Physicists Charles Kane and Eugene Mele are the recipients of the 2019 Frontiers of Knowledge Award, for discovering topological insulators, described as a new class of materials with extraordinary electronic properties.

The materials’ most promising applications lie in the future, though, with guaranteed conductivity and proof against any perturbation being among the conditions of interest for the development of quantum computers with exponentially more processing power.

The discovery of topological insulators brought to light new properties of matter that had always been there, but that no one had thought to seek. Shortly after Kane and Mele predicted the insulators’ 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.

Award committee members described the discovery as surprising and confirmed “the existence of new phases of matter and ways of manipulating their properties. In addition, 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.”

The impetus for Kane and Mele’s discovery was the characterization of graphene in 2004 as a single sheet of carbon one atom thick. The researchers 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,” Mele said. “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 at the time was that materials had to be one of two types: conducting or insulating.

Committee members also explained that metallic materials conduct electricity whereas insulators do not. That did not stop Kane and Mele from predicting 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 that fit the definition of a topological insulator might look. Just one year later, a laboratory came up with a combination of mercury and tellurium that displayed the properties predicted. Like graphene, though, it was a one-atom-thick 2D material, making it hard to synthesize. The breakthrough came a decade later with the discovery that 3D topological insulators actually exist in nature, among them cadmium telluride, a crystalline compound used in the manufacture of solar cells.

Expressing surprise at this finding, Mele said he and Kane initially thought that topological insulators could only occur at energy scales too small to be directly useful, but then they discovered this could be done in 3D materials at routinely accessible energy ranges.

“In fact,” Mele said, “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.”

Kane said that in topographical insulators the conducting surface is very special because it cannot be destroyed.

“It is very robust,” he said. “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, Kane said, “the flow of electric charge in the surface conductor is more organized than in an ordinary conductor and that might enable a smoother, more efficient flow, without overheating.”

Kane completed a B.S. in physics at the University of Chicago in 1985 and his Ph.D. at Massachusetts Institute of Technology in 1989. After three years working as a postdoctoral associate at IBM’s T.J. Watson Research Center in New York, he joined the faculty at the University of Pennsylvania as assistant professor in the Department of Physics and Astronomy. He is now a Christopher H. Browne Distinguished Professor of Physics in the School of Arts and Sciences.

Mele completed a B.S. in physics at St. Joseph’s University in Philadelphia in 1972 and then went on to earn a Ph.D. in physics from the Massachusetts Institute of Technology in 1978. One of his first professional posts was as an associate scientist at the Xerox Webster Research Center in New York. Since 1981, he has held successive appointments at the University of Pennsylvania as assistant professor (1981-1985), associate professor (1985-1989), and professor of physics (1989-2017). This last year, he took up the position of Christopher H. Browne Distinguished Professor of Physics in Penn's Department of Physics and Astronomy.

Of the laureates who have been recognized for their accomplishments with this award over the past 10 years, seven have gone on to win a Nobel Prize.