Superconducting properties and c-axis superstructure of Mg 1-x Al x B 2

The superconducting and structural properties of a series of Mg1-xAlxB2 samples have been investigated. X-ray diffraction results confirmed the existence of a structural transition associated with a significant change in interboron layer distance as reported previously by Slusky Moreover, transmission-electron-microscopy observations revealed the existence of a superstructure with doubled lattice constant along the c-axis direction. We propose that this superstructure is probably related to the structural transition. The modifications of the superconducting transition temperature T-c, the normal-state resistivity, and the upper critical field B-c2(0) by Al doping are discussed in terms of Al-substitution-induced changes in the electronic structure at the Fermi energy.