The superconducting-gap of ${\mathrm{M}\mathrm{g}\mathrm{B}}_2$ has been studied by high-resolution angle-resolved photoemission spectroscopy. The results show that superconducting gaps with values of 5.5 and 2.2 meV open on the $\sigma$ band and the $\pi$ band, respectively, but both the gaps close at the bulk transition temperature, providing a definitive experimental evidence for the two-band superconductivity with strong interband pairing interaction in ${\mathrm{M}\mathrm{g}\mathrm{B}}_2$. The experiments validate the role of $k$-dependent electron-phonon coupling as the origin of multiple-gap superconductivity as well as the high transition temperature of ${\mathrm{M}\mathrm{g}\mathrm{B}}_2$.

• Received 6 April 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.127001