Figure 1: A conceptual illustration of a Majorana fermion, which is also its own antiparticle. A Majorana fermion's multipole response to electromagnetic waves provides information about the Cooper pairs in a topological superconductor, two RIKEN physicists have predicted. © 2025 RIKEN Condensed Matter Theory Laboratory
A possible method for probing the properties of exotic particles that exist on the surfaces of an unusual type of superconductor has been theoretically proposed by two RIKEN physicists1.
When cooled to very low temperatures, two or more electrons in some solids start to behave as if they were a single particle.
This can give the material some exotic properties. For example, superconductivity arises in some materials because electrons form into couples known as Cooper pairs that move through the material without facing any electrical resistance.
Now, Yuki Yamazaki from the RIKEN Condensed Matter Theory Laboratory and Shingo Kobayashi from the RIKEN Center for Emergent Matter Science have theoretically proposed a method that could provide information about these Cooper pairs in an exciting type of superconductor that has only recently been discovered-a topological superconductor.
In conventional superconductors, Cooper pairs form due to interactions between the electrons and atomic vibrations, and they have a relatively simple symmetric geometry.
In contrast, the Cooper pairs in topological superconductors exhibit a more complex symmetry. "This symmetry in turn gives rise to special quantum states on the surface of the material known as Majorana fermions," explains Yamazaki.
First predicted by Ettore Majorana in 1937, the Majorana fermion is a particle that is identical to its antiparticle.
A pair of Majorana fermions appear on the surfaces of time-reversal symmetric topological superconductors. They are said to be 'time-reversal symmetric'-that is, they would behave the same if time were reversed. They are also characterized by an electromagnetic response that varies depending on direction, known as a Majorana multipole response.
But in a few special materials, Cooper pairs break this time-reversal symmetry so that Majorana fermions no longer form pairs.
"In time-reversal-symmetry-breaking topological superconductors, a single Majorana fermion appears on the boundary," says Yamazaki. "It doesn't interact with external fields because it's electrically neutral."
This lack of interaction with fields makes it difficult to probe these isolated Majorana fermions. To find a way to investigate them, Yamazaki and Kobayashi have theoretically extended the concept of Majorana multipole responses to time-reversal-symmetry-breaking topological superconductors.
In this way, they showed how the electromagnetic response of Majorana fermions can provide insights into the properties of the Cooper pairs in the underlying superconducting material.
"Our research has identified the fundamental electromagnetic properties of Majorana fermions in topological superconductors," says Yamazaki. "However, further investigation is required to explore their influence on actual physical quantities and to establish techniques for detecting them."
