The BBVA Foundation Frontiers of Knowledge Award in the Basic Sciences category has gone in this eleventh edition to physicists Charles Kane and Eugene Mele, “for their discovery of topological insulators, a new class of materials with extraordinary electronic properties: (…) they behave as conductors on the surface, but as insulators in the interior,” according to the committee’s citation.
5 March, 2019
The discovery of topological insulators brought to light new properties of matter that had always been there, but that no one had thought to look for. Shortly after Kane and Mele predicted their existence, in 2005, a multitude of materials were experimentally verified to be insulators of this kind. The study of their properties and potential applications, including the development of tomorrow’s quantum computers, is now among the hottest topics in physics research.
The “surprising discovery” of topological insulators, has, says the committee, confirmed “the existence of new phases of matter and ways of manipulating their properties. Moreover, the basic principles behind topological insulators have important implications beyond condensed matter physics, for instance in the generation of efficient photonic and electronic devices, or quantum information processing.”
Inspiration from graphene
Charles Kane (Urbana, Illinois, United States, 1963) and Eugene Mele (Philadelphia, Pennsylvania, United States, 1950) work at the University of Pennsylvania, and are longstanding collaborators. The seed that led to their discovery of topological insulators was the characterization of graphene in 2004 as a single sheet of carbon one atom thick. Mele and Kane realized that graphene had the peculiarity of being neither an electricity conductor nor an insulator. It stood instead “at a critical point between these two states,” explained Mele on the phone yesterday after hearing of the award. “We began to study the problem and that led us to the concept of this new insulating phase of matter.”
The standard wisdom in the physics of the time was that materials had to be one of two types: conducting or insulating. “Metallic materials conduct electricity whereas insulators do not,” explains the committee. However “Kane and Mele predicted in 2005 that this simple classification fails for a new class of materials called topological insulators, whose existence was experimentally confirmed soon thereafter.”
The new laureates postulated in 2006 how a real material might look that fit the definition of a topological insulator. And just one year later, a laboratory came up with a combination of mercury and tellurium that displayed the properties predicted. However it was, like graphene, a one-atom-thick two-dimensional material, making it hard to synthesize. The real boom in the area had to wait until the next decade, and the discovery that 3D topological insulators actually exist in nature; among them cadmium telluride, a crystalline compound used in the manufacture of solar cells.
Kane and Mele were the first to express surprise at this new finding: “We initially thought that [topological insulators] could only occur at energy scales too small to be directly useful, but then we discovered one could do this in three-dimensional materials at energy ranges that are routinely accessible,” says Mele. “In fact since then we have discovered that this phenomenon is not that rare in nature, it is just that people had not thought to ask the question or look for it before.”
Robust materials resistant to impurities
An essential characteristic of these materials is that their surface conductivity is “fundamentally robust,” the committee remarks. What this means in essence is that topological materials are not affected by the presence of the impurities or other perturbations that can alter the performance of conventional conductors.
As Kane explains it, “[in topographical insulators] the conducting surface is very special because it cannot be destroyed. It is very robust, so for that reason there may be things you can do with it that you can’t do with ordinary conductors. It is a new phase of matter, an insulator that is guaranteed to have this conducting state on its surface, and also it is topological, that is, it can be deformed without losing its conductivity.”
It is this property that opens the door to improvements in today’s electronic devices, enabling, for example, their further miniaturization. In topological insulators, “the flow of electric charge in the surface conductor is more organized than in an ordinary conductor,” he adds, “and that might enable a smoother, more efficient flow, without overheating.”
Yet the most promising applications lie in the future. Guaranteed conductivity, proof against any perturbation, is among the conditions of interest for the development of quantum computers with exponentially more processing power.
Applications as yet unimagined
For Mele, “the biggest pay-off may be things that we haven’t really thought of. We have a new palette of materials and when you hand that to people who are clever they will do clever things with them. If I could travel ahead 50 years in a time machine, I would like to know what kinds of new devices have been developed that are informed by our basic research.”
Both awardees, however, defend the value of their discovery above and beyond its potential applications: “What drives me is the beauty of what nature can do,” says Kane. “Certainly major technical applications could emerge from this, but what fascinates me is discovering what nature can do with these seemingly simple building blocks. This topic arose out of curiosity about how matter could arrange itself. At the time, we had no idea it was going to develop into such a broad enterprise…”.
“If I had only been willing to do work on things I knew were going to be technologically relevant in the short-term, I never would have discovered topological insulators. What made that possible was the fact of having the opportunity and the luxury of being able to follow my interests and my passion,” Kane concludes.
Mele shares his view, adding that “what is so nice about this work is that the underlying thinking is very mathematical; elegant, simple and pretty. And that fusion of an underlying mathematical, simple structure and its connection with things that have a real-world technical pay-off is what we are striving for in science.”