Revealing 1d ballistic charge and spin currents in second order topological insulators
One of the greatest recent achievement in Condensed matter physics is the discovery of a new class of materials, Topological Insulators (TI), whose bulk is insulating, while the edges conduct current in a quasi-ideal way. In particular, the 1D edges of 2DTI realize the Quantum Spin Hall state, where current is carried dissipationlessly by two counter-propagating ballistic edge states with a spin orientation locked to that of the propagation direction (a helical edge state). This opens many possibilities, ranging from dissipationless charge and spin transport at room temperature to new avenues for quantum computing. We propose to investigate charge and spin currents in a newly discovered class of TIs, Second Order Topological Insulators (SOTIs), i.e. 3D crystals with insulating bulk and surfaces, but perfectly conducting (topologically protected) 1D helical “hinge” states. Bismuth, despite its well-known semimetallic character, has recently been shown theoretically to belong to this class of materials, explaining our recent intriguing findings on nanowires. Our goal is to reveal, characterize and exploit the unique properties of SOTIs, in particular the high velocity, ballistic, and dissipationless hinge currents. We will probe crystalline bismuth samples with refined new experimental tools. The superconducting proximity effect will reveal the spatial distribution of conduction paths, and test the ballisticity of the hinge modes (that may coexist with non-topological surface modes). High frequency and tunnel spectroscopies of hybrid superconductor/Bi circuits will probe their topological nature, including the existence of Majorana modes. We will use high sensitivity magnetometers to detect the orbital magnetism of SOTI platelets, which should be dominated by topological edge currents. Lastly, we propose to detect the predicted equilibrium spin currents in 2DTIs and SOTIs via the generated electric field, using single electron transistors-based electrometers.