For the first time quasicrystal surfaces produced by in situ cleavage in ultrahigh vacuum have been investigated by scanning tunneling microscopy. Twofold and fivefold surfaces of icosahedral AlPdMn single quasicrystals have been studied. The surfaces were found to be rough. Their structure is determined by cluster aggregates formed on the basis of an elementary cluster whose contrast behavior and diameter of about 1 nm point to the Mackay-type cluster. Crack propagation occurs along zones of lower strength between clusters. This supports the cluster approach to quasicrystal structure.

• Received 7 June 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.3827