Aperiodic crystals: structure, dynamics and electronic properties

• Applicants: Professor Dr. Michael Engel; Professor Dr. Sander van Smaalen; Professor Dr. Wolf Widdra
• Subject Area: Experimental Condensed Matter Physics ; Mineralogy, Petrology and Geochemistry
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 406658237

Aperiodic crystals are materials that possess long-range order but lack lattice periodicity. They comprise incommensurately modulated structures, incommensurate composite crystals and quasicrystals. They are found in a large variety of systems, ranging from the elements to proteins. Although progress has been made in understanding their fascinating structures, the impact of aperiodic order on physical properties is still largely unexplored. The objective of the project is to provide a comprehensive understanding of the relationship between the aperiodic long-range order and physical properties, in particular the interrelation between phonon, phason, and electronic degrees of freedom. Indeed phason modes are diffusive-like excitations that are characteristic of aperiodic order, but whose interrelation with lattice dynamics and electronic properties is still not well understood. To go beyond the existing understanding, a joint effort is needed as proposed here. Six leading teams with complementary experimental and theoretical expertise in the field of aperiodic systems will together focus on the three aspects atomic structure, dynamics and phason modes, as well as electronic properties. Through a study of five well-chosen systems out of the different families of aperiodic systems, we aim at a generalized understanding of aperiodic order. We will employ the new possibilities of large-scale facilities and supplement the experimental study by state of the art atomic scale simulations, using either the DFT approach or model Hamiltonians. The specific systems are: (i) The phosphate tungsten bronzes that form two-dimensional CDW systems with soft phonon modes and phason excitations; (ii) Rb2ZnCl4 as an example of an incommensurately modulated phase, in which, for the first time, phason modes can be studied when the modulation goes from harmonic to strongly anharmonic; (iii) the incommensurate composite crystals [Sr]1+x[TiS3], exhibiting high structural order; (iv) recently discovered two-dimensional dodecagonal oxide quasicrystals, where phason flips can be atomically resolved; and (v) the Tsai-type binary icosahedral quasicrystal family. This ambitious project will gather the competences of three French and three German expert teams. Six PhDs working in close collaboration will tackle this challenging problem. The exchange of PhD students, the joint experiments, and the close cooperation with theory and structure simulations among six French-German expert teams will allow for a coherent approach and ensures broad expertise covering all aspects of aperiodic crystals. It will shine new light on aperiodic crystals and their specific properties that might pave the way also for new applications in fields such as ionic conduction, high-Tc superconductors, electronic and thermal transport, and thermoelectric materials.

• DFG Programme: Research Grants
• International Connection: France
• Partner Organisation: Agence Nationale de la Recherche / The French National Research Agency
• Cooperation Partners: Professor Dr. Marc de Boissieu; Dr. Alain Pautrat; Dr. Olivier Perez; Dr. Pierre Rodière