Evidence for two superconducting energy gaps in MgB₂ by point-contact spectroscopy

Experimental support is found for the multiband model of the superconductivity in the recently discovered system MgB₂ with the transition temperature T_c = 39 K. By means of Andreev reflection, evidence is obtained for two distinct superconducting energy gaps. The sizes of the two gaps (Δ_S = 2.8 meV and Δ_L = 7 meV) are, respectively, smaller and larger than the expected weak coupling value. Because of the temperature smearing of the spectra the two gaps are hardly distinguishable at elevated temperatures, but when a magnetic field is applied the presence of two gaps can be demonstrated close to the bulk T_c in the raw data.