Evidence for Two Superconducting Gaps in ${\mathrm{MgB}}_2$

We have measured the Raman spectra of polycrystalline ${\mathrm{MgB}}_2$ from 25 to $1200{\mathrm{cm}}^{-1\mathrm{}}$. A superconductivity-induced redistribution in the electronic Raman continuum was observed. Two pair-breaking peaks appear in the spectra, suggesting the presence of two superconducting gaps. The measured spectra were analyzed using a quasi-two-dimensional model in which two $s$-wave superconducting gaps open on two sheets of Fermi surface. For the gap values we have obtained ${\Delta }_1=22\mathrm{}{\mathrm{cm}}^{-1\mathrm{}}$ ( $2.7\mathrm{meV}$) and ${\Delta }_2=50\mathrm{}{\mathrm{cm}}^{-1\mathrm{}}$ ( $6.2\mathrm{meV}$). Our results suggest that a conventional phonon-mediated pairing mechanism occurs in the planar boron $\sigma$ bands and is responsible for the superconductivity of ${\mathrm{MgB}}_2$.