Phonon spectra in pure and carbon doped MgB2 by inelastic X-ray scattering

We review measurements of phonon spectra in MgB2. After discussing the relevance of phonon measurements to superconductivity, we provide an introduction to our technique of choice, inelastic X-ray scattering (IXS). We then discuss the measured spectra in MgB2, both the pure material, and the carbon doped material, Mg(B1−xCx)2 for x = 0.02, 0.05 and 0.125. The results (phonon dispersion and line-width) for the pure material are well described quantitatively by calculation, though there remain some open questions. In contrast, the spectra of the carbon doped material seems to be only qualitatively understood, with the hardening of the E2g mode on carbon doping being about three times larger than calculations suggest for x = 0.125.