Elucidating superconductivity and intermediate valence of elemental Eu under high pressure

Elemental Eu is a superconductor under high pressure, P ≥ 80 GPa, up to T_c = 2.75 K at 142 GPa. We elucidate the possible connection between pressure induced 4f configurational changes, Eu²⁺(4f⁷, J=7/2) <-> Eu³⁺ (4f⁶, J=0), and superconductivity. Notably high pressure L_III XAS and more recently also ME isomer shift measurements do not show the theoretically expected 2⁺ - 3⁺ transition, but intermediate valence states saturating at v ≅ 2.6 between ∝ 20 and 70 GPa, and undergoing between 31 and 37 GPa a transformation into a complex incommensurately modulated monoclinic structure ∝ α-U, which is followed by an other hitherto unidentified low-symmetry complex structure. We discuss how the Brillouin zone - Fermi surface interaction, driven by the non integer valence electron count, may stabilize low-symmetry complex structures, which tend to favor the occurrence of superconductivity through bandstructures with overlapping electron and hole bands.