Pressure-induced superconductivity in europium metal

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 ≃ 1.8 K; T c increases linearly with pressure to 2.75 K at 142 GPa. Eu metal thus becomes the 53 rd known elemental superconductor in the periodic table.Bernd Matthias once mused that all nonmagnetic metals might become superconducting, if only they be cooled to a sufficiently low temperature [1]. Of the 92 naturally occurring elements in the periodic table, there are 30 known elemental superconductors at ambient pressure and 22 more that become superconducting under high pressure [2]. An intriguing question is whether the remaining 40 elemental solids become superconducting in some temperature/pressure range. One could, in fact, pose this same question for all solids.Across the entire lanthanide series, only its first member, La, superconducts at ambient pressure. The reasons for this appear to be twofold: (1) the local-moment magnetism in all lanthanides except La, Yb, and Lu leads to strong pair-breaking effects, and (2) as for non-superconducting Sc or Y, the relatively weak d-character of the conduction electrons for heavy lanthanides like Lu results in an only diminutive pairing interaction. Since compressing the lattice enhances the d-electron concentration, it is not surprising that Sc, Y, and Lu all become superconducting under pressure [3]; indeed, the vast majority of transition metals superconduct.In contrast, the pressure-induced superconductivity observed for Ce metal above 2 GPa arises from the suppression of its magnetism [4]. The fact that all lanthanides other than La, Ce, and Lu do not superconduct under pressure, in spite of their enhanced d-electron concentration, is a tribute to the stability of their strong local-moment magnetism. At sufficiently high pressures, however, one would anticipate that the lanthanide valence should increase as electrons are successively squeezed out of the 4f shell into the s, p, d-conduction band. The first two lanthanide metals to do this would likely be Eu and Yb since both are divalent at ambient pressure, in contrast to all others which are trivalent.Whereas trivalent Yb would exhibit a strong local magnetic moment by virtue of its 4f 13 configuration with J = L + S = 3 + 1 2 = 7/2, trivalent Eu would be left with a 4f 6 electron shell where S = L = 3 and thus J = L − S = 0. Under sufficient pressure, therefore, the divalent antiferromagnet Eu would be expected to become a trivalent weak Van Vleck paramagnet and a good candidate for superconductivity [5], perhaps at temperatures as high as 10 -15 K as for the ...

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

“…1 The list of pressure-induced elemental superconductors even includes elements, which, at ambient pressure, are non-metallic (e.g., sulfur 2 and iodine 3 ), magnetic (e.g., iron 4 ), or both non-metallic and magnetic (e.g., oxygen 5 ). The lanthanide series contains many of the last remaining hold-outs against pressure-induced superconductivity.…”

Effect of Pressure on Superconducting Properties

“…with one 4f electron promoted to the open valence band (or conduction band), at a high pressure [38]. Johansson and Rosengren predicted that superconductivity is possible in europium at high pressure, and much later superconductivity was indeed found by Debessai et al [40].…”

Determination of pressure properties of superconducting systems based on characteristic ratios 2Δ(0)/Tc and ΔC(Tc)/CN(Tc)

“…The corresponding values of | x 0 |, χ , and χ s , derived within the model are given, and the possible type of the T c vs pressure dependence is indicated. Additionally, the values of ν 0 g , and T 0 = T c (0, 0) derived for a given T D (the Debye temperature) are included 5,19,21,39,40 . The data for Nb (1) is taken from refs 15,22 , and that for Nb (2) from ref.…”

“…There are many materials that undergo a superconducting phase transition under high pressure 5,39,40 , with some examples listed in Tables 4 and 5. Please note that the experimental data originates from two different sources, and therefore, although both may list the same elements, there is some variation in the experimentally measured values.…”