In the late 1980s, researchers discovered that copper-based materials called cuprates superconduct at temperatures reachable with affordable liquid nitrogen. Most other superconductors only work a few degrees above absolute zero, making cuprates uniquely practical for many applications. Yet nearly 40 years later, physicists still do not agree on why these materials behave this way. The central puzzle is a strange state called the pseudogap, which appears above the temperature where cuprates become superconducting. In this regime, measurements find far fewer electrons at the energies where they should be present. Milan Allan of Ludwig Maximilian University of Munich and Ingmar Swart of Utrecht University in the Netherlands recently addressed this gap in a Science study by listening to the faint electrical crackle of current flowing into a cuprate sample.
The missing electrons
Allan describes the pseudogap as a phenomenon of missing electrons, noting that "everybody knows that from different probes, you see that there’s these vanishing electrons, but nobody knows why." Over the decades, two main competing explanations have emerged to account for this behavior. One hypothesis suggests that electrons get stuck in structures such as charge density waves, freezing into patterns while superconductivity develops separately. The alternative explanation tightly links the pseudogap to superconductivity itself, proposing that electrons form pairs before the material becomes superconducting. These pairs, known as Cooper pairs, are what carry current in a superconductor. If they appear early, they act as a precondition for superconductivity, creating a waiting room where pairs exist but cannot yet move together in lockstep. Settling this debate proved extremely hard because different instruments all see something, but their readings are never conclusive. Allan stated that the data is always something kind of incoherent and hard to grasp, leaving researchers without a direct view of the pairs.
Listening to the crackle
The team needed a way to see pairs directly, and they figured out how to extract this information from electrical noise. By analyzing the faint crackle of current flowing into the cuprate, the researchers could distinguish between the two competing models. This method provided a clearer picture than previous probes, which often produced conflicting or ambiguous results. The findings support the idea that the pseudogap is linked to the formation of Cooper pairs rather than independent structural changes. This approach offers a new tool for studying high-temperature superconductivity, allowing physicists to probe the electronic states more precisely. While the mystery is not fully solved, this work provides concrete evidence for one side of the long-standing debate. The ability to listen to electrical noise represents a significant shift in how researchers investigate these complex materials.
