Magnetic fields are generally known to destroy superconductivity in a material. However, in exceptional cases they can lead to what is known as "reentrant superconductivity"— where superconductivity disappears as expected but then unexpectedly returns when the magnetic field is increased further. This behavior is sometimes seen in bulk, three-dimensional materials, but now, in a study published in Science Advances, a team led by the RIKEN Center for Emergent Matter Science (CEMS) in Japan has seen the phenomenon in a very thin conducting layer at the boundary between two insulating oxide materials. Because oxide interfaces can be precisely engineered and controlled, the discovery provides a new platform for investigating unconventional forms of superconductivity and the quantum mechanisms that allow it to survive under unusual conditions.
Reentrant superconductivity is a counterintuitive phenomenon. Superconductivity normally arises when electrons form Cooper pairs. In conventional superconductors, the two electrons in a pair have opposite spins. A magnetic field tends to align the electron spins, which can disrupt the delicate pairing responsible for conventional superconductivity. Thus, when reentrant superconductivity does happen, it indicates that something more complex is going on, such as more complex quantum mechanisms.
Similar effects have been observed in a few complex bulk materials, where they are often associated with non-standard forms of superconductivity. In the new study, the researchers created a very thin conducting layer at the boundary between two oxide materials, cooled it to temperatures close to absolute zero, and measured its electrical resistance while applying magnetic fields. This approach allowed them to track directly when the material entered or left the superconducting state. Observing this behavior in a well-defined two-dimensional interfacial system places it in a distinct physical regime.
The results show that the magnetic field does not act only as a simple destructive force in this system. Instead, the superconducting state appears to depend on a delicate balance among electronic effects at the oxide interface. This makes the material system valuable as a controllable setting for studying superconductivity in situations that are difficult to access in more complex bulk materials. The observed behavior also points to the possibility that the superconducting state is influenced by effects beyond the conventional description of superconductivity, which can now be explored in this system.
According to first author Denis Maryenko of RIKEN CEMS, "We are quite excited by this study, as it shows an unexpected behavior of superconductivity. We found this behavior in an extremely thin electronic system at the boundary between two otherwise insulating oxide materials, giving us a new way to ask why superconductivity can sometimes recover under conditions where we would normally expect it to keep fading. This gives us a new experimental platform for investigating why superconductivity can survive and even re-emerge under conditions where conventional theories would predict its disappearance."
"In the longer term," he adds, "insights from such studies may help guide the search for new superconducting materials and future low-loss electronic or quantum devices."
