Molecular Dynamic Studies of Atomic Jumps in d–Al-Cu-Co Dietmar Bunz, Gabriele Zeger, Johannes Roth, Martin Hohl, Hans-Rainer Trebin Institut f¨ ur Theoretische und Angewandte Physik, Universit¨at Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany Abstract The structure of quasicrystals permits phason flips: atoms may jump to equivalent alternate positions. The jumps represent the discrete representation of the phason strain in the continuum description. We have studied jumps in one of the Burkov models for decagonal Al-Cu-Co quasicrystals in molecular dynamics simulations by direct detection and with the help of correlation functions. In the decagonal structures we find correlated jumps of two atoms within a layer. These are equivalent to flips predicted in a tiling description for the model. We will also give an overview of correlated atomic motions in different quasiperiodic layers. Keywords: Molecular dynamics; Quasicrystals; Atomic jumps; Correlation functions 1. Introduction Describing a quasicrystal by the atomic decoration of a quasiperiodic tiling, phason motion becomes evident by a reshuffling of certain local tile configu- rations, along with their decoration. Katz and Ka- lugin suggested that these phason flips lead to a new mode of self-diffusion [1]. Triggered by the paper many groups have focused their interest on this sub- ject both experimentally and theoretically [2, 3, 4]. To study atomic flips in quasicrystals Coddens et al. performed neutron scattering experiments first on i- Al-Pd-Mn and i-Al-Cu-Fe and most recently on a d-Al-Ni-Co quasicrystals [5, 6, 7]. They can explain the temperature dependence of line widths and peak heights only by assuming the existence of correlated simultaneous jumps of several atoms, as is expected for phasons [8]. Flip diffusion has often been studied in pure tiling models [9, 10], but for many quasicrys- tal models such as the binary icosahedral model [11] it is not exactly known what phason flips look like [12]. Roth has shown that molecular dynamics (MD) simulations are a good tool to study atomic jumps in icosahedral quasicrystals [13]. We therefore inves- tigated the specific atomic jumps in decagonal Al- Cu-Co by means of MD simulations using one of the Burkov models (BII) [14], since for this model the atomic motions in phason flips are well known [15]. 2. Model and Simulations The BII model, which is based on the atomic de- coration of a T¨ ubingen triangle tiling, consists of two plane decagonal quasiperiodic layers, which are alternatively stacked along a perpendicular tenfold screw axis with a stacking period of 4.18 ˚ A. A binary approximant of this model in projection both along and perpendicular to the decagonal axis is shown in Fig. 1. The most prominent motifs are rings, which are formed by ten atoms in projection along the deca- gonal axis (Fig. 1 left). The midpoints of all atomic ten-rings are left empty in the BII model, whereas in Burkov model I (BI) [14] one Al atom is placed in the center of those atomic ten-rings, which consist of both Al and transition metal (TM) atoms. The smallest distances within the layers are 2.46 ˚ A for Al-Al and Al-TM and 2.89 ˚ A for TM-TM. The pha- son flip of the BII model is shown in Fig. 2 left for a single layer in the tiling description [15]. During each such a flip two atoms, at least one of them an Al atom, have to move correlated. If among them there are two Al atoms involved, as in Fig. 2, the jump length is 0.94 ˚ A, which is smaller than the interato- mic distances within the layers of the BII model. The MD simulations were carried out with the pro- gram package IMD which is suitable for massively parallel computers [16]. In the simulation we took