Image courtesy of Shou-Cheng Zhang
Schematic of two-dimensional topological insulator showing spin-polarized electrons traveling in four "lanes" at the two edges of the device, with an "insulating" band in the middle.
One of the more prescient predictions ever offered about the progress of technology was the famous "Moore's Law"—based on the 1965 prediction by Intel co-founder Gordon E. Moore that the number of transistors in integrated circuits would double every year (later corrected to a doubling every two years and sometimes reckoned as a doubling of computer power every eighteen months). One might say that Moore's Law, in broadly predicting the rapid advance of modern computing, has defined much of the course of contemporary technological development, which has been driven perhaps more by progress in integrated circuits than by any other single technology. As we all know, these integrated circuits, or "chips," lie at the heart of virtually every electronic device, from iPhones to supercomputers, and now play key roles in present-day automobiles, appliances, and countless other everyday products.
Moore's original forecast was rather modest, covering just ten years, through 1975. In reality, Moore's Law has endured far longer. But one thing seems clear: it probably cannot go on indefinitely. As chips become more crowded with transistors—the latest Intel chip hosts some 291 million of them—they will come up against ultimate limits of power usage and, especially, heat. Even though industry is pursuing multiple strategies to mitigate these problems, eventually, like downtown streets at rush hour, conventional chips will simply become too crowded with billions of electrons to operate efficiently. If we continue to rely on conventional semiconductor technology, it is generally agreed that Moore's Law will ultimately hit a wall.
That is one driver for the current intense interest among scientists in new, unusual electronic materials and devices such as "photonic crystals"—which raise the possibility of perhaps someday replacing hot electrons with cooler photons in circuitry (see "Beyond the Transistor"). Another approach would be to create materials that "tame" electrons to such perfection that the wasteful scatterings, collisions, and "recombinations" that generate resistance and unwanted heat in electronic devices are virtually eliminated.
Among the most advanced of these advanced new materials are those known as "topological insulators." Broadly defined, these are bulk insulators that conduct electricity in a very special way on their surface. The materials are garnering excited attention among a host of researchers these days, for several reasons.
In the first place, it's thought that they may one day help make quantum computing possible. Quantum computing might be able to get around the heat problem by relying on the more subtle quantum-mechanical properties of particles rather than on traditional electronics. Topological insulators are also seen as a possible key to advancing the related field of "spintronics," a new kind of potentially more efficient electronics that relies on electrons' "spin." There is also a great deal of purely scientific fascination with these materials—first conceived of only about six years ago—which represent nothing less than a new, previously undiscovered state of matter.
In effect, topological insulators use a kind of quantum-mechanical ‘trick’ to compel electrons to behave in an extremely ‘disciplined’ fashion, minimizing the wasteful scattering that must eventually be eliminated if we are to move beyond the limits of today’s conventional semiconductors”.
They are also remarkably clever. In effect, topological insulators use a kind of quantum-mechanical "trick" to compel electrons to behave in an extremely "disciplined" fashion, minimizing the wasteful scattering that must eventually be eliminated if we are to move beyond the limits of today's conventional semiconductors.
Now a team of researchers from SLAC National Accelerator Laboratory and Stanford University has fabricated a working "gated" topological insulator that has properties of a nanoscale transistor. It is an important early step toward the development of working electronics based on these materials. Their research was published online by Nature Nanotechnology.
Keep in mind that the goal of these materials is to "discipline," for lack of a better word, the flow of electrons. In conventional circuits and semiconductors, electrons are continually encountering impurities and barriers that cause them to scatter, colliding with other electrons and with positively charged "holes," diminishing efficiency and generating heat. The aim of the topological insulator is to interdict this wasteful motion, by confining electrons to certain definite "lanes."
Stanford University's Shou-Cheng Zhang, one of the originators of the topological insulator concept, has drawn an analogy with highway traffic. (Zhang was one of three scientists recently awarded the American Physical Society’s prestigious Buckley Prize in Condensed Matter Physics for work in this area, along with Charles L. Kane of the University of Pennsylvania and Laurens W. Molenkamp of the University of Würzburg.)
"Just as we have learned from basic traffic control, it would be much better if we could spatially separate the counterflow directions into two separate lanes," Zhang and co-author Xiao-Liang Qi wrote in Physics Today, so as to avoid “random collisions.”
One way to do this—we are not yet talking here about topological insulators per se—is with a powerful magnetic field at very low temperatures. A magnetic field that is perpendicular to a super-cooled conductor will generate a dual flow of electrons at the opposite edges of the conductor, each moving in the opposite direction, with an insulating band in the middle. You have two separate traffic lanes. Moreover, if the flowing electrons encounter a barrier such as an impurity in their "lane," they will simply flow around it instead of scattering. That's the effect we're looking for.
