Effect of heavy Al doping on MgB2: A point-contact study of crystals and polycrystals

We present the results of point-contact spectroscopy (PCS) measurements in single-phase Mg1-x Al (x) B-2 crystals and polycrystals with x up to 0.32, recently grown at ETH (Zurich) and University of Genova, respectively. The differential conductance curves of our 'soft' point contacts were measured as a function of temperature from 4.2 K up to the Andreev critical temperature T-c(A), at which the normal-state conductance is recovered. The gap amplitudes, Delta(sigma) and Delta(pi), were obtained through a two-band BTK fit of the normalized conductance curves. The results extend previous data to the range of heavy A1 doping and show that contrary to expectations, no gap merging is observed up to x = 0.32 (i. e., down to T-c(A) = 9 K). The analysis of the gap trends within the two-band Eliashberg theory shows that band filling is the main effect of A1 doping, even though a small increase in interband scattering might be necessary to account for the experimental data in single crystals.