Vibrational spectroscopy of MgB₂ by neutron inelastic scattering

Neutron inelastic scattering measurements have been performed on superconducting MgB₂ up to 80 and 130 meV over a wide range of scattering angles (3.14° - 127,5°) at the temperature range between 23 k and 270 K. In the measurements by E_i = 80 meV, we found fine structures in the phonon density of states of MgB₂, which has been anticipated by theoretical calculation, especially in the energy range 30 to 40 meV and 45 to 55 meV. Moreover, in the measurements by E_i = 130 meV, the temperature dependence of the integrated intensity of the generalized phonon density-of-states, where we made multi-phonon corrections, shows a clear anomalous behavior (the integrated intensity increases with decreasing temperature) in the energy range 75 to 80 meV and 87 to 91 meV. Since these energy regions are close to the energy of E_2g phonon mode (B in-plane bond stretching) and B_1g phonon mode (B out-of-plane bond bending), this behavior can be interpreted as an evidence for a substantial contribution to the electron-phonon coupling from these phonons.