High pressure experiments play a significant role in the field of superconductivity in a number of ways, one of the more important being to expand the number of known superconductors and enhance their transition temperatures T_c to record values. Of the 53 known elemental superconductors, fully 23 only become superconducting under sufficiently high pressures and include Fe, O, Si, Cs, Ca, Sc and Eu. In this talk I discuss the recent discovery of superconductivity in Eu metal near 2 K for pressures above 80 GPa (800,000 bar) and the implications this has for changes in Eu's magnetic state.

Neaton and Ashcroft have emphasized that pressures sufficiently high to bring ion cores into contact cause dramatic changes in the electronic properties. In the case of the alkali metals, in fact, the free-electron character of their conduction electrons is destroyed and the electron-phonon interaction enhanced. Indeed, Li is found to become superconducting at temperatures above 14 K at 30 GPa, reverting to semiconducting behavior above 70 GPa. T_c for Ca rises to 25 K at 160 GPa, a record value for an elemental superconductor. Studies on the trivalent d-electron elements Sc, Y, La, and Lu to extreme pressures push T_c to 20 K for Sc and Y and reveal an interesting correlation between T_c and the free volume available to the conduction electrons outside the ion cores, as suggested many years ago by Johansson and Rosengren.