Researchers have for the first time directly visualized the quantum behavior that causes superconductivity, a state in which pairs of electrons conduct electricity with zero resistance at extremely low temperatures.
But what they observed was surprising.
In a study published on April 15th, physical review letterThe researchers took images of individual atoms forming pairs in a specially prepared gas cooled to near absolute zero. This is the unattainable limit of the coldness of all things. This system, known as Fermi gas, allows scientists to replace electrons with atoms, allowing them to study superconductivity in a highly controlled environment.
Unexpected quantum ‘dance’ between paired particles
After the atoms paired up, researchers discovered something unusual. The pair did not act independently. Instead, they moved in a coordinated manner, with each pair’s position influenced by nearby pairs. This is a behavior not predicted by the Nobel Prize-winning theory of superconductivity 70 years ago.
“Our experiments showed that there is something qualitatively missing from this theory,” says Tariq Yefsa, head of experimental research at the Laboratoire Kassler-Brossel at the French National Center for Scientific Research (CNRS) in Paris. Yefser and other experimental physicists at CNRS collaborated on the new study with theoretical physicists including Shiwei Zhang of the Simmons Foundation Flatiron Institute.
The discovery adds an important piece to the puzzle of how superconductivity works and could guide efforts to create room-temperature superconductors, a long-standing goal that could dramatically improve the energy efficiency of power grids and electronics.
What is superconductivity and why is it important?
Superconductivity typically appears in certain metals when they are cooled to extremely low temperatures, much colder than those found naturally on Earth. When these materials drop below a critical temperature, their electrical resistance suddenly disappears. This happens because electrons form pairs and move together. This is often compared to dancers moving in sync across a ballroom floor.
This phenomenon was first described in the 1950s by physicists John Bardeen, Leon Cooper, and John Robert Schriefer.
Limitations of classical BCS theory
However, the BCS theory (named after its creator) provides only a rough explanation. It is not possible to fully describe all types of superconductors or capture all aspects of the associated behavior. Scientists have long suspected that the theory was missing important details, but the gap remained unclear.
“According to BCS theory, superconductivity occurs because electrons tend to form pairs,” says Zhang, a senior researcher and group leader in the Center for Computational Quantum Physics (CCQ) at the Flatiron Institute. “But it’s a rough theory and tells us nothing about how the pairs interact.” According to BCS theory, these pairs operate independently. This means that their positions should not depend on each other.
New imaging technique reveals interacting pairs
To investigate this missing piece, experimental physicists at CNRS worked closely with theorists at CCQ to study how these pairs influence each other.
Using a newly developed imaging technique, the research team took detailed snapshots of the positions of the paired atoms. They worked with a gas of lithium atoms cooled to just a few billionths of a degree below absolute zero. At such extreme temperatures, atoms behave as fermions, the same category of particles as electrons, making them ideal stand-ins for studying superconductivity.
The images showed that the paired atoms were not randomly distributed. Instead, their positions were linked, and each pair maintained a certain distance from the other, similar to couples avoiding collisions on a dance floor. This behavior reveals an additional layer of organization not included in traditional BCS frameworks.
A new perspective inside the quantum “ballroom”
“The BCS theory gives you a view from outside the ballroom, where you hear the music and see the dancers coming out, but you don’t see what’s going on inside the ballroom,” Yehusa says. “Our approach is like taking a wide-angle camera inside the ballroom, so you can see how the dancers pair up and look out for each other to avoid bumping into each other.”
To verify this finding, Zhang and Yuan-Yao He, a former postdoctoral fellow at CCQ and Institute of Modern Physics at Northwest University, China, performed detailed quantum simulations of the same system. The simulations matched experimental data and confirmed newly observed behaviors, such as the spacing between paired “dancers.”
Impact on future superconductors
These results improve scientists’ understanding of superconductors and other quantum materials consisting of fermions. Such insights are essential for designing materials that are superconducting at high temperatures.
In the 1980s, researchers discovered a class of materials known as high-temperature superconductors. The material operates at temperatures around liquid nitrogen, which is still chilly -196 degrees Celsius (-321 degrees Fahrenheit). Still, scientists still don’t fully understand why these materials work at relatively high temperatures.
Researchers hope to deepen their fundamental understanding of superconductivity and ultimately develop materials that function at everyday temperatures, revolutionizing energy transmission and computing technologies.
“By understanding this simple case, we can fine-tune our tools to study more complex systems,” Zhang says. “And in more complex systems, new levels of matter are being explored, and this has led to many technological advances in the past.”
