Evidence for Single-gap Superconductivity in Mg(B_(1-x)C_x)₂ Single Crystals with x=0.132 from Point-Contact Spectroscopy

We report the results of the first directional point-contact measurements in Mg(B_{1-x}C_{x})_2 single crystals with 0.047 <= x <= 0.132. The two-gap superconductivity typical of MgB_2 persists up to x=0.105. In this region, the values of the gaps Delta_{sigma} and Delta_{pi} were determined by fitting the Andreev-reflection conductance curves with a two-band Blonder-Tinkham-Klapwijk (BTK) model, and confirmed by the single-band BTK fit of the sigma- and pi-band conductances, separated by means of a magnetic field. At x=0.132, when T_{c}=19 K, we clearly observed for the first time the merging of the two gaps into one of amplitude Delta~3 meV.