Pressure-Induced Superconducting State of Europium Metal at Low Temperatures

Abstract

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} approximately 1.8 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.

Magnetism of europium under extreme pressures

“…The critical pressure at which Eu loses magnetic order is estimated to be 84 GPa, extrapolated from the data at 81 GPa and 82.5 GPa. It is significant that this pressure is the same as that (84 GPa) where superconductivity in Eu first appears at 1.8 K in magnetic susceptibility measurements; in the same study no superconductivity was observed above 1.4 K at 76 GPa [11]. While it is evident that pressure suppresses magnetism and induces superconductivity in Eu near 80 GPa, the precise phase boundaries remain to be accurately determined.…”

“…The high purity Eu sample (99.98% metal basis) is from the Materials Preparation Center of the Ames Laboratory [30], and from the same sample batch used in superconductivity studies in Ref. [11]. Due to its high reactivity, the sample was loaded into the diamond anvils cells in an Ar filled glove box together with 2-3 ruby spheres.…”

“…Because of their strong 4f local magnetic moments, they are normally not superconducting. Only Ce [9,10] and Eu [11] are known to become superconducting under pressure. At ambient pressure both Eu and Ce possess local magnetic moments.…”

“…It has long been believed that sufficient pressure would drive Eu to a trivalent 4f 6 state with only weak Van Vleck paramagnetism where superconductivity might appear, as in Am (5f 6 ) [23]. Indeed, Eu was found to be superconducting above 80 GPa with a critical temperature T c ≈ 2 K [11]. However, both T c and its small positive pressure derivative dT c /dP to 142 GPa lie well below the values found in the nonmagnetic trivalent spd-electron metals Sc, Y, La, and Lu [24].…”

Low-temperature electrical resistivity of praseodymium at pressures up to 120 GPa

“…However, Ce and Eu, both elements with unstable valences, have been demonstrated to become superconducting under pressure. The most recently discovered elemental superconductor, Eu, becomes superconducting above ∼80 GPa, 8 following a possible pressure-induced valence change from a strongly magnetic Eu 2+ (J = 7/2) configuration to weak Van Vleck paramagnetism in Eu 3+ (J = 0). Cerium metal undergoes an isostructural phase transition from the fcc γ-phase, characterized by Curie-Weiss local magnetic moment behavior, to the α-phase when subjected to a pressure of approximately 0.7 GPa at room temperature.…”