Two-Dimensional Organic Crystals with High Spin-Orbit Interaction – ORGANI’SO

Symmetry is a far-reaching concept in view of the generation of novel classical and quantum phases. Playing with the crystal and time (a)symmetries, as can be done in crystalline substances in the presence of a sizable spin-orbit interaction, has focused a great deal of research in the past years, which has put strong emphasis on the topology of the electronic band structure of novel materials. Topological insulators (TIs) are such materials, characterized by a dissipation-less electronic conduction at their edges, and in principle an insulating behavior in their interior. In these compounds, most of which were previously known as good thermoelectrics (TEs), the strong spin-orbit coupling is brought by high atomic number elements. The prospect for applications based on them, in the fields of spintronics, micro-electronics, metrology, and thermoelectricity, raises the key questions of their production cost and eco-friendliness. The available inorganic materials are not well-positioned in these respects. Our proposal consists in transposing the concept of a TI to crystalline organic materials, which will meet the future technological requirements for low-cost and eco-friendly materials, besides being flexible. The project targets unambiguous signatures of an organic topological insulator (OTI), and proof-of-principle meso- and nano-devices for electronic and thermal transport. The project also targets original insights into the unique quantum spin Hall state, by means of high resolution spin-sensitive imaging. The chosen strategy is strongly multi-disciplinary, and covers a broad range from materials science, to surface physics, electronic device physics, and computational chemistry and physics. The consortium gathers experts from three leading academic research laboratories, bringing complementary approaches and backgrounds. Institut FEMTO-ST will lead the chemistry side of the work-plan. Institut Néel will be in charge of combined structural, vibrational, and electronic transport studies, and will also conduct the theoretical work. IPCMS will perform low-temperature microscopy and spin-polarized spectroscopy.

Novel covalent organic networks will be prepared following innovative chemistry routes implemented in solution, yielding systems with good structural ordering across tens of nanometers, subsequently dispersed on different kinds of surfaces, for the purpose of structural and spectroscopic microscopies and electronic transport devices. Scanning tunneling microscopy and spectroscopy, with and without spin-polarized tips, will allow for the first high resolution combined mapping of the electronic and spin texture of the OTIs. The robustness of the quantum edge states will be probed by exploring the role of defects of various kinds. Electronic transport in OTI will be probed through ribbons fixed between the microscope tip and the support, which will give the opportunity to understand the influence of bending on electronic transport. Thermocurrents will be studied in devices prepared by nano-fabrication techniques, and rationalized in terms of the vibrational properties, which will be addressed by Raman spectroscopy. The experimental results will be interpreted in the light of spin-polarized density functional theory calculations and tight-binding modeling. Continuous feedback between experiments and theory will provide an efficient means to widen the understanding of the formation and physical properties of OTIs.