Point-contact Andreev-reflection spectroscopy in segregation-free Mg1-xAlxB2 single crystals up to x = 0.32

We present new results of point-contact Andreev-reflection (PCAR) spectroscopy in single-phase Mg_1-xAl_xB₂ single crystals with x up to 0.32. Fitting the conductance curves of our point contacts with the two-band Blonder-Tinkham-Klapwijk model allowed us to extract the gap amplitudes Δ_σ and Δ_π. The gap values agree rather well with other PCAR results for Al-doped crystals and polycrystals up to x = 0.2 reported in the literature, and extend them to higher Al contents. In the low-doping regime, however, we observed an increase in the small gap Δ_π on increasing x (or decreasing the local critical temperature of the junctions, T_c^A) which is not as clearly found for other samples. On further decreasing T_c^A below 30 K, both the gaps decrease and, up to the highest doping level x = 0.32 and down to T_c^A = 12 K, no gap merging is observed. A detailed analysis of the data within the two-band Eliashberg theory shows that this gap trend can be explained as being mainly due to the band filling and to an increase in the interband scattering which is necessary to account for the increase in Δ_π at low Al contents (x<0.1). We suggest interpreting the following decrease of Δ_π for T_c^A<30 K as being governed by the onset of the inhomogeneity and disorder in the Al distribution that partly mask the intrinsic effects of doping and are not taken into account in standard theoretical approaches.