To most people, solid-state or "condensed matter" physics doesn't sound like a field with many practical, everyday applications. Yet, beginning with the invention of the transistor in 1947 and continuing into our modern age of smartphones and fiber optics, it's been at the heart of many of the technological wonders that inhabit our daily lives. Charles Kane, Eugene Mele, and Shoucheng Zhang are continuing that trend with the discovery of an entirely new state of matter, materials known as topological insulators (TIs). Not only are TIs fascinating in themselves, but their potential applications may eventually surpass even the first transistor in importance.
Sometimes in science, as with the transistor, the experimental work comes first, inspiring new theoretical insights. It was the other way around with topological insulators, with theory preceding laboratory confirmation. The notion of TIs began with a pair of 2005 papers in Physical Review Letters by University of Pennsylvania colleagues Charles Kane and Eugene Mele, emerging from their studies of graphene, the two-dimensional form of carbon.
Kane and Mele proposed that under proper conditions, graphene might exhibit some rather intriguing characteristics in its two-dimensional electronic band structure. They showed how a new version of a well-known physics phenomenon called the Hall effect (in this case, a quantum spin Hall effect) could make graphene, and perhaps other materials, an insulator on the inside but a conductor on the outside edges. They dubbed such materials "topological insulators." The theory was elegant and insightful, but the predicted effects too subtle in graphene to be experimentally verified.
Meanwhile, Shoucheng Zhang of Stanford University, independently studying the spin Hall effect, began seeking TI candidate materials other than graphene and found that mercury telluride crystals could fit the bill. By 2007, as Kane and Mele continued to develop and expand upon their theoretical work, now joined by Zhang, experimenters such as Laurens Molenkamp in Germany and the team of M. Zahid Hasan at Princeton University proved the reality of TIs in the lab. Other theorists and experimentalists have shown that compounds involving bismuth, such as bismuth antimonide, bismuth selenide, and bismuth telluride, also exhibit the TI phase.
It may all sound hopelessly esoteric, but the confirmation of topological insulators has the potential to not only revolutionize the electronics, semiconductor, and computer industries but also lead to major new discoveries in other areas of physics. One prospect is confirming the existence of an exotic particle called a Majorana fermion, which could make possible practical superconductivity and quantum computers. TIs could also speed the realization of spintronics, electronic devices based on manipulating electron spin rather than charge.
Based at different institutions, these three creative scientists provide a classic case study of the collaborative process at the heart of science. Although each has made significant contributions on his own, by working together and building upon each other's contributions and ideas, the trio has spawned an entirely new field of study with importance and implications that are only beginning to be fully understood.
The theories of condensed matter physics may be obscure to the layperson, but their realization and use in the everyday world are vital to everyone. The work of Charles Kane, Eugene Mele, and Shoucheng Zhang serves as an elegant demonstration of how beautiful theories can lead to elegant experiments and ultimately to practical benefits for us all.
Information as of April 2015