High-Spin State Suggests Unconventional Superconductivity

Electron interactions in superconductors are dictated by a quantum property called spin. In an ordinary superconductor, electrons carry a spin of one-half and pair up and flow uninhibited with the help of vibrations in the atomic structure. The theory is well tested and can describe the behavior of most superconductors, but new research from the University of Maryland and its collaborators reports evidence for a new type of superconductivity in the material YPtBi, one that seems to arise from spin-3/2 particles.

“No one had really thought that this was possible in solid materials,” says Johnpierre Paglione, a UMD physics professor and senior author on the team’s paper, in Science Advances. “High-spin states in individual atoms are possible but once you put the atoms together in a solid, these states usually break apart and you end up with spin one-half. ”

Finding that YPtBi was a superconductor surprised the researchers as most superconductors start out as reasonably good conductors with a lot of mobile electrons, an ingredient that YPtBi lacks. According to the conventional theory, YPtBi would need about a thousand times more mobile electrons in order to become superconducting at temperatures below 0.8 K. And yet, upon cooling the material to this temperature, the team saw superconductivity happen anyway, the first sign that something exotic was going on inside the material.

After discovering the anomalous superconducting transition, researchers made measurements that provided insight into the underlying electron pairing, focusing on a telling feature of superconductors—their interaction with magnetic fields. As a material undergoes the transition to a superconductor, it will try to expel any added magnetic field from its interior. But the expulsion is not completely perfect. Near the surface, the magnetic field can still enter the material but then quickly decay away. How far it goes in depends on the nature of the electron pairing, and it changes as a material is cooled down further and further.

To probe this effect, the researchers varied the temperature in a small sample of the material while exposing it to a magnetic field more than ten times weaker than the Earth’s. A copper coil surrounding the sample detected changes to the superconductor’s magnetic properties and allowed the team to sensitively measure tiny variations in how deep the magnetic field reached inside the superconductor.

The measurements revealed an unusual magnetic intrusion. As the material warmed from near absolute zero, the field penetration depth for YPtBi increased linearly instead of exponentially as it would for a conventional superconductor. This effect, combined with other measurements and theory calculations, constrained the possible ways that electrons could pair up. The researchers concluded that the best explanation for the superconductivity was electrons disguised as particles with a higher spin—a possibility that hadn’t even been considered before in the framework of conventional superconductivity.

The discovery of this high-spin superconductor has given a new direction for research, according to the team. “We used to be confined to pairing with spin one-half particles,” says Hyunsoo Kim, lead author and a UMD assistant research scientist. “But if we start considering higher spin, then the landscape of this superconducting research expands and just gets more interesting.”

The research will continue at UMD’s Center for Nanophysics and Advanced Materials, Condensed Matter Theory Center and the Joint Quantum Institute, in collaboration with Ames Laboratory at Iowa State University, the Lawrence Berkley National Laboratory, the University of Otago and the University of Wisconsin. Many open questions remain, according to the team, including how such pairing could occur in the first place. “When you have this high-spin pairing, what’s the glue that holds these pairs together?” asks Paglione. “There are some ideas of what might be happening, but fundamental questions remain–which makes it even more fascinating.”