Physicists open a new path to a strange form of superconductivity

The researchers published a new theoretical framework.

Physicists have identified a mechanism responsible for creating oscillating superconductors, called density pair waves. The results, which highlight an atypical state of high superconductivity observed in certain materials such as high-temperature superconductors, are published in Physical review letters.

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

The first author of the study is Pedro Castro, a graduate student in physics from Emory. Co-authors are Daniel Shaffer, a postdoctoral fellow in the Santos group, and Yi-Ming Wu of Stanford University.

Santos is a theoretical scientist specializing in condensed matter physics. It studies the interactions of quantum matter — small things like atoms, photons, and electrons — that don’t behave according to the laws of classical physics.

Superconductivity, or the ability of some materials to conduct electricity without losing energy when cooled to an extremely low temperature, is one example of interesting quantum behavior. This phenomenon was discovered in 1911 when Dutch physicist Heike Kamerling Onnes showed that mercury lost its electrical resistance when cooled to 4 Kelvin, or negative 371 degrees. F. This is about a temperature Uranusthe 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 wander more or less independently. They collide with other particles, causing them to change speed and direction and dissipate energy. However, at lower temperatures, electrons can organize into a new state of matter.

Louise Santos, assistant professor of physics at Emory University, is the lead author of the study. Credit: Emory University

“They form pairs bound together in a collective state that act like a single entity,” Santos explains. “You can think of them as soldiers in an army. If they are moving in isolation they are easy to deflect. But when they are walking together at a steady pace, it is very difficult to destabilize them. This collective state carries the current in a powerful way.”

Superconductivity holds enormous potential. In theory, it could allow electric current to travel through the wires without heating them up or losing energy. These wires can then carry much more electricity, and with much more efficiency.

“One of the great hallows of physics is room-temperature superconductivity, which is practical enough for everyday applications,” says Santos. “This breakthrough could change the shape of civilization.”

Many physicists and engineers are working on this front line to revolutionize how electricity is transmitted.

Meanwhile, superconductivity has already found applications. Superconducting coils run on the magnetic energy used in magnetic resonance imaging (MRI) machines for medical diagnosis. A handful of magnetic levitation trains are now in operation in the world, built on superconducting magnets ten times stronger than ordinary electromagnets. The magnets repel when the two identical poles face each other, generating a magnetic field capable of lifting and propelling the train.

The Large Hadron Collider, a particle accelerator that scientists use to research the basic structure of the universe, is another example of technology that works through superconductivity.

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

One focus of Santos’ 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 a so-called exotic phenomenon is oscillating superconductivity, when paired electrons dance in waves, changing capacitance.

In an unrelated project, Santos asked Castro to investigate certain properties of van Hove singularities, structures in which many electronic states become close in energy. The Castro Project revealed that singularities appear to have the right kind of physics for seeding oscillating superconductors.

That prompted Santos and his collaborators to dig deeper. They have discovered a mechanism that would allow dancing wave states of superconductivity to emerge from van Hove singularities.

“As theoretical physicists, we want to be able to predict and classify behavior in order to understand how nature works,” says Santos. “Then we can start asking the relevant technology questions.”

Some high-temperature superconductors — which operate at temperatures three times colder than a home freezer — have this dancing wave behaviour. Figuring out how this behavior emerges from van Hove singularities provides a basis for experimentalists to explore the world of possibilities it presents.

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

Reference: “Emergence of the Supercharged Chern Wave and Pair Density by High-level Van Hove Singularities in the Haldane-Hubbard Model” by Pedro Castro, Daniel Schafer, Ye-Ming Wu, and Louise H. Santos, July 11, 2023, Available here. Physical review letters.
DOI: 10.1103/PhysRevLett.131.026601

The work was funded by the US Department of Energy’s Office of Basic Energy Sciences.