Stanford physicist’s quest for the perfect keys to unlock the mysteries of superconductivity
For decades Z-X Shen has ridden a wave of curiosity about the strange behavior of electrons that can levitate magnets.
Zhi-Xun Shen vividly remembers his middle school physics teacher demonstrating the power of X-rays by removing a chunk of radioactive material from a jar stored in a cabinet, dropping it into a bucket and having students put their hands between the bucket and a phosphor screen to reveal the bones hidden beneath the skin and flesh.
“That left an impression,” Shen recalled with a grin. Sometimes he wonders if that moment set the stage for everything that followed.
Shen did not, he admits, have a strong interest in physics. There wasn’t much incentive to study in mid-1970s China. The country was in the grip of the Cultural Revolution of 1966, which had shut down all the universities and left most of the nation, including the town south of Shanghai where his parents worked in medicine, in poverty. But as Shen and his mother watched his brother board a bus to the countryside for “reeducation” at a forced labor camp one cold morning, she turned to him and said, “You are our hope for a college education.”
Still, given the family’s circumstances, college seemed like an impossible dream. Then an unlikely series of events changed everything.
In 1977, the Cultural Revolution ended and universities re-opened.
When the same inspiring middle school teacher organized a physics competition, then-16-year-old Shen entered and came in first at every level – school, district, city and province. It was fascinating and built his self-confidence, cementing his feeling that physics was the field for him, but where could it possibly lead?
Shen won a college spot before graduating high school but held back a year on the advice of his father, then entered the physics program at Fudan University in Shanghai.
And in his third year as a physics major, he took an entrance exam for a program just launched by Chinese-American Nobel laureate Tsung-Dao Lee that brought a limited number of Chinese students to the U.S. for advanced studies in physics.
That’s how, in March 1987, Shen found himself in a jam-packed, all-night conference session that came to be known as the Woodstock of Physics, where nearly 2,000 scientists shared the latest developments related to the discovery of a new class of quantum materials known as high-temperature superconductors. These exotic materials conduct electricity with zero loss at much higher temperatures than anyone had thought possible, and expel magnetic fields so forcefully that they can levitate a magnet. Their discovery had revolutionary implications for society, promising better magnetic imaging machines for medicine, perfectly efficient electrical transmission for power lines, maglev trains and things we haven’t dreamed up yet.
“I was able to get there early and get a seat in the room where the talks were going on,” Shen recalled. “To me, it was the most exciting thing – a completely new frontier of science suddenly opened up.”
A revolution of tools
In another extraordinary stroke of luck, he happened to be in a perfect position to jump into this new frontier, not just to probe the quantum states of matter that underlie superconductivity but to develop ever-sharper tools for doing so.
As a PhD student at Stanford University, he’d been using extremely bright X-ray beams to investigate related materials at what is now SLAC National Accelerator Laboratory, just up the hill from the main campus. As soon as the meeting ended, he set about applying the technique he’d been using, called angle-resolved photoemission spectroscopy, or ARPES, to the new superconductors.
More than three decades later, with many important discoveries to his credit but the full puzzle of how these materials work still unsolved, Shen is the Paul Pigott Professor of Physical Sciences at Stanford’s School of Humanities and Sciences and a professor of photon science at SLAC. He and his colleagues are putting the finishing touches on what may be the world’s most advanced system for probing unconventional superconductors and other exotic forms of matter to see what makes them tick.
Key parts of the system are just a few steps away from the X-ray beamline at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL) where Shen carried out those first experiments. One of them is a recently upgraded setup where scientists can precision-build samples of superconducting material one atomic layer at a time, shuttle them through a tube and a vacuum chamber into the SSRL beamline without exposing them to air and make measurements with many times higher resolution than was ever possible before. The materials they build are also transported to the world’s first X-ray free-electron laser, SLAC’s Linac Coherent Light Source, for precision measurements not possible by other means.
These experimental setups were designed with a singular purpose in mind: to unravel the weirdly collaborative behavior of electrons, which Shen and others believe is the key to unlocking the secrets of superconductivity and other phenomena in a broad range of quantum materials.
Shen’s quest for answers to this riddle is driven by his curiosity about “how this remarkable phenomenon that shouldn’t have happened, happened,” he said. “You could argue that it’s a macroscopic quantum phenomenon – nature desperately trying to reveal itself. It only happens because those electrons work together in a certain way.”
The first superconductors, discovered in 1911, were metals that became perfectly conducting when chilled below 30 kelvins, or minus 406 degrees Fahrenheit. It took about 50 years for theorists to explain how this worked: Electrons interacted with vibrations in the material’s atomic lattice in a way that overcame the natural repulsion between their negative charges and allowed them to pair up and travel effortlessly, with zero resistance. What’s more, these electron pairs overlapped and formed a condensate, an altogether different state of matter, whose collective behavior could only be explained by the nonintuitive rules of quantum mechanics.
Scientists thought, for various reasons, that this could not occur at higher temperatures. So the discovery in 1986 of materials that superconduct at temperatures up to minus 225 degrees Fahrenheit was a shock. Weirder still, the starting materials for this form of superconductivity were insulators, whose very nature would be expected to thwart electron travel.
In a perfect metal, Shen explained, each of the individual electrons is perfect in the sense that it can flow freely, creating an electrical current. But these perfect metals with perfect individual electrons aren’t superconducting.
In contrast, the electrons in materials that give rise to superconductivity are imperfect, in the sense that they’re not free to flow at all. But once they decide to cooperate and condense into a superconducting state, not only do they lose that resistance, but they can also expel magnetic fields and levitate magnets.
“So in that sense, superconductivity is far superior,” Shen said. “The behavior of the system transcends that of the individuals, and that fascinates me. You and I are made of hydrogen, carbon and oxygen, but the fact that we can have this conversation is not a property of those individual elements.”
Although many theories have been floated, scientists still don’t know what prompts electrons to pair up at such high temperatures in these materials. The pursuit has been a long road – it’s been 33 years since that crazy Woodstock night – but Shen doesn’t mind. He tells his students that a grand scientific challenge is like a puzzle you solve one piece a time. Better tools are gradually bringing the full picture into focus, he says, and we have already come a long way.
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