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Abstract
At ambient pressure there are 29 elemental superconductors in the periodic table, none of which is an alkali metal. The first alkali metal to become superconducting under high pressure is Cs followed years later by Li. Alkali metals are believed to be exemplary free-electron systems. The fact that an alkali metal becomes superconducting at all is surprising and is a result of the fact that under pressure it shows marked deviations from free-electron behaviour where, counterintuitively, bands narrow and gaps widen. For this reason the alkali metals are among the most interesting systems known to study in high-pressure experiments and superconductivity is one of their most fascinating properties.
Acknowledgements
The author would like to express his appreciation to N. Ashcroft, S. Deemyad, J. Hamlin and W. Pickett for critically reading the first draft of this paper. Thanks are due J. Hamlin for technical support and K. Syassen for providing the revised alkali phase diagram in (lower). The author also gratefully acknowledges the support of the National Science Foundation through grant DMR-0404505.