The superconducting gaps of C-substituted and Al-substituted MgB2 single crystals by point-contact spectroscopy



We studied the effects of carbon and aluminum substitutions on the gaps of the two-band superconductor MgB2 by means of point-contact measurements in Mg(B1–xCx)2 and Mg1–yAlyB2 single crystals with 0 ≤ x ≤ 0.132 and 0 ≤ y ≤ 0.21. The gap amplitudes, Δω and Δπ, were determined by fitting the conductance curves of the point contacts with the standard Blonder-Tinkham-Klapwijk (BTK) model generalized to the two-band case. Whenever possible, their values were confirmed by the independent fit (with a single-band BTK model) of the partial contribution of the two bands to the conductance, separated by means of a suitable magnetic field B*. In C-substituted crystals, the two gaps remain clearly distinct up to x ∼ 0.10, but at x = 0.132 we observed for the first time their merging into a single gap Δ ≃ 3 meV with a gap ratio 2Δ = kBTc close to the standard BCS value. In Al-substituted crystals, we found no evidence of this gap merging. Instead, Δπ reaches the value 0.4 meV at y = 0.21, where Δπ saturates at about 4 meV. These results are compared with other recent experimental findings in polycrystals and with the predictions of the models for multiband superconductivity. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)