Polarisation-resolved Raman scattering on MgB2 single crystals reveals a sharp pair-breaking peak, which appears in the electronic continuum below Tc. This peak reaches an average maximum Raman shift of 105±1 cm-1 (2Δ(0)/kBTc = 4.0 ± 0.1) and shows a small anisotropy of 2-5 cm-1 as a function of polarisation. The temperature dependence follows that predicted from BCS theory and we find no evidence of a second gap feature below 2Δ. We conclude that the temperature dependence, polarisation dependence and magnitude of the superconducting gap observed by Raman scattering are consistent with a slightly anisotropic s-wave gap in a conventional BCS superconductor.
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