High Tc superconductivity in a critical range of micro-strain and charge density in diborides

Expansion of the superlattice of boron layers, with AB2 structure, due to different intercalated A atoms has been studied to understand the emergence of high Tc superconductivity in the diborides. The structure of these metal heterostructures at the atomic limit (MEHALs) (with A = Al, Mg, Ti, Hf, Zr) has been measured by synchrotron x-ray diffraction. The increasing atomic radius of the intercalated A ions induces an increase of (1) the separation between the boron layers and (2) the tensile micro-strain e of the B–B distance within the boron layers. The results show that the superconductivity in these MEHALs appears in a critical region in a phase diagram controlled by two variables, the micro-strain and the charge density (e, ρ).