Physicists open a new path to an exotic form of superconductivity

Physicists have identified a mechanism for the formation of oscillating superconductivity called pair density waves. Physical review letters published the discovery, which provides new insight into an unconventional high-temperature superconducting state seen in some materials, including high-temperature superconductors.

“We discovered that structures known as Van Hove singularities can produce modulating, oscillating states of superconductivity,” said Luiz Santos, assistant professor of physics at Emory University and senior author of the study. “Our work provides a new theoretical framework for understanding the origin of this behavior, a phenomenon that is not well understood.”

The superconductivity puzzle

Santos is a theorist specializing in condensed matter physics. He studies the interactions between quantum materials – tiny things like atoms, photons and electrons – that do not behave according to the laws of classical physics.

Superconductivity, or the ability of certain materials to conduct electricity without energy loss when cooled to a super-low temperature, is an example of exciting quantum behavior. The phenomenon was discovered in 1911 when the Dutch physicist Heike Kamerlingh Onnes showed that mercury lost its electrical resistance when cooled to 4 Kelvin or minus 371 degrees Fahrenheit. That’s about the temperature of Uranus, the coldest planet in the solar system.

It took scientists until 1957 to come up with an explanation for how and why superconductivity occurs. At normal temperatures, electrons roam more or less independently of each other. They bump into other particles, causing them to change speed and direction and lose energy. At low temperatures, however, electrons can organize themselves into a new state of matter.

“They form pairs that are bound together into a collective state that behaves as a single entity,” explains Santos. “You can think of them as soldiers in an army. If they move in isolation, they’re easier to divert. But when they march together in lockstep, it’s much harder to destabilize them. This collective state carries power in a robust way.”

The holy grail of physics

Superconductivity has enormous potential. In theory, it could allow electric current to move through wires without heating them or losing energy. These wires would then be able to carry much more electricity, much more efficiently.

“One of the holy grails of physics is superconductivity at room temperature that is practical enough for everyday applications,” says Santos. “That breakthrough could change the shape of civilization.”

Many physicists and engineers are working on this frontline to revolutionize the way electricity is transmitted.

Meanwhile, superconductivity has already found applications. Superconducting coils drive electromagnets used in magnetic resonance imaging machines (MRI) for medical diagnostics. A handful of magnetic levitation trains are now in operation in the world, built on superconducting magnets which is 10 times stronger than ordinary electromagnets. The magnets repel each other when the matching poles face each other, generating a magnetic field that can levitate and propel a train.

The Large Hadron Collider, a particle accelerator which scientists use to probe the fundamental structure of the universe, is another example of technology going through superconductivity.

Superconductivity continues to be discovered in more materials, including many that are superconducting at higher temperatures.

An accidental discovery

A focus of Santo’s research is how interactions between electrons can lead to forms of superconductivity that cannot be explained by the 1957 description of superconductivity. An example of this so-called exotic phenomenon is oscillatory superconductivity, when the paired electrons dance in waves and change amplitude.

In an unrelated project, Santos asked Castro to investigate specific properties of Van Hove singularities, structures where many electronic states become close in energy. Castro’s project revealed that the singularities appeared to have the right kind of physics to create oscillatory superconductivity.

That prompted Santos and his associates to dig deeper. They revealed a mechanism that would allow these dancing wave states of superconductivity to arise from Van Hove singularities.

“As theoretical physicistswe want to be able to predict and classify behaviors to understand how nature works,” says Santos. “Then we can start asking questions with technical relevance.”

A few high temperature superconductors-which operates at temperatures roughly three times as cold as a household freezer – has this dancing wave behavior. The discovery of how this behavior can emerge from Van Hove’s singularities provides a basis for experimentalists to explore the realm of possibilities it provides.

“I doubt that Kamerlingh Onnes was thinking about levitation or particle accelerators when he discovered superconductivity,” says Santos. “But everything we learn about the world has potential applications.”