Is Delta_(pi)-gap-only superconductivity possible in Mg_(1-x)Al_(x)B₂ and Mg(B_(1-y)C_(y))₂ alloys?

Using density-functional-based method, we study the k-resolved sigma- and pi-band holes in Mg_{1-x}Al_{x}B_{2} and Mg(B_{1-y}C_{y})_{2} alloys. We find that the calculated profiles of the loss of sigma- and pi-band holes in these two systems as a function of impurity concentration are in qualitative agreement with experiments, as expected. We also describe its implications vis-a-vis superconductivity in Mg_{1-x}Al_{x}B_{2} and Mg(B_{1-y}C_{y})_{2}.