Anharmonic and non-adiabatic effects in MgB2: Implications for the isotope effect and interpretation of Raman spectra

We review the published calculations of the anharmonic effects in MgB2 and show that different results are mainly related to the various degrees of approximation involved in the calculations. When all the leading order terms in anharmonic perturbation theory are included the magnitude of anharmonic effects is marginal. This result is in good agreement with the phonon dispersion measured by inelastic X-ray scattering showing weak anharmonic phonon frequency shift. However, Raman spectra display a feature having E2g symmetry at ∼12 meV above the available X-ray phonon dispersion near Γ. Raman data can be explained if dynamical effects beyond the adiabatic Born–Oppenheimer approximation and electron lifetime effects are included in the phonon self-energy, without invoking anharmonicity. Finally, we discuss the implications of weak anharmonicity for the interpretation of the isotope effect and conclude that the isotope effect is the most important unresolved issue in the physics of MgB2.