Jan 20 2016
Scientists have developed a unique type of bismuth-based nanoribbons, whose semiconductor properties were effectively controlled by applying a magnetic field. This novel technology shows promise for developing new quantum computers and spintronic devices, and would also aid in studying exotic physics concepts. These nanoribbons were developed using a new class of materials, known as topological insulators.
Yong P. Chen, a Purdue University associate professor of physics and astronomy and electrical and computer engineering, collaborated with Ph.D student Luis A. Jauregui and other scientists for the study. Chen stated that topological insulators, unlike standard materials that are either conductors or insulators, function both as an insulator and conductor. While they are capable of conducting electricity on the surface, they also act as insulator from within.
According to the team, the materials could possibly be employed for quantum computers and spintronic devices that are more powerful than the current-generation of technologies. In the study, a magnetic field was applied to promote a helical mode of electrons, which may help in regulating the electrons spin state. The results of the study have been outlined in a research paper, published in the advance online publication of the journal, Nature Nanotechnology.
Researchers used bismuth telluride to fabricate the nanoribbons. This material is used in commercial thermoelectric refrigerators and other similar solid-state cooling technologies.
The study demonstrated that when a magnetic field is applied, the nanoribbons undergo a topological transition between a material that lacks a band gap on the surface and a material that has one.
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. 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. 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.
Yong P. Chen, Associate Professor of Physics and Astronomy and Electrical and Computer Engineering, Purdue University
Study authors include Jauregui; Michael T. Pettes, an assistant professor in the Department of Mechanical Engineering at the University of Connecticut and a former postdoctoral researcher at the University of Texas at Austin; Li Shi, BF Goodrich Endowed Professor in Materials Engineering at the University of Texas at Austin; Leonid P. Rokhinson, a Purdue professor of physics and astronomy and electrical and computer engineering; and Chen.
In the study, the team were able to record how nanoribbons effectively determined the Aharonov-Bohm oscillations. Such oscillations are realized when electrons are conducted in reverse directions in circle-like paths around the nanoribbons. The nanoribbon’s structure, which shares the same topological surface with a cylinder, was integral to these findings. This is because such nanowires aid in studying electrons as they pass in a circular direction across the nanoribbons. The electrons can only conduct on the nanowire surface, tracing out a circular direction.
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.
Luis A. Jauregui, PHD Student
In order to show a new oscillation change on the topological insulator surfaces, the scientists induced the electrons spin helical mode, which made the insulator to switch from constructive interference to destructive interference and vice versa.
This provides very definitive evidence that we are measuring the spin helical electrons. 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.
Luis A. Jauregui, PHD Student
Further, this oscillation was shown as a function of gate voltage, which provides another means to change from high to low conduction.
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.
Yong P. Chen, Associate Professor of Physics and Astronomy and Electrical and Computer Engineering, Purdue University
This type of gate-dependent oscillation in nanoribbons was never observed before, further correlating it to the bismuth telluride’s topological insulator band structure. It is said that nanoribbons exhibit a topological protection function, which inhibits the back scattering of electrons on the surface, and facilitates high conductivity, a unique feature that is not found in other standard semiconductors and metals. While the nanoribbons were being cooled to minus 273oC, the measurements were performed.
“We have to operate at low temperatures to observe the quantum mechanical nature of the electrons,” Chen said.
In the future studies will be performed to further explore the nanowires,to enable a better understanding of the exotic physics can be obtained to make effective topological quantum calculations. The aim of the study is to link the nanowires with superconductors to realize a combination of hybrid topological insulator, and superconducting devices. Superconductors are materials that can conduct electricity without any resistance.
Increased integration of the superconductor and topological insulators would make it possible to develop a practical quantum computer, which is not highly sensitive to impurities in the environment and to perturbations, which have posed a number of difficulties till date. If developed the technology would be able to do computations by means of the laws of quantum mechanics. This way, computers would become relatively faster and better than that of traditional PCs, and would enable code breaking, database searches, and other tasks to be performed at a much higher speed.
The study was supported with funding from the National Science Foundation, the U.S. Defense Advanced Research Projects Agency, National Science Foundation, the Purdue Center for Topological Materials, Department of Energy, and Intel Corp.