Evidence for Two Superconducting Energy Gaps inMgB2by Point-Contact Spectroscopy

Experimental support is found for the multiband model of the superconductivity in the recently discovered system ${\mathrm{MgB}}_2$ with the transition temperature $T_c=39\mathrm{}\mathrm{K}$. By means of Andreev reflection, evidence is obtained for two distinct superconducting energy gaps. The sizes of the two gaps ( ${\Delta }_S=2.8\mathrm{}\mathrm{meV}$ and ${\Delta }_L=7\mathrm{}\mathrm{meV}$) 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.