Researchers have created nanoribbons of an emerging class of materials known as topological insulators and used a magnetic field to control their semiconductor properties. This represents a step toward harnessing this technology for the study exotic physics and to build new spintronic devices or quantum computers.
Unlike ordinary materials that are either insulators or conductors, topological insulators are paradoxically both at the same time. They are insulators on the inside but conduct electricity at the surface, said Yong Chen, a Purdue University associate professor of physics and astronomy and electrical and computer engineering who worked with doctoral student Luis Jauregui and other researchers. These materials could be used for ‘spintronic’ devices and practical quantum computers that are far more powerful than today's technologies.
In this new study, the researchers used a magnetic field to induce a so-called ‘helical mode’ of electrons, a capability that could make it possible to control the spin state of electrons in topological insulators. As detailed in a research paper in Nature Nanotechnology, this meant they could induce the nanoribbons to undergo a ‘topological transition’, switching between a material possessing a band gap on the surface and one that does not.
"Silicon is a semiconductor, meaning it has a band gap, a trait that is needed to switch on and off the conduction, the basis for silicon-based digital transistors to store and process information in binary code. Copper is a metal, meaning it has no band gap and is always a good conductor," Chen explained. "In both cases the presence or absence of a band gap is a fixed property. What is weird about the surface of these materials is that you can control whether it has a band gap or not just by applying a magnetic field, so it's kind of tunable, and this transition is periodic in the magnetic field, so you can drive it through many 'gapped' and 'gapless' states."
The nanoribbons are made of bismuth telluride, the material behind solid-state cooling technologies such as commercial thermoelectric refrigerators. "Bismuth telluride has been the workhorse material of thermoelectric cooling for decades, but just in the last few years people found this material and related materials have this amazing additional property of being topological insulators," he said.
A key advance was that the researchers were able to use the nanoribbons to measure so-called Aharonov-Bohm oscillations, by conducting electrons in opposite directions in ring-like paths around the nanoribbons. The structure of the nanoribbon – a nanowire that is topologically the same as a cylinder – is key to this discovery, because it allows the electrons to be studied as they travel in a circular direction around the ribbon. The electrons conduct only on the surface of the nanowires, tracing out a cylindrical circulation.
"If you let electrons travel in two paths around a ring, in left and right paths, and they meet at the other end of the ring then they will interfere either constructively or destructively depending on the phase difference created by a magnetic field, resulting in either high or low conductivity, respectively, showing the quantum nature of electrons behaving as waves," Jauregui said.
"What is weird about the surface of these materials is that you can control whether it has a band gap or not just by applying a magnetic field, so it's kind of tunable, and this transition is periodic in the magnetic field, so you can drive it through many 'gapped' and 'gapless' states."Yong Chen, Purdue University
The researchers demonstrated a new variation on this oscillation in topological insulator surfaces by inducing the spin helical mode of the electrons. The result is the ability to flip from constructive to destructive interference and back.
"This provides very definitive evidence that we are measuring the spin helical electrons," Jauregui said. "We are measuring these topological surface states. This effect really hasn't been seen very convincingly until recently, so now this experiment really provides clear evidence that we are talking about these spin helical electrons propagating on the cylinder, so this is one aspect of this oscillation."
Findings also showed that this oscillation is a function of ‘gate voltage’, representing another way to switch conduction from high to low. "The switch occurs whenever the circumference of the nanoribbon contains just an integer number of the quantum mechanical wavelength, or 'fermi wavelength,' which is tuned by the gate voltage of the electrons wrapping around the surface," Chen said.
This was the first time researchers had seen this kind of gate-dependent oscillation in nanoribbons and further correlates it to the topological insulator band structure of bismuth telluride. The nanoribbons are said to possess ‘topological protection’, preventing electrons on the surface from back scattering and inducing high conductivity, a quality not found in metals and conventional semiconductors. They were fabricated by researchers at the University of Texas at Austin.
The measurements were performed while the nanoribbons were chilled to about -273°C. "We have to operate at low temperatures to observe the quantum mechanical nature of the electrons," Chen said.
Future research will include further investigation of the nanoribbons as a platform for studying the exotic physics needed for topological quantum computations. Researchers will aim to connect the nanoribbons with superconductors, which conduct electricity with no resistance, for hybrid topological insulator-superconducting devices. By combining topological insulators with a superconductor, researchers may be able to build a practical quantum computer that is less susceptible to the environmental impurities and perturbations that have presented challenges thus far. Such a technology would perform calculations using the laws of quantum mechanics, making for computers that are much faster than conventional computers at certain tasks such as database searching and code breaking.
This story is adapted from material from Purdue University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.