Structure of the superconducting gap in MgB₂ from point-contact spectroscopy

We have studied the structure of the superconducting gap in MgB₂ thin films by means of point-contact spectroscopy using a gold tip. The films were produced by depositing pure boron on a sapphire substrate, using e-beam evaporation, followed by reaction with magnesium vapour. The films have a T_c of 38.6 ± 0.3 K and resistivity of about 20 μΩ cm at 40 K. The point-contact spectra prove directly the existence of a multi-valued order parameter in MgB₂, with two distinct values of the gap, Δ₁ = 2.3 ± 0.3 meV and Δ₂ = 6.2 ± 0.7 meV at 4.2 K. Analysis of the spectra in terms of the Blonder–Tinkham–Klapwijk model reveals that both gaps close simultaneously at the T_c of the film. Possible mechanisms that can explain the intrinsic coexistence of two values of the gap are discussed.