The research from University of Maryland (UMD) is said to reveal effects that are profoundly different from anything that has been seen before with superconductivity. According to the team, it has uncovered 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," said professor Johnpierre Paglione of UMD and senior author on the study. "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. "
Unlike YPtBi, superconductors start out as reasonably good conductors, so the findings surprised the team. 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 Kelvin. And yet, upon cooling the material to this temperature, the team saw superconductivity happen anyway. This was a first sign that something exotic was going on inside this material.
After discovering the anomalous superconducting transition, the researchers made measurements which gave them insight into the underlying electron pairing. They studied their interaction with magnetic fields and found that as the material undergoes the transition to a superconductor, it attempts to expel any added magnetic field from its interior. But the expulsion is not completely perfect, the researchers revealed. Near the surface, the magnetic field can still enter the material, but will quickly decay away. How far it goes in depends on the nature of the electron pairing, and changes as the material is cooled down further and further.
To probe this effect, the researchers said they 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 team said that the measurement revealed an unusual magnetic intrusion. As the material warmed from 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.
The researchers claimed that the discovery of the high-spin superconductor has given a new direction for this research field. "We used to be confined to pairing with spin one-half particles," said 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."
For now, many open questions remain, 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?" questioned Paglione. "There are some ideas of what might be happening, but fundamental questions remain-which makes it even more fascinating."
