Abstract
We studied the effects of co-doping with Li and Al on the energy gaps ofMgB2 by performing point-contact Andreev-reflection spectroscopy (PCAR) in polycrystallineMg1−x(AlαLi1−α)xB2 sampleswith x≤0.4. Even though the lattice parameters and the critical temperatureof the compound simply scale with the effective Al contentαx, irrespective of the Li concentration, the energy gaps do not. Inparticular, for a given effective Al content, the comparison withMg1−y(Al)yB2 withy = αx shows that theσ bandgap is practicallythe 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 theoutcomes of first-principles calculations of the effects of Li and Al co-doping on theelectronic structure of magnesium diboride.
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