Carbon and aluminum doped MgB2
MgB2 is a two-gap superconductor in which the quasi two dimensional s band and the isotropic p band almost separately contributes to the superconductivity. The influence of impurity scattering and/or chemical doping on the physical properties of such a two-gap system is widely studied. The only on site substitutions by non magnetic elements which are known so fare are C for B and Al for Mg. Both these elements act not only as the scattering centers but they also dope MgB2 with one extra electron filling the s band with strongly coupled holes, thus decreasing their density of states and Tc. Recently, Kortus et al.  proposed a model incorporating the interband scattering and at same time the band filling in MgB2. The former effect leads to approaching of the two gaps by increasing of Dp and decreasing Ds while the latter suppresses both gaps proportionally with Tc. This model is general with no prediction on the strength of band filling and interband scattering effects in a particular doped material. The published experiments bring rather controversial picture as far as the strength of both effects and the development of two gaps in the Al and C-doped MgB2 is concerned.
But for the highest dopings Dp decreases in both cases. The solid lines are theoretical calculations for the case of a pure band filling effect and no interband scattering. The dashed lines show the calculations including also the interband scattering with the rate IB = 1000 x cm-1 (or 2000 y cm-1). Comparing all here presented gap values obtained on different types of polycrystals and single crystals it is evident, that the the decreased Tc and the evolution of the two gaps upon both substitutions are mainly the consequence of the band filling effect. However, the energy gaps in Al-doped samples reveal stronger tendency to approach each other and their evolution cannot be accounted without an interband scattering but it is smaller than in the presented calculations.
P. Szabó, Ho¾anová, P. Samuely (Inst. Exp. Physics, Koice, Slovakia)