Optimising topological insulators
A new class of materials is ready to take condensed matter systems by storm. TIs behave as insulators internally, while at their surface they form conducting states, resulting in novel states of electronic matter. EU-funding of the 'Topological effects in matter with strong spin-orbit coupling' (TEMSSOC) project enabled scientists to find new ways to create, manipulate and probe topological phases of condensed matter systems. Part of research was geared toward using light to drive ordinary insulators into a topological state. Scientists used graphene to describe two-dimensional TIs as it displays much of the same physics.Its spin-orbit coupling leads to a novel state of electronic matter, known as quantum spin hall state, characterised by conductive states circulating around an insulating bulk. New topological phases were predicted in the presence of strong mechanical strain. Besides studying topological phases, work was also devoted to developing new platforms for TI manufacture. To this end, project members used strained graphene with strong pseudomagnetic fields to create fractional TIs. Furthermore, they investigated TI surface transport properties in the presence of strong warping deformation of their Fermi surface. These turned out to strongly depend on the hexagonal warping amplitude, except for compounds based on bismuth. Terahertz photons were used to probe edge-state helical structure and robustness. When excited at circularly polarised photons, the electron spin adopted the spin orientation imposed by the right- or left-circularly polarised light. This means that the spin could be systematically manipulated, depending on the light that is used. It also delivered important insight into how currents can be induced in TIs. Another way to probe edge modes was to create tunable backscattering by combining external magnetic field and Rashba spin-orbit coupling, controlled by a local gate. TEMSSOC should foster TI study in the EU as these compounds have so far been mostly studied in the United States. Besides condensed matter systems, project findings should also be useful for high-energy and nuclear physics.
