Tunneling systems and disorder in quasicrystals

Atomic tunneling has been found in icosahedral ribbons of ${\mathrm{Al}}_5$ ${\mathrm{Li}}_3$Cu and ${\mathrm{Mg}}_3$ ${\mathrm{Zn}}_3$ ${\mathrm{Al}}_2$, as well as in a single-grain equilibrium quasicrystal of ${\mathrm{Al}}_{5.1}$ ${\mathrm{Li}}_3$Cu, from measurements of the low-frequency sound velocity and damping below 1 K. The density of states of tunneling systems depends on the sample preparation and correlates with the amplitude of phason strain in the samples. The extent of disorder in quasicrystals is appreciably closer to that of crystals than to metallic glasses.