Stanford theoretical physicist Shoucheng Zhang dies at 55
Zhang was a rare theorist who concerned himself with the implications of his abstract ideas about new quantum states of matter on experiments and future technologies.
Shoucheng Zhang, the J.G. Jackson and C.J. Wood Professor in Physics at Stanford University whose research on the quantum physics of many interacting electrons led to the predictions of new phenomena and exotic states of matter, died on Dec. 1. He was 55.
“We are saddened to learn of the passing of Shoucheng Zhang, a distinguished physicist who has made tremendous scientific contributions,” said Stanford Provost Persis Drell, who also is a professor of physics at the university. “His passing is a loss for his family, his colleagues at the university and for his field.”
Zhang’s death was unexpected and followed a “battle with depression,” according to his family. “I have lost a dear friend whose infectious enthusiasm for new experiences and love of exploring ideas and scholarship across all disciplines are irreconcilable with his tragic end,” said Steven Kivelson, a professor of physics at the Stanford Institute for Theoretical Physics and Zhang’s former advisor and collaborator.
A dual appreciation
Zhang’s colleagues recalled his “extraordinary creativity” and his wide-ranging intellect, which explored everything from novel materials and quantum gravity to artificial intelligence. He also had a deep appreciation of both the mathematical beauty of physics and the nuts and bolts of implementing abstract ideas in the real world, said Kathryn Ann Moler, Stanford vice provost and dean of research.
“Many of us aspire to achieve both,” said Moler, who is also a professor of applied physics and of physics. “Shoucheng not only had that aspiration, but he also put it into practice very effectively, time and time again.”
As a graduate student, Zhang studied supergravity but later switched his attention to the problem presented by the fractional quantum Hall effect because he felt there was a greater likelihood that it could be experimentally confirmed within his lifetime.
First discovered in 1982, the fractional quantum Hall effect is a physical phenomenon that involves unusual variations in the electrical resistance of a thin strip of certain superconducting materials when cooled to super low temperatures and exposed to strong magnetic fields.
In 1988, Zhang, working with Kivelson and Hans Hansson, both then at the State University of New York (SUNY) at Stony Brook, derived a topological quantum field theory that elegantly captures the physics of the fractional quantum Hall effect. “One day Shoucheng came to visit and he said, ‘Look what I figured out.’ He then sketched out a basic idea of the theory and all of these mysterious features of the fractional quantum Hall effect just dropped in your lap incredibly simply,” Kivelson said. “That’s not how physics usually works. You usually slave away at things. But on that day Shoucheng’s idea was just so focused and so perfect.”
The next year, in collaboration with Dung-Hai Lee at IBM, and Kivelson, Zhang applied this field theoretic approach to make a host of predictions concerning the global properties of quantum Hall systems, including the prediction that they should act as insulators and as metals under certain conditions. These and several other predictions were confirmed experimentally soon after.
For Zhang, science’s ability to make testable predictions about nature was a wondrous thing. “The most glorious role a theory can have is to predict,” Zhang once said. “It reveals the level of our understanding.”
Zhang had a talent for making important and successful predictions. “He had great taste in science – just a really good sense of what’s interesting and what’s not and why,” Moler said.
In 2005, Zhang’s team, in parallel with another group at the University of Pennsylvania, proposed a new state of matter, called the quantum spin Hall insulator, in which electrical current flows only along the edges and in a direction dictated by their electrons’ quantum spin states. Electrons with identical spins travel in the same direction together, while electrons with the opposite spin move in the opposite direction.
The following year, Zhang’s group predicted for the first time how a quantum spin Hall insulator could be made by stacking layers of mercury telluride and cadmium telluride. This prediction was soon confirmed experimentally, prompting a flurry of research worldwide. Significantly, topological insulators are impervious to the processes by which power is dissipated as heat when electric currents are passed through traditional semiconducting materials. Zhang believed this property might one day be harnessed to extend Moore’s Law and make smaller and more powerful computer chips.
“Right now, inside conventional semiconductors, it’s like you have a Ferrari in a crowded marketplace. It keeps bumping into its surroundings, so it dissipates all of its energy,” Zhang once explained. “But there are no collisions in topological insulators. They’re like a highway system for electrons.”
Zhang once again demonstrated the power of theory in 2017, when his team helped discover some of the most compelling evidence yet for the “Majorana fermion,” a particle that has the unusual distinction of also being its own antiparticle. The Majorana fermion was predicted in 1937, but firm evidence of its existence had remained elusive for 80 years. Zhang’s group predicted not only which type of material the Majorana fermion could be found in but also its experimental signature. Both predictions were confirmed by experimentalists at UCLA, which resulted in a paper published in Science.
Zhang thought that Majorana fermions could be used to construct robust quantum computers unaffected by environmental noise, and in recent months he advocated for the creation of a new center at Stanford devoted to the study of topological physics and quantum information. Zhang was slated to be its first director.
“The center is the culmination of Shoucheng’s life work,” Moler said. “He saw it as his way of supporting the next generation of scientists who would carry on this work.”
A distinguished scholar
In his personal life, Zhang is remembered as a loving husband and father who cherished quality time with his family. “On our family vacations together, he loved to take us to the most beautiful natural sights on Earth, to share with us stories of ancient history in every region we visited, and to encourage our latest ideas and interests,” his family said in a statement.
Zhang’s favorite poem was “Auguries of Innocence” by William Blake, which reads in part:
To see a World in a Grain of Sand
And a Heaven in a Wild Flower
Hold Infinity in the palm of your hand
And Eternity in an hour
The poem expressed Zhang’s life mission for boundless exploration and discovering beauty, according to his wife and children. “Motivated by his desire to witness the glory of God through scientific research, he brought an infectious spirit of curiosity to the entire world,” the family’s statement reads.
Zhang graduated from Freie Universität in Berlin in 1983 with a bachelor’s degree in physics. In 1987, he received his doctoral degree from SUNY Stony Brook. He was a postdoctoral researcher at the Institute for Theoretical Physics at the University of California, Santa Barbara, and a research staff member at IBM’s Almaden Research Center in San Jose, California, before joining Stanford as a professor in 1993. Zhang also held an appointment at the Stanford Institute of Materials and Energy Sciences (SIMES), a collaboration between Stanford University and SLAC National Accelerator Laboratory.
Zhang was a member of the U.S. National Academy of Sciences, the American Academy of Arts and Sciences, and a foreign member of the Chinese Academy of Sciences. He received numerous awards over the course of his career, including the Oliver Buckley Prize and the Dirac Medal in 2012 and the Benjamin Franklin Medal in Physics in 2015.
Zhang is survived by his wife, Barbara; his children, Stephanie and Brian; and his daughter-in-law, Ruth Fong.