AVS2001 Session SS1-ThM: Quasicrystals

Thursday, November 1, 2001 8:20 AM in Room 121

Thursday Morning

Start	Invited?	Item
8:20 AM	Invited	SS1-ThM-1 Peter Mark Memorial Award Lecture - Surface Preparation and Electronic Properties of d-AlNiCo E. Rotenberg (Lawrence Berkeley National Laboratory); W. Theis, K. Franke (Free University, Germany); K. Horn (Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany); P. Gille (Maximilians-Universitat Munich, Germany) Apart from their striking structural properties, i.e. perfect long range order without translational symmetry, quasicrystals exhibit fascinating physical properties unlike those of ordinary metallic alloys. As the properties of all metals are determined by the valence states near the Fermi level, we have undertaken a program to prepare clean quasicrystalline surfaces and characterize these states by angle-resolved photoemission. The samples were characterized by ESCA, xray photoelectron diffraction (XPD) and LEED. Our k-dependent photoemission intensity maps from the tenfold and two inequivalent twofold surfaces from d-Al_71.8Ni_14.8Co_13.4 shows the existence of dispersing s-p- and d-derived valence states. We use an extension of the concept of the Brillouin zone in order to characterize the observed dispersion. Maps from all three surface orientations yield a consistent picture of free- electron-like constant energy surfaces in 3D k-space. We find the centers of these energy surfaces to be located at the positions of dominant electron diffraction spots as expected from the quasi-Brillouin zone model. Dispersion arises from delocalized quasicrystal valence states, while a large non-dispersing intensity in our spectra could be attributed to either localized states or scattering from the hierarchy of densely spaced Bragg planes in quasicrystals.
9:00 AM		SS1-ThM-3 Structure, Morphology and Dynamics of High Symmetry AlNiCo-Surfaces W. Theis, H.R. Sharma, K. Franke, A. Riemann, S. Fölsch, K.H. Rieder (Freie Universität Berlin, Germany); P. Gille (Ludwig-Maximilians-Universität München, Germany) Although many aspects of the bulk structure and dynamics of decagonal quasicrystal are well understood, knowledge regarding their surface structure is much more limited. Employing highly surface sensitive elastic He-atom scattering (HAS) and high resolution low energy electron diffraction (SPA-LEED), we have investigated 10- and 2-fold surfaces of Al_71.8Ni_14.8Co_13.4. The observed SPA-LEED and HAS diffraction patterns reveal that the top most surface layer retains the full symmetry extrapolated from the bulk. Both SPA-LEED and HAS show weak superstructure peaks. In the diffraction patterns of the 2-fold surface we observe extremely weak streaks corresponding to a 16 Å periodicity in addition to the strong peaks reflecting the main 8 Å periodicity. The spots in the odd lines of the diffraction pattern are shifted with respect to the even lines by 0.5 Å^-1. He-atom scattering from the 10-fold surface reveals a low corrugation, extremely few defects, an average terrace width in the order of 100 Å, and a predominant step height of 2 Å. Low temperature (5 K) scanning tunneling microscopy (LT-STM) confirms this step morphology showing a high density of rough 2Å-steps without any tendency towards step pairing and revealing regions of two different surface termination and repeating 5-fold symmetric motives. Surface phonons of the 10-fold surface have been investigated by inelastic He-atom scattering. In experiments with cooled and heated samples and He-atom kinetic energies ranging from 8 to 35meV the (acoustic) Rayleigh mode was observed. Strong single phonon signatures are limited to wavevectors q < 0.5 Å^-1 and the phonon linewidths are resolution limited up to q = 0.25 Å^-1, beyond which they increase rapidly, similar to the behavior recently observed in the corresponding bulk phonons. ¹ ¹ F. Dugain, et al, Eur. Phys. J. B. 7, 513 (1999).
9:20 AM		SS1-ThM-4 Tribological Properties of Quasicrystals and Quasicrystal Approximants C. Mancinelli (Carnegie Mellon University); J.S. Ko (Merck & Co.); A.J. Gellman (Carnegie Mellon University) An experimental comparison has been made between the properties of the surfaces of an Al₇₀Pd₂₁Mn₉ quasicrystal and its Al₄₈Pd₄₂Mn₁₀ approximant. This investigation has attempted to identify connections between the quasicrystalline structure and bulk properties. The Al₇₀Pd₂₁Mn₉ sample was a single grain icosahedral quasicrystal cut to expose its five-fold symmetric (000001) surface. The approximant was polycrystalline β-phase Al₇₀Pd₄₂Mn₁₀, which has a CsCl-type cubic structure. Surfaces of both were prepared under ultra-high vacuum (UHV) conditions and then used for comparative measurements of their frictional properties and oxidation rates. The aluminum based quasicrystals and their approximants are oxidized by reaction with O₂ to form a thin film of aluminum oxide that ultimately passivates their surfaces. The oxidation process was studied using Auger Electron Spectroscopy to measure the oxide film thickness following controlled exposures to both O₂ and H₂O. The interesting difference between the two samples is that the