Pressure-Induced Superconducting State of Europium Metal at Low Temperatures

Divalent Eu ($4f^7$, $J=7/2$) possesses a strong local magnetic moment which suppresses superconductivity. Under sufficient pressure it is anticipated that Eu will become trivalent ($4f^6$, $J=0$) and a weak Van Vleck paramagnet, thus opening the door for a possible superconducting state, in analogy with Am metal ($5f^6$, $J=0$) which superconducts at 0.79 K. We present ac susceptibility and electrical resistivity measurements on Eu metal for temperatures 1.5–297 K to pressures as high as 142 GPa. At approximately 80 GPa Eu becomes superconducting at $T_c\simeq 1.8\mathrm{K}$; $T_c$ increases linearly with pressure to 2.75 K at 142 GPa. Eu metal thus becomes the 53rd known elemental superconductor in the periodic table.

Figure 1

Quasi-four-point electrical resistance measurements versus temperature for Eu metal at 37, 48, 61, 73, 81, 91, and 62 GPa, taken in that order. Inset shows data near 2 K for 73, 81, and 91 GPa on a highly expanded scale. $T_c$ is determined from the superconducting onset as the temperature is increased (see arrows).

Figure 2

Real part of the ac susceptibility versus temperature for Eu metal as pressure is increased from 76 to 142 GPa. The superconducting transition appears at 84 GPa and shifts slowly under pressure to higher temperatures. $T_c$ is determined from the temperature at the transition midpoint. The inset shows raw ${\chi }^{\prime }(T)$ data at 118 GPa (see Ref. 24).

Figure 3

Superconducting transition temperature of Eu metal versus pressure from electrical resistivity ($\oplus$) and ac susceptibility ($■$) data in Figs. 1, 2, respectively. Vertical error bars give 20%–80% transition width. Solid straight line is guide to the eye.