# Stanford physicist wins prestigious Dirac Medal

At the peak of the Olympic Games, the gold medal of theoretical physics has just been announced. Physics Professor **SHOUCHENG ZHANG** and two other physicists were awarded the Dirac Medal from the International Centre for Theoretical Physics for their research on a novel type of matter known as a “topological insulator.”

Topological insulators are one of the most exciting topics in condensed-matter physics, because of both their conceptual beauty and their exciting practical applications. The materials function as insulators on the inside, but they conduct electricity on their surface; this conductivity is “topologically protected,” so the state of the electrons passing on the surface cannot be changed or destroyed. Because the conducting electrons arrange themselves along the surface – “spin-up” electrons travel in only in one direction, “spin-down” electrons go only the opposite – the material could be useful for building a practical spintronic device that reads an electron’s spin, rather than its charge.

Unlike most exotic phases of matter, topological insulators were predicted theoretically before they were discovered experimentally. Zhang predicted the first topological insulator material in mercury telluride, which was confirmed experimentally soon after by a group at the University of Würzburg in Germany. Later, Zhang’s group predicted other topological insulator materials, including bismuth telluride, which has been extensively studied at Stanford. There is a worldwide race to apply topological insulators to integrated circuits in order to extend the life of Moore’s law.

“I have deeply admired Dirac ever since I was a student,” Zhang said Wednesday. “He pioneered a novel scientific method – to discover the truth of nature by searching for mathematical beauty. Discovery of topological insulator is a triumphant tribute to Dirac’s inspiring style and to his ubiquitous Dirac equation.”

The Dirac Medal is named after the British Nobel Prize-winning theorist Paul Dirac. He is best known for his unification of quantum mechanics with Einstein’s theory of special relativity, leading to the discovery of the Dirac equation and the prediction of anti-matter. The medal was first awarded in 1985 and is given each year on Dirac’s birthday, Aug. 8. This year’s medal, which comes with a $5,000 prize for each medalist, marks the 110th anniversary of his birth.

—*by Bjorn Carey of the Stanford News Service*