Physicists have listened to the faint electrical crackle of current flowing into a cuprate material to crack a nearly 40-year-old superconductor mystery.
Superconductors carry electricity without resistance. Most operate only a few degrees above absolute zero, but copper-based cuprates discovered in the late 1980s superconduct at higher temperatures reachable with affordable liquid nitrogen.
Nearly 40 years later, physicists still do not agree on why. Central to this high-temperature superconductivity mystery is a strange state known as the pseudogap.
“The pseudogap is a phenomenon of missing electrons,” Milan Allan said. Above the superconducting temperature, measurements find far fewer electrons at the energies where they should belong.
“Everybody knows that from different probes, you see that there’s these vanishing electrons, but nobody knows why,” Allan said.
Physicists have debated two main explanations for decades. The first hypothesizes that electrons freeze into patterns like charge density waves while superconductivity develops separately.
The alternative explanation links the vanishing electrons directly to superconductivity. “Other people think they’re missing because they form pairs,” Allan said.
These electron pairs, called Cooper pairs, carry current in a superconductor. If pairs appear before the material becomes superconducting as it cools, they act as a waiting room where pairs exist without moving together in lockstep.
Settling this question proved extremely hard. “Lots of different experimental signatures are kind of lumped inside this,” Allan said. Different instruments show readings that remain incoherent and hard to grasp.
A team of physicists led by Milan Allan of Ludwig Maximilian University of Munich and Ingmar Swart of Utrecht University in the Netherlands needed a way to see pairs directly. They figured out how by listening to the faint electrical crackle of current flowing into a cuprate, as detailed in a recent Science study.
