Inelastic-neutron-scattering measurements of phonons in icosahedral Al-Li-Cu

Inelastic-neutron-scattering measurements on a single grain of icosahedral Al-Li-Cu are presented. Within experimental error the acoustic-phonon dispersion at small, but finite, q is independent of direction, confirming the isotropic elasticity expected for icosahedral crystals. By following the longitudinal-phonon dispersion curve along a fivefold axis we can identify the location of ``quasi-Brillouin-zone boundary.'' The measurements also suggest the presence of an optical or localized mode along the twofold axis at approximately 10--11 meV. We compare these results to theoretical calculations for one- and two-dimensional quasiperiodic systems.