A superconductor made by a metal heterostructure at the atomic limit tuned at the `shape resonance': MgB2*

We have studied the variation of superconducting critical temperature T c as a function of charge density and lattice parameters in Mg 1- x Al x B 2 superconducting samples. The AB 2 heterostucture of metallic boron layers (intercalated by A = magnesium, aluminum layers, playing the role of spacers) is made by direct chemical reaction. The spacing between boron layers and their charge density are controlled by chemical substitution of Mg by Al atoms. We show that high T c superconductivity is realized by tuning the chemical potential at a `shape resonance' according with the patent for `high-temperature superconductors made by metal heterostructures at the atomic limit'. The energy width of the superconducting shape resonance is found to be about 400 meV.