TEMSSOC

TOPOLOGICAL EFFECTS IN MATTER WITH STRONG SPIN-ORBIT COUPLING

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 Obiettivo del progetto (Objective)

'This theoretical project addresses novel quantum phases of matter whose low energy properties are dominated by topological excitations: Majorana fermions in topological insulators, skyrmions in chiral magnets and magnetic monopoles in pyrochlore magnets. It is motivated by the recent experimental discovery of topological insulators (HgTe/CdTe,Bi1-xSbx or Bi2Se3), the mounting evidence for skyrmions in the chiral magnet MnSi (anomalous Hall effect measurements in 2009) and the prediction of magnetic monopoles in spin ice (by Roderich Moessner and co-workers in 2008). During the outgoing phase in UC Berkeley, we shall consider the interplay of interaction and spin-orbit effects in topological insulators both in bulk materials and in their heterostructures. The fellow researcher has previously studied heterostructures combining superconductors and ferromagnets. Majorana fermions are expected to appear when such heterostructures are deposited on top of a three-dimensional topological insulator. Moreover A. Vishwanath (outgoing host researcher) initiated the research on spin crystals and skyrmions in chiral magnets. During the return phase in MPIPKS Dresden, we shall investigate the properties of the magnetic monopoles predicted by Roderich Moessner (scientific in charge of this project) in spin ice (Dy2Ti2O7,Ho2Ti2O7). The mechanisms of the anomalous Hall effect in itinerant pyrochlore ferromagnets (Nd2Mo2O7, Pr2Ir2O7) will also be considered. Finally the present project will foster the study of topological insulators in the EU community, while these compounds have been almost exclusively studied in the USA so far. The present project is also relevant for other branches of physics since, in spite of a longstanding and on-going hunt, Majorana fermions, skyrmions and monopoles are still to be found in high-energy and nuclear physics.'

Introduzione (Teaser)

Topological insulators (TIs) are considered a very promising material class for developing novel computing devices. EU-funded scientists demonstrated new methods to tune their unique properties by using light or strain.

Descrizione progetto (Article)

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.