Our starting point is the discovery that electron waves in a metal can have an internal twist. The unusual properties of topological matierals were recognized by the 2016 Nobel prize in physics, and may open new ways of encoding and processing information in the future. Electron waves can be twisted in space, distinguishing them well from untwisted ones, and similar to knots on a string such twists are thought to be very robust. These electron waves are clearly of different character than those in ordinary metals. The natural question of this young research field is whether or not this difference in wavefunctions leads to distinct physical properties, and if these properties can be useful in technology. In our project MiTopMat, we develop new processes to build prototype devices from conductors that host these topological electron waves, so called Dirac/Weyl semi-metals. These new materials exist only in small particles so far, and new concepts to fabricate them into functional devices are critical. MiTopMat develops a new approach based on focused ion beam milling. Here, an ion beam is focused onto a nanometric spot on the particle, and the impacting ions very locally cut the target material. In a way quite similar to carving a figurine out of wood, we cut away most of the target particle until only a precise microdevice in the desired shape remains.
The main goal we address here is to test these ideas of using topological materials in applications. Can we realize a microdevice in which electrons, due to their topological nature, do something radically different compared to copper or silicon, which are normal, non-topological metal? If this can be robustly demonstrated, naturally new technologies can emerge that exploit these differences. To this end, we screen multiple materials in which topological electrons have been predicted and compare their behavior in microdevices. A key distinction is a principle called bulk-surface correspondence that inherently links electron waves on the surface and deep inside the bulk. We could show that magnetic fields can be used to shift electron charge from the bulk into the surface and back, which is a first signature of an electron process that could not happen in silicon.
MiTopMat was able to gather key, pioneering data in the field of topological microelectronics, and we found some remarkable electronic responses never observed before. Our data now serves as critical input to the refinement of our theoretical modeling, as it showed that we have to advance our models of topological transport to incorporate the complexity and richness of real materials.
