Raman scattering spectra from the alternating-current face of thick single crystals were measured in zz, xz, and xx polarizations. In zz and xz polarizations, a threshold at around 29/cm forms in the below TC continuum but no pair-breaking peak was seen, in contrast to the sharp pair-breaking peak at around 100/cm in xx polarization. The zz and xz spectra were consistent with Raman scattering from a dirty superconductor while the sharp peak in the xx spectra argues for a clean system. Analysis of the spectra resolves this contradiction, placing the larger and smaller gap magnitudes in the σ and π bands and indicating that relatively strong impurity scattering was restricted to the π bands.

c-Axis Raman Scattering Spectra of MgB2 - Observation of a Dirty Limit Gap in the pi Bands. J.W.Quilty, S.Lee, S.Tajima, A.Yamanaka: Physical Review Letters, 2003, 90[20], 207006 (3pp).