Effect of Li–Al co-doping on the energy gaps of MgB2

We studied the effects of co-doping with Li and Al on the energy gaps of MgB2 by performing point-contact Andreev-reflection spectroscopy (PCAR) in polycrystalline Mg1−x(AlαLi1−α)xB2 samples with x≤0.4. Even though the lattice parameters and the critical temperature of the compound simply scale with the effective Al content αx, irrespective of the Li concentration, the energy gaps do not. In particular, for a given effective Al content, the comparison with Mg1−y(Al)yB2 with y = αx shows that the σ bandgap is practically the same while the π bandgap is higher. A clear gap merging is observed in the most doped sample (x = 0.4) when Tc<20 K. The results are discussed within the two-band Eliashberg theory and compared to the outcomes of first-principles calculations of the effects of Li and Al co-doping on the electronic structure of magnesium diboride.