We report the results of the first directional point-contact measurements in Mg(B_1-xC_x)₂ single crystals with 0 <= x <= 0.132 and in Mg_1-yAl_yB₂ crystals with 0 <= y <= 0.21, obtained at ETH (Zurich) by means of a high-pressure technique in a cubic anvil. We made the point contacts by using the "soft" technique described elsewhere [1]. The Andreev-reflection features of our conductance curves were measured in magnetic fields up to 9 T, applied either parallel to the c axis or to the ab plane of the crystals. By fitting the normalized conductance with the Blonder-Tinkham-Klapwijk (BTK) model generalized to the two-band case we obtained the dependence of the gaps Δ_π and Δ_σ on the C or Al content (x or y). This dependence was confirmed by applying to the junctions a suitable magnetic field B^* able to remove the contribution of the π-band gap to the total conductance, without appreciably affecting the σ-band one [2]. This allowed the separate determination of the gaps via a single-band three-parameter BTK fit.
In C-substituted crystals up to x ~ 0.10, the two-gap nature of superconductivity characteristic of pure MgB₂ is preserved. At x = 0.132 we clearly and reproducibly observed for the first time the theoretically predicted merging of Δ_π and Δ_σ into a single gap Δ ~ 3 meV with a gap ratio 2Δ/k_BT_c close to the standard BCS value [3].
In Al-substituted crystals, we found no evidence of gap merging: Δ_π reaches the value 0.4 meV at y=0.21, where Δ_σ saturates at about 4 meV. The behaviour of Δ_π(y) and Δ_σ(y) cannot be simply explained by the two-band model and can be due to the presence in these crystals of a phase segregation of Al-rich layers.

[1] R.S. Gonnelli et al., Phys. Rev. Lett. 89, 247004 (2002).
[2] R.S. Gonnelli et al., Phys. Rev. B 69, 100504(R) (2004).
[3] R.S. Gonnelli et al., cond-mat/0407265.

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