The Superconducting Gap in MgB_2: Electronic Raman Scattering Measurements of Single Crystals

We have performed polarisation-resolved Raman scattering measurements on MgB$_{2}$ single crystals to determine the magnitude, symmetry and temperature dependence of the superconducting gap. A single sharp peak due to Cooper pair breaking appears in the electronic continuum below $T_c$, reaching an average maximum Raman shift of $105 \pm 1$ cm$^{-1}$ ($2\Delta(0) / k_{B}T_{c} = 3.96 \pm 0.09$) and showing up to 5 cm$^{-1}$ anisotropy between polarised and depolarised spectra. The temperature dependence of $2\Delta$ follows that predicted from BCS theory, while the anisotropy decreases with decreasing temperature. We conclude that our Raman results are consistent with a slightly anisotropic \textit{s}-wave gap in a conventional BCS superconductor.