Role of the E2g phonon in the superconductivity of MgB2: a Raman scattering study

Temperature-dependent Raman scattering studies in polycrystalline MgB2 (10<T<300 K) reveal that the E2g phonon does not experience any self-energy renormalization effect across the superconducting critical temperature TC≈39 K, in contrast with most of the current theoretical models. In the presence of our results, those models must be reviewed. The analysis of the temperature dependence of the E2g phonon frequency yields an isobaric Grüneisen parameter of |γE2g|≲1, smaller than the value of 3.9 obtained from isothermal Raman experiments under pressure. It is suggested that this apparent disagreement can be explained in terms of pressure-induced changes of the topology of the Fermi surface.