Superconductivity of nano-crystallineMgB2

Abstract

Partial altervalent/aliovalent chemical substitutions for Mg or B have beenattempted to modify the Fermi-level density-of-states and to alter thelattice parameters, thus varying the superconducting transition temperature(Tc)of MgB2. However, different from Cu oxide superconductors in which replacement of La by Y inLa2CuO4 formsY Ba2Cu3O7−δ andraises Tc from 35 to 93 K, most of the substitutions inMgB2 studied todate depress Tc, and at higher replacements completely suppress the superconductivity ofMgB2. Such diminution and loss of superconductivity inMgB2 arise from the subtle interplay between the competing/cooperating effects of the electronicand lattice structural variations, which are induced by the different charge andatomic radii of the substituents. In this work, we experimentally separate latticestructural effects from electronic contributions to superconductivity by exploiting thenanosize dependence of the lattice structure to modify the structural parameterswithout resorting to chemical doping. It is found that the superconductivity ofMgB2 isextremely sensitive to lattice parameter variation, such that contraction of the Mg–Mg bond dramaticallydepresses Tc and eventually results in the loss of superconductivity as the averagecoordination of Mg to B falls from 12 to 8 for nano-crystallineMgB2 of 2.5 nm in diameter.