Electron-phonon effects on the Raman spectrum in MgB2

The anomalous features of the Raman spectroscopy measurement in MgB2 represent a still unresolved puzzle. In particular, highly debated are the origin of the huge E-2g phonon linewidth, the nature of the low energy (omega <omega(E2g)) background and the evolution of the Raman spectra with Al doping. In this paper we compute the self-energy of the E-2g phonon mode in a fully self-consistent way, taking into account electron-phonon effects on the electronic properties. We show that all the anomalous features can be naturally understood in a framework where the whole electron-phonon spectrum alpha F-2(omega) gives rise to significant damping processes for the electronic excitations and consequently for the E-2g phonon itself. The two-peak structure as function of the Al doping is ascribed to finite bandwidth effects arising as the Fermi level approaches the sigma band edge.