Synthetic Spin-Orbit Coupling
There is a strong current interest in realizing topological superconductivity (TSC) with super-semi nanowires. Apart from the fundamental interest in topological matter, qubits based on excitations could be extremely well-protected against noise-induced dephasing. So far, the study has focused on topological insulators or III-V materials with strong spin-orbit interaction coupled to s-wave superconductors. Experimentally, the presence of TSC is expected to enhance the zero-bias conductance above a critical magnetic field. However, the requirement for strong B-fields places restrictions on the superconductor that can be used - the requirement for strong spin-orbit interaction is only met by a handful of III-V materials such as InAs and InSb, and only very thin layers of epitaxially-grown superconductor are compatible with the magnetic fields required to reach the topological phase. More recent theories have shown that the requirement for strong intrinsic spin-orbit interaction can be relaxed if this can be engineered using spatially varying external magnetic fields, for example using arrays of magnets of submicron size - opening up the field to a whole new set of cleaner material platforms such as Si or GaAs, or reducing the fields required to reach the topological phase in conventional III-V and V-VI materials. More generally, the ability to use nanomagnets to control spin-orbit interaction – or simply to create a spatially varying magnetic field - would be an important resource in the field of spin-based quantum information processing.
Magnetic Topological Insulators
Magnetic topological insulators in the quantum anomalous Hall regime host ballistic chiral edge channels. When proximitized by an 𝑠-wave superconductor, these edge states offer the potential for realizing topological superconductivity and Majorana bound states without the detrimental effect of large externally applied magnetic fields on superconductivity. Realizing well-separated unpaired Majorana bound states requires magnetic topological insulator ribbons with a width of the order of the transverse extent of the edge state, however, which is expected to bring the required ribbon width down to around 100nm. In this regime, it is known to be extremely difficult to retain the ballistic nature of chiral edge channels and realize a quantized Hall conductance. We are interested in imaging magnetically-doped V-VI compounds to observe how spatial distribution of currents as two-dimensional surface states evolve into the one-dimensional edge channels. Capturing images at different carrier density and magnetic field should reveal precisely how this transition takes place, and by comparing with topography we can understand how it is affected by ionised surface and bulk impurities, edge disorder, and variations in crystal morphology (see MAGMA for more details).