Periodic Reporting for period 4 - MiTopMat (Microstructured Topological Materials: A novel route towards topological electronics)
Reporting period: 2022-06-01 to 2023-03-31
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.
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.
Methodically, we have made significant advances in the microfabrication of complex materials. One outcome of MiTopMat is a standardized flow process to fabricate high-quality hybrid nanodevices from smallest samples of chemically complex materials. This versatile approach is already accelerating materials testing today. Forced by the pandemic to readjust some goals to the new reality, our team developed an innovative approach to obtain free-standing microstructures. These structures are cantilevered off the edge of a silicon chip, and can be controllably bent and distorted in a cryostat, in order to study the role of bending distortions on topological materials. This is most insightful as a) topological properties are thought to be robust and bending distortions are an ideal way to probe this; and b) recent theories predicted a unique behavior of topological electrons in bent crystals that has close similarities of that under extreme magnetic fields - much stronger than we have available in the laboratory. Bending 3D crystals was a difficult challenge, until now. Focusing on the Dirac semi-metal Cd3As2, our team demonstrated how transport on the quantum level is affected by extreme bending - with the remarkable observation that it sustains massive bending levels with minimal change.
We further searched for new materials beyond the previously known Cd3As2 for topological transport, with a focus on (Ta,Nb)(As,P). Our fabrication enriches the elemental superconductor Niobium at the surface, which paves a new route towards topological superconducting materials - a highly sought-after property in the field of quantum computation. Yet we also made a few important discoveries in other materials: The magnetoresistance of PdCoO2, when cut into microscopic bars, oscillates as a function of the applied magnetic field. These oscillations are directly linked to the wave-like nature of electrons. Surprisingly, this phenomenon persists up to temperatures of T~60K, which could lead to new atomic scale magnetic field sensors we currently investigate. Further, we found a field-switchable diode in CsV3Sb5, a Dirac semi-metal that hosts a corner-sharing triangular lattice of Vanadium. Under high magnetic fields we used to probe it, the material suddenly conducted electric currents better in one direction than in its reverse - a property known as a diode. The remarkable part here is that the forward direction, in which the material conducts well, can be switched by the magnetic field. This entirely novel response highlights the new functionalities to expect from advanced materials.
We further searched for new materials beyond the previously known Cd3As2 for topological transport, with a focus on (Ta,Nb)(As,P). Our fabrication enriches the elemental superconductor Niobium at the surface, which paves a new route towards topological superconducting materials - a highly sought-after property in the field of quantum computation. Yet we also made a few important discoveries in other materials: The magnetoresistance of PdCoO2, when cut into microscopic bars, oscillates as a function of the applied magnetic field. These oscillations are directly linked to the wave-like nature of electrons. Surprisingly, this phenomenon persists up to temperatures of T~60K, which could lead to new atomic scale magnetic field sensors we currently investigate. Further, we found a field-switchable diode in CsV3Sb5, a Dirac semi-metal that hosts a corner-sharing triangular lattice of Vanadium. Under high magnetic fields we used to probe it, the material suddenly conducted electric currents better in one direction than in its reverse - a property known as a diode. The remarkable part here is that the forward direction, in which the material conducts well, can be switched by the magnetic field. This entirely novel response highlights the new functionalities to expect from advanced materials.
The crystalline microdevices developed in MiTopMat push the technical boundaries of how we can shape electronic devices in complex materials. We have developed workflows and procedures that standardize this time-consuming prototyping process and greatly enhance the efficiency. While initially every microstructure was a unique project, using the methods of MiTopMat now turns microstructure characterization into a simple and standard approach allowing us to quickly screen multiple materials. Taking a step back, these results and novel methodologies showcase advanced electronic functionality that will benefit our society when they are introduced into actual technology. The vision is that advanced functionality can be achieved efficiently and at low power in a novel material which today requires us to emulate such response by the design of a digital circuit using standard materials. We know very well how to fabricate silicon and copper, and made amazing progress with it, however it is abundantly clear that progress in this area is exhausted. The beyond-silicon age is coming up, yet we do not know which materials class will define the next section. MiTopMat contributed to this search by its investigations of topological electronics on hybrid chips directly; but at the same time it also developed methods and tools to accelerate materials investigations in the future.