Superconductors carry current without resistance, but most function only a few degrees above absolute zero. In the late 1980s, copper-based materials called cuprates were found to be superconductive at higher temperatures – achievable with affordable liquid nitrogen. But almost 40 years later, physicists still don’t agree why.
In a recent Science study, a team of physicists led by Milan Allan of Ludwig Maximilian University of Munich and Ingmar Swart of Utrecht University in the Netherlands found some answers by listening to the faint electrical crackle of current flowing into a cuprate.
The missing electrons
At the heart of the mystery of cuprate high-temperature superconductivity is a strange state called a pseudogap. “The pseudogap is a phenomenon of missing electrons,” Allan said. Above the temperature at which cuprates become superconducting, some of their electrons appear to disappear – measurements show that there are far fewer of them at the energies at which they should be. “Everyone knows that you can see from different probes that there are these disappearing electrons, but no one knows why,” Allan said.
Over the decades, physicists have put forward two competing explanations. The first hypothesis assumes that the electrons get stuck in some structure. In this view, electrons solidify into patterns like charge density waves, and superconductivity develops separately. The alternative explanation links this phenomenon closely to superconductivity. “Other people think they are missing because they form pairs,” Allan said. Electron pairs, so-called Cooper pairs, are what transport the current in a superconductor. If these pairs appear as the material cools, before the material becomes superconducting, they would be a prerequisite for superconductivity, a kind of waiting room in which pairs exist but cannot yet move together in lockstep.
This proved extremely difficult to sort out. “A lot of different experimental signatures are kind of summarized in this,” Allan said. Different instruments all see something, but their readings, Allan said, are never conclusive. “It’s always something disjointed, hard to understand,” he said. What the researchers needed was a way to see couples directly, and Allan and his colleagues figured it out.
