Superconducting Gap inMgB2: Electronic Raman Scattering Measurements of Single Crystals

Polarization-resolved Raman scattering measurements were performed on ${\mathrm{MgB}}_2$ 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 a maximum Raman shift of $105\pm 1{\mathrm{cm}}^{-1\mathrm{}}$ $\left[2\Delta \right(0){/k}_B{T}_c=3.96\mathrm{}\pm 0.09]$ and showing up to $5{\mathrm{cm}}^{-1\mathrm{}}$ anisotropy between polarized and depolarized spectra. The temperature dependence of $2\Delta$ follows that predicted from BCS theory, while the anisotropy decreases with decreasing temperature. It is concluded that the Raman results are consistent with a slightly anisotropic $s$-wave gap in a conventional BCS superconductor.