Theory of high-TCsuperconductivity: transition temperature

It is demonstrated that the transition temperature ( T C ) of high- T C superconductors is determined by their layered crystal structure, bond lengths, valency properties of the ions, and Coulomb coupling between electronic bands in adjacent, spatially separated layers. Analysis of 31 high- T C materials (cuprates, ruthenates, ruthenocuprates, iron pnictides, organics) yields the universal relationship for optimal compounds, k B T C0 = β/ℓζ, where ℓ is related to the mean spacing between interacting charges in the layers, ζ is the distance between interacting electronic layers, β is a universal constant and T C0 is the optimal transition temperature (determined to within an uncertainty of ± 1.4 K by this relationship). Non-optimum compounds, in which sample degradation is evident, e.g. by broadened superconducting transitions and diminished Meissner fractions, typically exhibit reduced T C < T C0 . It is shown that T C0 may be obtained from an average of the Coulomb interaction forces between the two layers.