Measured tunneling current and its dependence on two applied magnetic fields: the fans of red/yellow curves each correspond to a fingerprint of the conducting edge states. Image: University of Basel, Department of Physics.Physicists from Switzerland and the US have developed a technique that can create an individual fingerprint of the current-carrying edge states occurring in novel materials such as topological insulators or 2D materials. They describe this new technique in a paper in Nature Communications.
While insulators do not conduct electrical currents, some special materials exhibit peculiar electrical properties: though not conducting in their bulk, their surfaces and edges may support electrical currents due to quantum mechanical effects, and do so without any loss of electrical energy. These so-called topological insulators have attracted great interest in recent years due to their remarkable properties. In particular, their robust edge states are very promising since they could lead to great technological advances.
Similar edge-conducting properties also appear when a two-dimensional (2D) metal is exposed to a strong magnetic field at low temperatures. When the so-called quantum Hall effect is realized, current is thought to flow only at the edges of the 2D metal, where several conducting channels are formed.
Until now, it was not possible to address these numerous current-carrying states individually or to determine their positions separately. With their new technique, however, the physicists can obtain an exact fingerprint of the current carrying edge states with nanometer resolution.
This advance is reported by researchers at the Department of Physics and the Swiss Nanoscience Institute of the University of Basel, in collaboration with colleagues at the University of California, Los Angeles, Harvard University and Princeton University. In order to measure the fingerprint of the conducting edge states, the physicists, lead by Dominik Zumbühl at Basel, took advantage of scanning tunneling spectroscopy.
They used a gallium arsenide nanowire located at the sample edge, moving it parallel to the edge states under investigation. In this configuration, electrons may jump (tunnel) back and forth between a specific edge state and the nanowire as long as the energies in both systems coincide. Using an additional magnetic field, the scientists could control the momentum of the tunneling electrons and address individual edge states. From the measured tunneling currents, the position and evolution of each edge state may be obtained with nanometer precision.
This new technique is very versatile and can also be used to study dynamically evolving systems. Upon increasing the magnetic field, the number of edge states is reduced, and their distribution is modified. For the first time, the scientists were able to watch the full edge state evolution, starting from their formation at very low magnetic fields.
As the magnetic field increases, the edge states are first compressed towards the sample boundary, until eventually they move towards the inside of the sample and then disappear completely. Analytical and numerical models developed by the research team agree very well with the experimental data.
"This new technique is not only very useful to study the quantum Hall edge states," said Zumbühl. "It might also be employed to investigate new exotic materials such as topological insulators, graphene or other 2D materials."
This story is adapted from material from the University of Basel, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.