A point-contact study of the superconducting gaps in Al-substitutedand C-substituted MgB2 single crystals

We present the results of directional point-contact spectroscopy in state-of-the-art Mg1−xAlxB2 and Mg(B1−yCy)2 single crystals produced at ETHZ. Fitting the conductance curves of our point contacts, that always feature Andreev reflection structures, we obtained the doping dependence of the gap amplitudes. The results are discussed in comparison with other experimental findings and relevant theoretical predictions. We conclude that the physics of Al-substituted crystals at x≥0.09 might be governed by phase segregation, while C-substituted crystals unexpectedly show a doping-induced transition to single-gap superconductivity at y=0.132.