Raman-active E2g phonon in MgB2 : Electron-phonon interaction and anharmonicity

Abstract

Measurements of the Raman spectra in ${\mathrm{MgB}}_{2}$ up to 750 K are presented. The width of the ${E}_{2g}$ phonon line shows a pronounced maximum in the range of 350--400 K. In the same temperature range the frequency exhibits a sharp softening. The ${E}_{2g}$ phonon line is superimposed on the electronic background, whose frequency also strongly depends on temperature. The phonon self-energies and spectra of electronic Raman scattering were calculated, taking into account a renormalization of ab initio electronic structure due to an electron-phonon scattering. We obtained a satisfactory description of the temperature dependence of the phonon linewidth and proved that it is mainly determined by nonadiabatic effects. Since the wave vector of the ${E}_{2g}$ phonon is in the nonadiabatic range, the contribution of electrons to phonon damping dominates and determines its nonmonotonic behavior as a function of temperature. When the anharmonic contributions to the phonon frequency are taken into account, the total shift of the ${E}_{2g}$ phonon energy agrees with the experiment at 300 K, but the estimates of the anharmonic terms in the high-temperature range need further clarification.