New research into topological insulators could lead to quantum computers
Topological insulators are a newly discovered class of materials that behave as insulators in their interior, but whose surfaces contain conductive states at interfaces with other insulators. Surface states of topological insulators have an intriguing property: the direction of electron motion is inextricably linked to the spin orientation. These protected conducted states can break if electrons flip their spin. A benefit of this spin-selective behaviour is that it prevents a phenomenon called backscattering. Imperfections that would normally ruin the electronic properties of the material have little effect. However, topological surfaces retain dissipationless spin current and protection from backscattering across distances of a few microns. Backscattering mechanisms over longer distances that have been reported on mercury telluride (HgTe) remain unclear. To help with this, scientists initiated the project MAGNETOP (Probing the effect of time reversal symmetry breaking by the application of a local magnetic field in topological insulators). Work was geared towards investigating the effect of small magnetic fields on scattering, making sure to distinguish contributions between electric field fluctuations and local magnetic fields. Using scanning gate microscopy, the team probed electron transport and scattering at HgTe quantum wells. Results should not only enhance understanding of scattering mechanisms in quantum spin Hall states, but also demonstrate how edges interact with carriers in the bulk material as this becomes conductive. Scientists also investigated the effect of confining carriers to a Fabry-Pérot cavity in HgTe quantum wells. Bipolar junctions were created combining the effect of back-gate and top-gate voltage. The resulting quantum interference was detected in transport measurements. Results should provide further insight into the role of confinement in the edge states and their interaction with bulk carriers in both the absence and presence of a small magnetic field. In addition, results can be used to study the interplay between quantum Hall and quantum spin Hall effects in the presence of high magnetic fields. Experiments conducted by scanning microwave impedance microscopy on thin samples in high magnetic fields demonstrated unexpected conduction at edges. These results challenge existing views that high magnetic fields break the protected conducted states. MAGNETOP research on the behaviour of edge states under magnetic fields should help better understand electron scattering mechanisms. Manipulating magnetic fields on those states is important for future electronic circuits and transistors. All project results have been published in peer-reviewed journals.
Keywords
Topological insulators, quantum computers, magnetic fields, scattering, MAGNETOP, quantum spin Hall