rate of oxidation of the approximant is significantly higher than that of the quasicrystal in spite of the fact that the bulk Al concentration of the approximant is lower than that of the quasicrystal. Friction measurements were made under UHV conditions between pairs of quasicrystals and pairs of approximants whose surfaces were either clean or oxidized to varying degrees. The friction measurements were made between pairs of clean surfaces and between pairs of surfaces exposed to controlled oxidation. The friction between pairs of the approximant surfaces is roughly twice that measured between the quasicrystal surfaces under all conditions of surface oxidation. The results of this work do suggest that there is a direction connection between the quasicrystalline structure and low friction.
9:40 AM	Invited	SS1-ThM-5 Surface Properties of Quasicrystals P.A. Thiel (Iowa State University) There has been a systematic progression of knowledge, over the past 10 years, concerning the atomic arrangements and compositions at surfaces of bulk quasicrystals. Our group has been particularly involved in studying surfaces of the icosahedral aluminum-rich phases, both in ultrahigh vacuum and in air. The understanding of the surface structure of these materials is currently at a point where, we believe, it can be used as a springboard toward understanding more complex phenomena. Following a review of the picture that has evolved for the native surfaces in vacuum and in air, we will discuss more recent results for friction, wetting, and epitaxy.
10:20 AM		SS1-ThM-7 Auger Spectroscopy and the Electronic Structure of Quasicrystals V. Fournèe, J.W. Anderegg, A.R. Ross, T.A. Lograsso, P.A. Thiel (Iowa State University) So far, the electronic structure of QC has been probed primarily by photoelectron and x-ray spectroscopies. These studies mainly focussed on the existence of a structure induced minimum of the density of states located at the Fermi level, the so-called pseudogap. Auger spectroscopy, on the other hand, can yield unique insight into important aspects of the electronic structure of alloys such as screening and electron correlation effects. Here we will present our experimental work on the transition metals LMM Auger lines in several quasicrystalline phases, the Al-Pd-Mn and Al-Cu-Fe icosahedral QCs and the Al-Ni-Co decagonal QC. This include the core-core-valence and the core-valence-valence Auger spectra. From the line shape, line splitting and energies of the Auger spectra, it is possible to derive empirical parameters correlated with the extent of quasiatomic or bandlike behavior, which depends on the relationship between the local DOS and the value of the on-site correlation energy. Our first results in the i-Al-Pd-Mn clearly shows that the Mn-L3M45M45 has a bandlike behavior and reflects the self-convolution of the Mn-3d valence band. The width of the Mn-3d band is quite narrow in the QC, as compared to pure Mn. We also found a decrease of the 3d-containing Auger transition rates in the QC relative to the metal. This could be due to an increased localization of the 3d states in the quasicrystals, that will lower the probability for a 3d electron to fill a 2p photohole. This is consistent with the reduction of the intrinsic width of the Mn 2p3/2 core level observed by photoemission spectroscopy.
10:40 AM		SS1-ThM-8 Nucleation and Growth of Al and Ag Thin Films on Five-fold Surface of Icosahedral Quasicrystals T. Cai, V. Fournee, T.A. Lograsso, A.R. Ross, P.A. Thiel (Iowa State University) Aluminium and silver are deposited on the five-fold surface of icosahedral Al-Cu-Fe and Al-Pd-Mn quasicrystals. The nucleation and growth of thin metal films on quasicrystalline surface are studied using scanning tunnelling microscopy. Our first results on Al films deposited on Al-Cu-Fe show that, at low coverage, Al atoms form pentagonal stars, all adopting the same orientation, though with no evidence of order between the Al pentagonal stars. As Al coverage is increased, but still within the submonolayer regime, the density of Al pentagonal stars increases. The five-fold symmetry of the quasicrystalline substrate is responsible for arranging deposited atoms with a local five-fold symmetry, which is incompatible with the translation periodicity of the cubic lattice of the individual elements. With scanning tunnelling microscope, we expect to observe directly the transition from a pentagonal symmetry to the face-centred cubic symmetry as the metallic film develops on the quasicrystalline surface.
11:00 AM		SS1-ThM-9 Accessing the Role of Net Planes in Decagonal Al-Ni-Co C.J. Jenks, J. Bjergaard, P.A. Thiel, P. Canfield (Iowa State University); A. Cervellino, W. Steurer (ETH-Zurich, Switzerland) Net planes linking the periodic and quasiperiodic directions in decagonal Al-Ni-Co are discussed. These net planes are thought to play a critical role in the stability and growth of decagonal quasicrystals. To explore their potential roles we have studied single grain surfaces of decagonal Al-Ni-Co. We have used surface analytical techniques to study surface morphology which sheds light on the structural stability of these planes.