Phonon Density-of-States in MgB2

We report inelastic neutron scattering measurements of the phonon density-of-states in Mg11B2, which has a superconducting transition at 39.2 K. The acoustic phonons extend in energy to 36 meV, and there are highly dispersive optic branches peaking at 54, 78, 89 and 97 meV. A simple Born-von Karman model is able to reproduce the mode energies, and provides an estimate of the electron-phonon coupling of lambda~0.9. The estimated boron and magnesium contributions to the isotope effect are in qualitative agreement with experiment. The data confirm that a conventional phonon mechanism, with moderately strong electron-phonon coupling, can explain the observed superconductivity.