Universal behaviour of nonmetal–metal Mott transitions in fluid H, N, O, Rb, andCs

Abstract

The systematics of Mott transitions inlow-Z and alkali fluids H, N, O, Rb, and Cs are discussed. Such transitions have only beenobtained in elemental, monatomic, and disordered systems by using high pressures andtemperatures. By finding elements which undergo this transition, it is possible to testMott’s ideas in systems which are relatively simple. For fluid H, N, and O, 100 GPapressures and K temperatures are required and were achieved by dynamic shock compression. For fluidRb and Cs, 0.01 GPa and K are required and were achieved statically by Hensel et al. Despite the fact thatthese two groups of elements at ambient conditions are so different chemicallyand that pressures required to observe this transition differ by a factor of104 for the two groups, the metallic conductivities are essentially the same for all five and thedensity dependences of their semiconductivities are determined systematically by the radialextents of the electronic charge-density distributions of the various atoms. The latterobservation provides a physical understanding for the Herzfeld criterion of metallization. Itis the tuning of both high pressure and temperature which permits these two apparentlydifferent classes of elements at ambient conditions to become quite similar in nature atextreme conditions.