Cluster Model of Decagonal Tilings

A relaxed version of Gummelt's covering rules for the aperiodic decagon is considered, which produces certain random-tiling-type structures. These structures are precisely characterized, along with their relationships to various other random tiling ensembles. The relaxed covering rule has a natural realization in terms of a vertex cluster in the Penrose pentagon tiling. Using Monte Carlo simulations, it is shown that the structures obtained by maximizing the density of this cluster are the same as those produced by the corresponding covering rules. The entropy density of the covering ensemble is determined using the entropic sampling algorithm. If the model is extended by an additional coupling between neighboring clusters, perfectly ordered structures are obtained, like those produced by Gummelt's perfect covering rules.Comment: 10 pages, 20 figures, RevTeX; minor changes; to be published in Phys. Rev.

Size-effect-like distortions in quasicrystalline structures

We show in this paper that by applying size-effect distortions to a perfect Penrose tiling, on the basis that rhomb-edges which connect different types of vertices assume different lengths, we can obtain a diffraction pattern which shows remarkable similarity to the zero-level (h5 = 0) section observed in decagonal Al71Co13Ni16. In particular a central clearly delineated decagon is observed, on the inside of which there is reduced intensity, and on the outside of which there is enhanced intensity. Such a transfer of intensity is characteristic of size-effect distortions in crystals but for these systems it is necessary to have disorder involving (at least) two types of atoms. In the present case the effect is observed with all vertices occupied by a single scatterer and the system remains topologically equivalent to the Penrose pattern with long range quasicrystallinity

Geometrical Model of the Phase Transformation of Decagonal Al–Co–Ni to its Periodic Approximant

Based on computer simulations in direct as well as in reciprocal space, a geometrical model for the transformation from decagonal Al-Co-Ni to an orientationally twinned crystalline nanodomain structure is derived. Mapping the atomic positions of the quasicrystal onto the corresponding positions of its (4, 6)-approximant leads to a patchwork-like arrangement of crystalline nanodomains. The atomic displacements necessary to transform the quasicrystal into the nanodomain structure are determined locally. The optimum orientation of the approximant unit cells building the nanodomains is obtained by minimizing the sum of the corresponding displacements. Approximately 50% of the resulting atomic shifts are less than 1 Å, and more than 90% less than 1.5 Å. These results are verified by comparison with previous experimental observations. An intermediate state of the transformation is related to a one-dimensional quasicrystal. It is interpreted within the approach of a linear growth model. Slight changes of the approximant lattice parameters as induced by temperature strongly influence domain size and distribution. Correlations between the nanodomains are referred to the discrete periodic average structure common to both the decagonal phase and the approximant structure

Selection of optimized materials for CBRAM based on HT-XRD and electrical test results

Among emerging memory technologies that rely on the bistable change of a resistor, the conductive bridging random access memory (CBRAM) is of particular interest due to its excellent scaling potential into the sub-20 nm range and low power operation. This technology utilizes electrochemical redox reactions to form nanoscale metallic filaments in an isolating amorphous solid electrolyte. Ge chalcogenides are candidate materials for high performance solid electrolytes in combination with Ag as the preferred metal showing high mobility and switching speed. Due to the thermal budget for a back end of the line (BEOL) processing, the layer stack materials must withstand temperatures in the range of 300-450 degrees C. Pure GeS was stable up to 450 degrees C without crystallization. For GeSe, deleterious crystallization was observed. High temperature X-ray diffraction (HT-XRD) and electrical characterization with stepwise annealing were applied to characterize the thermal stability of Ag/GeSe and Ag/GeS material systems. The higher onset temperature for solid-state reactions found with HT-XRD in the Ag/GeS system is the key for the better electrical performance compared to the Ag/GeSe system. Even after thermal annealing with a peak temperature of 300 degrees C, excellent and stable yield numbers of more than 90% for memory elements were achieved for the sulfide, which qualifies this material system for a low temperature BEOL process

Geometrical Model of the Phase Transformation of Decagonal Al–Co–Ni to its Periodic Approximant

Based on computer simulations in direct as well as in reciprocal space, a geometrical model for the transformation from decagonal Al-Co-Ni to an orientationally twinned crystalline nanodomain structure is derived. Mapping the atomic positions of the quasicrystal onto the corresponding positions of its (4, 6)-approximant leads to a patchwork-like arrangement of crystalline nanodomains. The atomic displacements necessary to transform the quasicrystal into the nanodomain structure are determined locally. The optimum orientation of the approximant unit cells building the nanodomains is obtained by minimizing the sum of the corresponding displacements. Approximately 50% of the resulting atomic shifts are less than 1 Å, and more than 90% less than 1.5 Å. These results are verified by comparison with previous experimental observations. An intermediate state of the transformation is related to a one-dimensional quasicrystal. It is interpreted within the approach of a linear growth model. Slight changes of the approximant lattice parameters as induced by temperature strongly influence domain size and distribution. Correlations between the nanodomains are referred to the discrete periodic average structure common to both the decagonal phase and the approximant structure