Lattice dynamics of epitaxial strain-free interfaces

We report a systematic lattice dynamics study of the technologically important ${\mathrm{Fe}}_{3}\mathrm{Si}$/GaAs heterostructure for ${\mathrm{Fe}}_{3}\mathrm{Si}$ layer thicknesses of 3, 6, 8, and 36 monolayers. The Fe-partial phonon density of states obtained by nuclear inelastic scattering exhibits up to a twofold enhancement of the low-energy phonon states compared to the bulk material for layer thicknesses of 8 monolayers and below. First-principles calculations explain the observed effect by interface-specific phonon states originating from the significantly reduced atomic force constants and allow for achieving a comprehensive understanding of the lattice dynamics of epitaxial strain-free interfaces.