Evidence for Two Superconducting Gaps in $Mg{B}_2$

Abstract

We have measured the Raman spectra of polycrystalline MgB${}_2$ from 25 {\cm} to 1200 {\cm}. When the temperature was decreased below the superconducting transition temperature $T_c$, we observed a superconductivity-induced redistribution in the electronic Raman continuum. Two pair-breaking peaks appear in the spectra, suggesting the presence of two superconducting gaps. Furthermore, we have analyzed the measured spectra 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 ${\Delta }_1=22c{m}^{-1}$ (2.7 meV) and ${\Delta }_2=50c{m}^{-1}$ (6.2 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 MgB${}_2$.

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