The question is, could you achieve the same or a similar kind of effect without the magnetic field and ultimately at temperatures closer to room temperature?
The answer is that you can do so, partly by creating an indissoluble association between the direction of the electron's movement (technically, its momentum) and the electron's spin. Though not exactly "spinning" in the manner of a spinning top, electrons have a property known as angular momentum, or spin, typically represented by an up or down arrow and having a value of either +1/2 or -1/2.
Topological insulators are typically compounds composed of relatively heavy elements, such as mercury (atomic number = 80) or tellurium (atomic number = 52), whose electrons have what is known as strong "spin-orbit coupling." The outer electrons of these comparatively heavy atoms are moving at near relativistic (i.e., light) speeds and in the process generating strong local magnetic fields that impact the spin of the electrons moving across their surface. In effect, the spin-orbit coupling of the topological insulator has the special property of "locking" the surface electron's direction of travel to its spin.
Image courtesy of Stanford Institute for Materials & Energy Science
Microscopic image showing three-dimensional "gated" topological insulator device fabricated from (Bi0.50Sb0.50)2Te3 nanoplates.
Electrons flowing on the surface in one direction have a spin up; electrons flowing in the opposite direction have a spin down.
In a two-dimensional topological insulator (predicted by Zhang and colleagues in 2006 and experimentally demonstrated by a German-led research team a year later), you end up with four electron "lanes," two at each edge of the material: a forward moving electron flow with spin up and backward moving flow with spin down on one edge, and a mirror-image pair of lanes at the opposite edge. In between is an insulation "band" where electrons do not travel.
Here's the important point. If these flowing electrons encounter an impurity, they do not scatter or make a U-turn, as happens in normal conductors. They simply go around the barrier and continue flowing in the same direction. The reason is partly the quantum-mechanical "trick" that was mentioned earlier. Because spin and momentum are locked together, in order to reverse direction, the electron must also reverse spin. To go from spin-up to spin-down, the electron must turn 180°, in effect, either "clockwise" or "counter-clockwise." But in the weird world of quantum mechanics both of these turns, clockwise and counter-clockwise, are said to take place simultaneously. The effect is that they cancel each other out. So the electron cannot reverse direction and simply flows around the obstacle.
It is this kind of effect that is meant when scientists describe these systems as "topologically protected." You can bend the system without breaking it or changing its fundamental topological nature—just as, in a favorite example of topologists, you can in theory turn a coffee cup into a donut by sheer twisting and stretching. (In practice, of course, there must be a limit to the number of defects a topological system could accommodate and remain "topological.")
Topological insulators are an area of science where experimentation is continually rushing to catch up with theory. Topological insulators existed in the equations of physicists before they existed in the real world.
The most recent work, led by Yi Cui of the Stanford Institute for Materials & Energy Sciences, a joint institute of SLAC and Stanford University, has achieved two key breakthroughs in the experimental realm, working with three-dimensional versions of these materials. The work was supported in part by the DOE Office of Science.
First, the researchers were able to create an unprecedentedly effective topological insulator by "tuning" the precise composition of a ternary or three-element material composed of bismuth, antimony, and tellurium. The key here was maximizing the bulk insulation properties of the material. To the degree that the material conducts electrons in bulk, this conductivity tends to undermine the topological effect on the surface. So you want the material to be a very good bulk insulator. The tuning was accomplished with the help of the Advanced Light Source at Lawrence Berkeley National Laboratory in research led by SLAC's Zhi-Xun Shen, working with crystals fabricated by Stanford's Ian Fisher.
The second major accomplishment was to achieve a transistor gating effect on thin nanoplates of the same materials. Applying gate voltage to this material in one direction turned it into an n-type semiconductor; applying gate voltage in the opposite direction turned it into a p-type semiconductor. N-type semiconductors have an excess of electrons, while p-type semiconductors have a shortage of electrons, or, in different terms, an excess of positive "holes." Transistors are electronic "gates" made up of combinations of n-type and p-type materials.
Full-scale electronics based on topological insulators are no doubt some years away, but progress in these novel materials since they were first predicted in 2005-2006 has been extraordinarily rapid. And the study of these strange materials is uncovering new mysteries in the quantum behavior of matter that are likely to pave the way for further insights and applications in the years ahead.
—Patrick Glynn, DOE Office of Science, Patrick.Glynn@science.doe.gov
Research and Facility (Advanced Light Source): DOE Office of Science, Office of Basic Energy Sciences
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Stanford Institute for Materials & Energy Science
Advanced Light Source