X-ray absorption and optical spectroscopy studies of (Mg1-xAlx)B2

Abstract

X-ray absorption spectroscopy and optical reflectance measurements have been carried out to elucidate the evolution of the electronic structure in (Mg1-xAlx) B-2 for x=0.0, 0.1, 0.2, 0.3, and 0.4. The important role of B 2psigma hole states to superconductivity has been identified, and the decrease in the hole carrier number is quantitatively determined. The rate of the decrease in the hole concentration agrees well with the theoretical calculations. On the other hand, while the evolution of the electronic structure is gradual through the doping range, T-c suppression is most significant at x=0.4. These results suggest that the superstructure in (Mg1-xAlx)B-2, in addition to the sigma holes, can affect the lattice dynamics and contributes to the T-c suppression effect. Other possible explanations like the topological change of the sigma band Fermi surface are also discussed.