Scanning tunneling microscopy of an Al-Ni-Co decagonal quasicrystal

Abstract

The structure of an Al-Ni-Co decagonal (d-) quasicrystal has been investigated by scanning tunneling microscopy (STM). STM images with atomic-scale resolution have been obtained successfully for the surfaces of both tenfold and twofold planes. On the tenfold surface, large terraces and monoatomic-layer steps are formed. The symmetry of each layer is not decagonal but pentagonal and the two adjacent layers are related by the inversion symmetry. The step lines are rough, which can be attributed to the existence of many symmetrically equivalent low-energy steps. The atom adsorptions are often observed at locally symmetric sites. An analysis based on the high-dimensional description of the quasicrystalline structure has shown that the structure has nearly perfect quasiperiodic order for the decagonal quasicrystal. On the twofold surface, interlayer phason defects are observed, but the density of them is quite low. This fact indicates that the d-quasicrystal of the present sample is not in the random tiling state in which the configurational entropy related to the phason disorder stabilizes the quasicrystal.