SLAC Today logo

People: Shoucheng Zhang's Practical Philosophy

(Photo by Lauren Schenkman. Click for larger image.)

When he talks about physics, Shoucheng Zhang waxes philosophical. His carefully chosen words map out neat avenues of logic as late afternoon light suffuses his Stanford office, a spacious room with a thick autumn-toned rug and deep leather couches that invite long conversations. And having conversations about science is one of his biggest hobbies, admits the Stanford professor and Stanford Institute for Materials and Energy Science physicist. "I love talking to people," he says. "I like looking for good analogies to explain science and make it accessible." When asked about his research, Zhang zooms out—way out. He says he sees his work as part of a tradition that dates back to the Greeks—the search for what is "elementary."

This search, Zhang says, has branched into the separate disciplines known as high energy physics and condensed matter physics. High-energy physicists stalk the Alice-in-Wonderland quantum world for the "elementary particles"—which can't be divided into yet smaller parts—hidden inside atoms. Meanwhile, condensed matter physicists aren't looking for new building blocks, but new buildings. Beyond forming the familiar states of solid, liquid and gas, atoms often exhibit extraordinary collective behaviors. For Zhang, studying matter is part of our journey toward the elemental; high-energy physics and condensed matter physics aren't really as separate as they seem.

Zhang wrote about this idea in a mathematical lesson-cum-essay for a collection dedicated to the late physicist John Wheeler, who mentored giants such as Richard Feynman. With Wheeler's philosophical bent in mind, Zhang chose the title of his essay, "To See a World in a Grain of Sand," from the English poet William Blake. "I thought it was only fitting to use something poetic," Zhang says. "It means that the structure of subatomic particles is reflected in the systems they make up—the solid, the grain of sand."

He likens the idea to M.C. Escher's drawing of an endless waterfall; the viewer follows the waterfall to its base, only to find himself at the top again. This wide-minded approach to physics allowed Zhang to make his most recent discovery. His work encompasses the mysterious quality of electron spin to predict a material with extremely special characteristics.

"Anyone who tells you spin is simple obviously doesn't understand it," Zhang says with a genial laugh. He picks up his pencil and turns it point over eraser. "You agree that if I rotate it another 180 degrees, it's back to its original position?" He rotates the pencil. "Wrong. If this were an electron, you'd have to rotate it another 360 degrees—720 degrees total—for it to be back in the same state."

If spin sounds like another wacky idea best left to theorists and philosophers, think again. In a few years, Zhang's work with electron spin could be coming to a microprocessor near you.

As Moore's law states, chip-manufacturers can double computing power about every 18 months by cramming more and more transistors onto each processor. But as transistors shrink, they spend more energy generating heat—lap-scalding MacBook, anyone?—than actually crunching bits. An engineer might try to solve the problem by figuring out how to remove the heat. But the physicist's solution, Zhang says, "is to not generate any heat in the first place."

When a solid transports current, Zhang says, on the atomic level it looks like a Berlin night club around 3:00 a.m. "It's totally disordered, like a disco. You're the electron, trying to move from one side of the room to the other, but you keep bumping into people and losing energy."

How do you avoid dissipating energy? "The idea is to get the electrons to dance more coherently, so that instead of having a disco you have something that looks more like dancing from the 19th century, with everyone in pairs," Zhang says. He imagines the interior of a solid as an empty dance floor, the pairs of electrons twirling decorously along the walls. Some couples spin clockwise while moving clockwise around the room, while other couples spin counterclockwise while moving counterclockwise. No one bumps into each other, so they don't dissipate energy. Part of this "dance," the electrons' movement around the room, has been known for a while. It's called the quantum Hall effect, and occurs when a solid is placed in a strong magnetic field.

By adding spin, Zhang removed the need for the magnet. In his 2006 paper, he pointed to a real world material—mercury telluride—in which such a phenomenon could in principle occur. One year later, collaborators at the University of Würzberg confirmed the theory experimentally. The discovery of this new state of matter, a solid that exhibits the quantum spin Hall effect, was lauded by Science as a runner-up breakthrough of 2007.

Although a chip could be made out of this material, it would still need to be kept at temperatures lower than 10 kelvin, hobbling the material's usefulness to the computing industry. Now Zhang is busy dreaming up the material that could exhibit the quantum spin Hall effect at room temperature. He is working in conjunction with SIMES Director Z. X. Shen at the Stanford Synchrotron Radiation Lightsource to scope out possible candidates.

If Zhang succeeds, the philosophical could meet the practical in a very powerful way. "There are physicists who dream things up, and others who make things happen," he adds. "Even though we dreamers don't seem to do anything, we do contribute in our way."

—Lauren Schenkman
  SLAC Today, January 21, 2009