Superconductivity of nano-crystalline MgB2

Partial altervalent/aliovalent chemical substitutions for Mg or B have been attempted to modify the Fermi-level density-of-states and to alter the lattice parameters, thus varying the superconducting transition temperature (Tc) of MgB2. However, different from Cu oxide superconductors in which replacement of La by Y in La2CuO4 forms Y Ba2Cu3O7-δ and raises Tc from 35 to 93 K, most of the substitutions in MgB2 studied to date depress Tc, and at higher replacements completely suppress the superconductivity of MgB2. Such diminution and loss of superconductivity in MgB2 arise from the subtle interplay between the competing/cooperating effects of the electronic and lattice structural variations, which are induced by the different charge and atomic radii of the substituents. In this work, we experimentally separate lattice structural effects from electronic contributions to superconductivity by exploiting the nanosize dependence of the lattice structure to modify the structural parameters without resorting to chemical doping. It is found that the superconductivity of MgB2 is extremely sensitive to lattice parameter variation, such that contraction of the Mg-Mg bond dramatically depresses Tc and eventually results in the loss of superconductivity as the average coordination of Mg to B falls from 12 to 8 for nano-crystalline MgB2 of 2.5 nm in diameter