Scanning tunnelling microscopy with atomic resolution on the twofold surface of the icosahedral AlPdMn quasicrystal

Abstract

We have investigated the 2-fold surface of an icosahedral Al₇₀Pd₂₁Mn₉ quasicrystal by means of low-temperature scanning tunnelling microscopy (77 K). The sequence of the terrace step edge heights revealed a Fibonacci sequence with S = 2.40 Å and L = τS = 3.88 Å as fundamental terrace step heights. S and L can be related to the pentagon edge length of the below terrace Bergman polytope and the surface dissected Bergman polytope of the 5-fold surface. The atomic resolution STM images of the 2-fold surface have been compared to the M-model structure of the i-AlPdMn and the experimental STM surface structure of the 5-fold surface by analysis of the FFT spots. We could show that the positions of the FFT spots in the 2-fold surface are related to the FFT spots of the 5-fold surface by projections onto the fivefold axis and are in good correspondence to the model structure. We identified characteristic rectangular features on the 2-fold surface with edge length of 4.8, 7.7 and 12.6 Å corresponding to D₂, τD₂ and τ ²D₂ with D₂ being the pentagon edge length of the surface dissected Bergman polytope. We interpret our results as being a strong indication for the bulk termination of the 2-fold surface of the i-AlPdMn quasicrystal.

Acknowledgments

We would like to thank M. Feuerbacher and his group for providing us with high quality QC crystals. This work was partially supported by the European Community through the SMART Quasicrystal Project No. G5RD-CT-2001-00584, whose financial support is greatly appreciated.