Point-contact Andreev-reflection spectroscopy in segregation-freeMg1−xAlxB2 singlecrystals up to x = 0.32

Abstract

We present new results of point-contact Andreev-reflection (PCAR) spectroscopy in single-phaseMg1−xAlxB2 singlecrystals with x up to 0.32. Fitting the conductance curves of our point contacts with the two-bandBlonder–Tinkham–Klapwijk model allowed us to extract the gap amplitudesΔσ and Δπ. The gap values agree rather well with other PCAR results for Al-doped crystals and polycrystals upto x = 0.2 reported in the literature, and extend them to higher Al contents. In thelow-doping regime, however, we observed an increase in the small gapΔπ onincreasing x (or decreasing the local critical temperature of the junctions,TcA) which is not as clearly found for other samples. On further decreasingTcA below 30 K, both the gaps decrease and, up to the highest doping levelx = 0.32 anddown to TcA = 12 K, no gap merging is observed. A detailed analysis of the data within the two-band Eliashbergtheory shows that this gap trend can be explained as being mainly due to the band filling andto an increase in the interband scattering which is necessary to account for the increase inΔπ at low Alcontents (x<0.1). We suggest interpreting the following decrease ofΔπ for TcA<30 K as being governed by the onset of the inhomogeneity and disorder in the Al distributionthat partly mask the intrinsic effects of doping and are not taken into account in standardtheoretical approaches.