Structure and electronic structure of quasicrystalline surfaces

Dusanka Naumovic, DÈpartement de Physique, PÈrolles, UniversitÈ de

Fribourg, 1700 Fribourg, Switzerland.

The structure and electronic structure of different low-index surfaces of either quasicrystalline or approximant Al-Pd-Mn were studied by means of photoemission-based techniques such as X-ray photoelectron diffraction and ultraviolet photoelectron spectroscopy. We find that the two-, three- and five-fold surfaces of icosahedral Al-Pd-Mn exhibit all the symmetry elements of the icosahedral non-crystallographic group within the probed depth of approximately 20≈ (mean free path of photoelectrons). These surfaces can be modeled by single-scattering cluster calculations. Nevertheless it is difficult to distinguish between the pseudo ten-, or two-fold surfaces of an Al-Pd-Mn approximant and the corresponding faces of a decagonal quasicrystal. The icosahedral surfaces are characterised by a lowering of the density of states close to the Fermi edge, compatible with the opening of a pseudogap, as expected for a quasicrystal.

As the experiments are performed in ultra high vacuum (UHV), the samples are cleaned in situ by sputter/anneal cycles. The icosahedral or approximant surfaces are recovered after annealing at about 500-600∞C and are supposed to be bulk-like terminated, i.e. to have the same structure as the bulk. For annealing temperature lower than about 400∞C, ordered crystalline surfaces with bcc domains are formed. For temperatures above 600∞C, Mn or Pd enrichment of the surface is observed, corresponding to the formation of, respectively, a decagonal quasicrystalline overlayer or a crystalline layer with bcc domains. The (low-temperature) crystalline overlayers are characterised by a sharp Fermi edge, while the approximant and decagonal quasicrystalline surfaces have a lowered density of states.