For the first time, scientists have looked inside a superconductor and actually seen what the electrons are doing. And what they saw came as a surprise.
A team led by physicists at the French National Centre for Scientific Research (CNRS) in Paris, working with theorists at the Simons Foundation's Flatiron Institute in New York, has directly imaged the paired particles that give superconductors their almost magical properties — and discovered that the pairs are not behaving the way the textbooks say they should.
The findings, published on 15 April in the journal Physical Review Letters, fill in a gap in a 70-year-old, Nobel-prize-winning theory of how superconductors work.
A quick refresher: what is a superconductor?
Most materials resist the flow of electricity. That is why your phone charger gets warm and why the national grid loses a small slice of every kilowatt it transmits. But cool certain metals down far enough — typically to within a whisker of absolute zero, the coldest temperature physically possible — and something extraordinary happens. Their electrical resistance vanishes entirely.
That is superconductivity. And it matters, because materials with zero resistance can carry electricity without losing any energy as heat. They are already at work inside MRI scanners, the world's most sensitive particle accelerators, and the magnetic-levitation trains that glide between Shanghai and its airport. They are also a central ingredient in the race to build a useful quantum computer.
The catch is the temperature. Keeping things that cold is expensive and impractical. The dream — physicists call it the holy grail of the field — is a material that superconducts at room temperature.
The dancing pairs
The current best theory, known as BCS theory after the trio of American physicists who proposed it in 1957, says that superconductivity happens because electrons, normally a stand-offish bunch, find a way to couple up. Each pair then glides through the metal without bumping into anything. Crucially, the theory assumes the pairs ignore each other entirely — every couple does its own thing.
To test that, the team used a clever stand-in. Real superconductors are messy and hard to image, so the researchers built a tiny gas of lithium atoms cooled to a few billionths of a degree above absolute zero. At that temperature, the atoms behave like electrons, only bigger and easier to photograph.
Then they took the pictures. And the pairs were not ignoring each other at all.
"The BCS theory gives us a view from outside the ballroom, where we can hear the music and see the dancers come out, but we don't know what's going on in the ballroom," said Tarik Yefsah, the lead experimentalist, of the Laboratoire Kastler Brossel at CNRS. "Our approach is like taking a wide-angle camera inside the ballroom. Now we can see how the dancers are pairing up and paying attention to one another, so they don't bump into each other."
The paired atoms, in other words, were keeping a polite distance from one another, like couples on a crowded dance floor — a synchronised pattern no one had predicted.
What it means
Theoretical physicists at the Flatiron Institute, led by Shiwei Zhang, ran detailed quantum simulations that matched the experiment exactly, confirming that the dance is real.
"BCS theory tells us superconductivity arises because electrons have a tendency to pair," Zhang said. "But it's a rough theory, and it doesn't tell us anything about how the pairs interact." Now, for the first time, it does.
Nobody is promising a room-temperature superconductor tomorrow. But every glimpse of what is actually going on inside these strange materials brings the dream a little closer — and the prize, an electrical grid that loses no energy and computers that barely sip it, is enormous.
