TOPOSPIN · Scanning tunneling spectroscopy of topological interfaces for future spintronics

Scanning tunneling spectroscopy of topological interfaces for future spintronics

Progress in condensed matter physics is mainly driven by the discoveries of novel material systems which often require new concepts to be theoretically described. For example, discoveries of semiconductor heterostructures, high temperature superconductors and more recently, graphene opened new chapters in modern solid state physics. In this context, topological insulators and related topological phenomena have at least the same potential to reshape the fields of mesoscopic physics and nanotechnology. Due to their inverted band-structure, topological systems exhibit protected boundary states with various highly unique properties. The most pursued example of such topological boundary states are Majorana bound states, potentially the basic building blocks of topological quantum computer. The main goal of the TOPOSPIN project was to experimentally characterize properties of topological boundary states in systems that are believed to be most suitable for future quantum spintronic applications. Particular emphasis was indeed on the realization of Majorana bound states, zero energy excitation expected at the boundary of topological superconductor in one dimension. Variety of topological systems have been explored and in several cases clear signatures related to the topological nature of the boundary states were established. Three major directions explored within the project are listed below. -Establishing novel systems for realization of Majorana Fermions - We have developed a novel approach to realize Majorana bound states in chains of magnetic atoms on the surface of an s-wave superconductor. Our experimental efforts are motivated by model calculations which show that such chains can support topological superconductivity with Majorana end modes. One of the surprising results of the model is that even short chains consisting of only tens of atoms may host well separated Majorana modes provided certain conditions imposed by relative spin orientation of adjacent magnetic spins. Importantly, this system is well suited for spatially resolved studies: chains of magnetic atoms can be be easily assembled on the atomically flat superconducting surfaces and probed using scanning tunneling microscopy - Observing signatures of topological edge modes - Low dimensional topological systems are especially important for the field of quantum spintronics. We have studied Bismuth-bilayers, promising two-dimensional topological material and by means of scanning tunneling spectroscopy we have observed one-dimensional electronic edge states. On the material side the focus was on the Bi4Se3 as well as bulk Bi crystals. Careful comparison between experimental data and theoretical modeling was used to establish topological properties of these edge states. -Electrically tunable topological systems - In the return phase of the project we were focused on InAs/GaSb quantum well based two dimensional topological insulators. Due to their double-layered structure with electron and hole gases separated in two layers these topological system were expected, in theory, to have rich trivial-topological phase diagram. During the TOPOSPIN project we have, for the first time, achieved an in situ and continuous tuning between the trivial and topological insulating phases in this material by the means of electrical dual-gating. Furthermore it was established that magnetic field can be used to differentiate between topological and trivial phases in this system. Impact of the project The initial scientific goals of the TOPOSPIN project were focused on establishing of topological properties in interfaces between materials with inverted band topology and superconductors and ferromagnets. During the project various novel systems were explored and arguably the main scientific objectives were achieved the large degree. In addition, the objectives related to the transfer of knowledge activities, publication record, and other specified deliverables were overall completely fulfilled. The TOPOSPIN project was used to initiate at least one major research direction in the field of topological and quantum related phenomena (Magnetic atomic chains on the surface of a superconductor as a platform for Majorana bound states). The project also had a significant role in helping the fellow to obtain and independent professorship position in one of the major USA universities - California Institute of Technology (CALTECH). Furthermore it is to be expected that previously fruitful collaboration will be continued in the future between Delft University of technology, Princeton University and CALTECH.

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