The study of the anisotropic superconductor ${\mathrm{M}\mathrm{g}\mathrm{B}}_2$ using a combination of scanning tunneling microscopy and spectroscopy reveals two distinct energy gaps at ${\Delta }_1=2.3\mathrm{m}\mathrm{e}\mathrm{V}$ and ${\Delta }_2=7.1\mathrm{m}\mathrm{e}\mathrm{V}$ at 4.2 K. Different spectral weights of the partial superconducting density of states are a reflection of different tunneling directions in this multiband system. Temperature evolution of the tunneling spectra follows the BCS scenario [Phys. Rev. Lett. 3, 552 (1959)] with both gaps vanishing at the bulk $T_c$. The data confirm the importance of Fermi-surface sheet dependent superconductivity in ${\mathrm{M}\mathrm{g}\mathrm{B}}_2$ proposed in the multigap model by Liu et al. [Phys. Rev. Lett. 87, 087005 (2001)].

• Received 31 January 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.187002