$T$) phase diagram of $P$-induced superconductivity in EuFe${}_2$As${}_2$ single crystals via resistivity ($\rho$) measurements up to 3.2 GPa. As hydrostatic pressure is applied, the temperature $T_0$ where an antiferromagnetic (AF) transition of the Fe moments and a structural phase transition occur shifts to lower temperatures, and the corresponding resistive anomaly becomes undetectable for $P⩾2.5$ GPa. This suggests that the critical pressure $P_c$ where $T_0$ becomes zero is about 2.5 GPa. We have found that the AF order of the Eu${}^{2+}$ moments survives up to 3.2 GPa, the highest pressure in the experiments, without significant changes in the AF ordering temperature $T_N$. The superconducting (SC) ground state with a sharp transition to zero resistivity at $T_c~30$ K, indicative of bulk superconductivity, emerges in a pressure range from $P_c~2.5$ to $~3.0$ GPa. At pressures close to but outside the SC phase, the $\rho (T)$ curve shows a partial SC transition (i.e., zero resistivity is not attained) followed by a reentrant-like hump at approximately $T_N$ with decreasing temperature. When nonhydrostatic pressure with a uniaxial-like strain component is applied using a solid pressure medium, the partial superconductivity is continuously observed in a wide pressure range from 1.1 to 3.2 GPa.

Figure 1

Simplified schematic views of assemblies for the high-pressure resistivity measurements: (a) samples ${}^{\#}$1 (${}^{\#}$3) and ${}^{\#}$2, and Manganin ${}^{\#}$a and ${}^{\#}$b; (b) samples ${}^{\#}$4 and ${}^{\#}$5, and Manganin ${}^{\#}$c. Samples ${}^{\#}$1–${}^{\#}$3 and all Manganin wires are immersed in Daphne 7474 pressure medium, whereas samples ${}^{\#}$4 and ${}^{\#}$5 are fixed by cured Stycast 1266. An external load is applied parallel to the longitudinal axis of a pressure cell as indicated by arrows.

Figure 2

(a) $\rho$ vs $T$ of EuFe${}_2$As${}_2$ (sample ${}^{\#}$1; Daphne 7474) under several pressures up to 2.6 GPa. The anomaly at $T_0$ is due to an AF transition of the Fe moments and a structural phase transition. $T_N$ is an AF ordering temperature of the Eu${}^{2+}$ moments. (b) $d\rho /\mathit{dT}$ vs $T$ for $P⩽2.4$ GPa. Arrows indicate $T_0$ where $d\rho /\mathit{dT}$ exhibits its minimum. (c) Expanded view of $\rho$ vs $T$. $T_c^{\mathrm{on}}$ at 1.9 GPa indicates an onset temperature of the SC transition. The data in (b) and (c) are vertically shifted for clarity.

Figure 3

(a) Low-temperature expanded view of $\rho (T)$ data for EuFe${}_2$As${}_2$ (sample ${}^{\#}$3; Daphne 7474) under several pressures up to 3.2 GPa. As illustrated in the inset using the data at 2.9 GPa, $T_c^{\mathrm{on}}$ and $T_c^{\rho =0}$ are determined by the onset and $\rho =0$ of the SC transition. (b) $\rho (T)$ data at 2.5 GPa for several current values up to $I=200$ mA ($~500$ A/cm${}^2$). Shifts of SC transitions and resistive humps to lower temperatures as current increases may not be intrinsic, and are most likely attributed to a joule heating effect.

Figure 4

$P$-$T$ phase diagram of $T_0$, $T_N$, $T_c$, and $\Delta T_c$ ($=T_c^{\mathrm{on}}-T_c^{\rho =0}$) in EuFe${}_2$As${}_2$ (samples ${}^{\#}$1–${}^{\#}$3; Daphne 7474), deduced from the $\rho (T)$ data up to 3.2 GPa. PM, AFM, and SC indicate the paramagnetic, antiferromagnetic, and superconducting states, respectively. For the SC state, open and solid symbols indicate $T_c^{\mathrm{on}}$ and $T_c^{\rho =0}$, respectively. As indicated by an arrow, the critical pressure ($P_c$), where $T_0\to 0$, is about 2.5 GPa. Dashed curves are eye guides. For comparison, data from Refs. 8 and 13 are also shown.

Figure 5

Comparison of $\rho (T)$ data of EuFe${}_2$As${}_2$ for samples ${}^{\#}$1 (open circles) and ${}^{\#}$2 (dotted curves), simultaneously measured using Daphne 7474 pressure medium. The data are arbitrarily shifted in the longitudinal direction for clarity. The inset displays a resistance change ratio $\Delta R$ (left axis) and the corresponding pressure value (right axis) obtained at 4.2 K for Manganin ${}^{\#}$a and ${}^{\#}$b as a function of the externally applied load. Sample ${}^{\#}$1 and Manganin ${}^{\#}$a are aligned parallel to the load axis, and sample ${}^{\#}$2 and Manganin ${}^{\#}$b are in the perpendicular direction, as shown in Fig. 1a.

Figure 6

$\rho$ vs $T$ of EuFe${}_2$As${}_2$ at several pressures up to 3.2 GPa for samples ${}^{\#}$4 (a) and ${}^{\#}$5 (b), simultaneously measured using a Stycast 1266 pressure medium. Samples ${}^{\#}$4 and ${}^{\#}$5 are aligned parallel and perpendicular, respectively, to the load direction as in Fig. 1e. Figures (c) and (d) show the low-temperature expanded views for samples ${}^{\#}$4 and ${}^{\#}$5, respectively. (e) Pressure evolutions of $T_N$ and $T_c^{\mathrm{on}}$ for samples ${}^{\#}$4 ($▵$) and ${}^{\#}$5 ($\square$). For comparison, $T_N$ ($\blacksquare$) and $T_c^{\mathrm{on}}$ ($◯$) obtained using Daphne 7474 pressure medium for samples ${}^{\#}$1 and ${}^{\#}$3 are also shown. A shaded area represents a SC phase where the resistivity is zero under hydrostatic pressure.