Intercalate Superconductivity and van der Waals Equation

Superconductivity in two single-element intercalated compounds has been investigated with the van der Waals equation. For CuxTiSe2 and YBa2Cu3O6+x, the van der Waals term characterizing the attractive energy per particle (i.e., electrons), aN/V, is calculated from concentration-dependent transition temperature plots derived from experiment. It is shown that two times the attractive energy per intercalant valence electron (2aN(val)/V-unit) is equal to the energy gap predicted by BCS theory (Delta) for these superconductors. This realization allows another way to estimate the energy gap of superconducting intercalated insulators and semiconductors, this time, directly from physical real-space properties of the superconductor and the applied external pressure. The physical properties of importance are shown to be the intercalant concentration, transition temperature, and the number of intercalant valence electrons per unit cell volume.