Higher Superconductivity in the Compressed Lower Elements

N. W. Ashcroft, Cornell University

Early attempts to systematize the occurrence of superconductivity under normal conditions resolutely excluded Group I elements, viewed then as single wide-band systems. The recent observation of significant superconductivity in compressed lithium focuses attention both on the role of band-width (and on the core electrons which determine this in part) and also on ionic arrangement, some high density phases exhibiting a multi-band compensated electron structure. In this context there are certain similarities to the latecomer light element superconductor MgB2 whose structure leads to 8 electrons per cell. Within a framework of superconductivity driven by phonon exchange it may therefore be asked whether systems exist where the phonon energy scale is especially high, but also where compensated electronic structure is again present. These criteria lead to some common hydrides where the hydrogen content is notably large and for which an octet of electrons per cell is also the norm. Comparisons to the limiting case, metallic hydrogen itself, are interesting because the presence of a second (or even third) constituent leads to "chemical pre-compression" and attainment of metallic states, with large superconducting transition temperatures, are predicted to occur at pressures considerably lower than presently estimated for pure metallic hydrogen.

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