Features of the E 2g phonon mode in MgB2

A model estimate of the energy eigenvalues of the E (2g) phonon mode of the ion vibrations in the plane of boron atoms is obtained. It is shown that the anharmonic contribution to the energy of E (2g) monotonously increases with the increase in z = A (4)/A (2) (2) , where A (4) is the fourth-order anharmonic coefficient and A (2) is the harmonic constant. The results agree with the estimate in the Hartree-Fock approach. However, the discrepancy between these results and Raman spectroscopy data indicates the necessity to account for the influence of the electron-phonon interaction on the energy of E (2g) in more detail.