The relationship between the composition and the superconducting properties of MgxB2 films fabricated by molecular beam epitaxy were investigated. The MgxB2 films, with a composition gradient of Mg and B, were formed on c-plane Al2O3 (00▪1) substrates by controlling the in-plane distribution of the flux rate of Mg on the substrates. It was found that the composition of the films on the substrates, x (in MgxB2), gradually changed from 0.68 to 2.75. Though the composition was distributed widely, the films exhibited superconductivity in their whole area. Thin films with a critical temperature of 31–33K, critical current density of 0.8–1.1 x 107A/cm2 at 4.2K, and surface roughness of 2–3nm were obtained in a wide range of film compositions from stoichiometric to Mg-poor (i.e., x = 0.68–0.96).

Composition Dependence of Superconductivity and Surface Morphology of MgB2 Thin Films Grown by Molecular Beam Epitaxy. H.Yamamoto, A.Tsukamoto, H.Hasegawa, K.Saitoh, M.Okada, H.Kitaguchi: Physica C, 2005, 426-431[2], 1444-8