Topological particles in condensed matter
Topological insulators are an extraordinary class of materials. One fascinating aspect is that they behave like insulators in the bulk, but also conduct electricity on their surface. Moreover, these materials in the proximity of a superconductor show excitations that satisfy non-commutative statistics, the so-called Majorana fermions that could be used for quantum computing. Researchers working on the TOPOSPIN (Scanning tunneling spectroscopy of topological interfaces for future spintronics) project explored a variety of topological insulators. Their ultimate aim was to identify condensed matter systems where Majorana fermions can be detected at their outer boundaries under a scanning tunnelling microscope. Majorana fermions behave simultaneously like matter and antimatter. Their conflicting properties render these exotic particles neutral and minimise their interaction with the environment. This 'aloofness' spurred TOPOSPIN researchers to search for ways to engineer Majorana fermions at the edge of a one-atom-thick wire – just where they have been theoretically predicted to be. TOPOSPIN researchers investigated the conditions under which a chain of magnetic atoms on the surface of a superconductor can host Majorana fermions. One of the surprising results of their theoretical studies was that even a short chain consisting of only a few tens of atoms in a particular energy range could behave like a Majorana fermion. More importantly, the researchers succeeded in constructing a physical system of iron atoms deposited onto the surface of an ultrapure crystal of lead. After cooling the system to – 272 °C, they confirmed that superconductivity in the atomically thin wire matched the conditions needed to support a Majorana fermion at the end of the wire. Scanning tunnelling microscopy also allowed them to study topological states of matter at the edges of bismuth bilayers. Project researchers demonstrated the 1D structure of electrons propagating along the perimeter of the system. These low-dimensional topological systems are expected to play a key role in the realisation of Majorana fermions. TOPOSPIN findings open up new directions for topological insulator behaviour research. In addition to its work having implications for condensed matter physics, the project also helped established a fruitful collaboration between leading research institutes in Europe and the United States that is expected to continue in the future.
Keywords
Condensed matter, topological insulators, quantum computing, Majorana fermions, scanning tunneling
