Phonon Dispersion and Electron-Phonon Coupling in MgB2 and AlB2

We present a first principles investigation of the lattice dynamics and electron-phonon coupling of the superconductor MgB ₂ and the isostructural AlB ₂ within the framework of density functional perturbation theory using a mixed-basis pseudopotential method. Complete phonon dispersion curves and Eliashberg functions α²F are calculated for both systems. The main differences are related to high frequency in-plane boron vibrations, which are strongly softened in MgB ₂ and exhibit an exceptionally strong electron-phonon coupling. We also report on Raman measurements, which support the theoretical findings. Implications for the superconducting transition temperature are briefly discussed.