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 ${\mathrm{MgB}}_{2}$ with the transition temperature ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}39\mathrm{K}$. By means of Andreev reflection, evidence is obtained for two distinct superconducting energy gaps. The sizes of the two gaps ( ${\ensuremath{\Delta}}_{S}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.8\mathrm{meV}$ and ${\ensuremath{\Delta}}_{L}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}7\mathrm{meV}$) 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.