Abstract
Aside from their fascinating structure with aperiodic long range order and conventionally forbidden rotational symmetries, quasicrystals possess outstanding surface properties. It is of great interest to learn whether or not surface phenomena observed in periodic crystals are relevant in these new class of material. To investigate this, high symmetry surfaces of two distinct types of quasicrystals were examined by various surface sensitive techniques capable of providing information in both real and reciprocal space. The investigated surfaces are the 10-fold and two inequivalent 2-fold surfaces of decagonal (d) Al71.8Ni 14.8Co13.4 and the 5-fold surface of icosahedral (i) Al71.5Pd21Mn8.5. The techniques employed are highly surface sensitive He atom scattering (both elastic and inelastic), high resolution spot profile analyzing low energy electron diffraction, and low temperature (6 K) scanning tunneling microscopy. The investigations focus on structure, morphology, and dynamics (phonons).
It is found that the surfaces can be prepared with high structural quality suitable for He diffraction. Both He and electron diffraction show that the topmost surface layer of the surfaces retains the full symmetry extrapolated from their respective bulk. The diffraction spots are very sharp and their positions are related by the golden mean revealing a perfect long range,quasicrystalline order. The observed diffraction patterns can be fully explained by the respective bulk reciprocal basis vectors.
Helium diffraction from the 10-fold d-Al-Ni-Co surface reveals an average terrace width on the order of 100 Å and a predominant step height of 2 Å. Scanning tunneling microscopy confirms this step morphology by showing a high density of rough 2 Å -steps. Terraces are found to be atomically flat, having a variety of 5-fold symmetric motives, and possessing a quasiperiodic long range order. In agreement with the bulk structure model, a rhombic tiling of a large surface region is identified.
The 2-fold surfaces of the d-Al-Ni-Co show a weak 16 Å periodicity in addition to the main 8 Å periodicity. The 2-fold (001¯10)surface is found to develop facets of (10000)-equivalent orientation due to a lower surface energy of the (10000)- compared to the as-cut (001¯10)-surface.
The first result of surface phonons on quasicrystals is presented. The 10-fold d-Al-Ni-Co and 5-fold i-Al-Pd-Mn surfaces are found to possess well-defined Rayleigh modes with isotropic sound velocity of about 3750 m/s and 3250 m/s, respectively. The observed sound velocities are in good agreement with calculations based on the respective bulk data. | 
