An important method for determining the electron-phonon coupling uses the phonon widths measured in Raman scattering. In metallic systems a phonon can decay in an electron-hole pair excitation. The extra width due to this decay is a measure of the coupling. Theories (Allen's formula) extracting a coupling assume an average over all phonon wave vectors q. Since Raman measures q=0, it has been argued that Raman cannot be used for this purpose. We find (Phys. Rev. B 60, 6495 (1999)), however, that the orientational disorder violates the momentum conservation so efficiently that it is not a serious problem that Raman in principle is a q=0 technique.
We furthermore find (Phys. Rev. B 61, 8628 (2000)), that correlation effects can substantially reduce the extra width due to the decay in electron-hole pairs. If this is not taken into account, the electron-phonon coupling can be substantially underestimated for strongly correlated systems. This may explain part of the discrepance between the estimates of the electron-phonon coupling based on Raman scattering and photoemission.