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Physicists listened to electrical noise to crack a superconductor mystery
Tuning in to electronic noise can tell us whether electrons have paired up or not.
Superconductors carry electricity without resistance, but most only work a few degrees above absolute zero. In the late 1980s, copper-based materials called cuprates were found to superconduct at higher temperatures—reachable with affordable liquid nitrogen. Yet nearly 40 years later, physicists still don’t agree on 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 has found some answers by listening to the faint electrical crackle of current flowing into a cuprate.
Central to the mystery of cuprates’ high-temperature superconductivity is a strange state called the pseudogap. “The pseudogap is a phenomenon of missing electrons,” Allan said. Above the temperature at which cuprates become superconducting, some of their electrons seem to vanish—measurements find far fewer of them at the energies where they should be. “Everybody knows that from different probes, you see that there’s these vanishing electrons, but nobody knows why,” Allan said.
Over the decades, physicists have put forward two main competing explanations. The first hypothesizes that the electrons get stuck in some sort of structure. In this view, electrons freeze into patterns, such as charge density waves, and superconductivity develops separately. The alternative explanation tightly links this phenomenon to superconductivity. “Other people think they’re missing because they form pairs,” Allan said. Pairs of electrons, called Cooper pairs, are what carry current in a superconductor. If, as the material cools, these pairs appear before the material becomes superconducting, they would be a precondition of superconductivity, a waiting room of sorts, where pairs exist but can’t yet move together in lockstep.
Settling this proved extremely hard. “Lots of different experimental signatures are kind of lumped inside this,” Allan said. Different instruments all see something, but their readings, Allan said, are never conclusive. “It’s always something kind of incoherent, hard to grasp,” he said. What the researchers needed was a way to see pairs directly, and Allan and his colleagues figured it out.
The team decided to listen to shot noise—tiny fluctuations caused by the arrival of individual charges that form the stream of an electric current. Allan says it resembles rain drumming on the roof. “If you imagine that you have raindrops and then all of a sudden you make the drops twice as heavy, there’ll be more noise,” Allan said. “With the electrons it’s similar. If you have two electrons coming always together, because they’re associated with pairing, then it will be more noise.”
The researchers did this measurement with a scanning tunneling microscope, which passed current between a needle-sharp tip and a cuprate sample, letting them listen to the noise at specific energy levels and different locations on the surface. In conventional superconductors, pairing produces a clean doubling of the noise. In cuprates, the signal is much weaker, because single electrons keep flowing alongside the pairs.
Making the instrument sensitive enough to discern the pairs took years. “What it took is very talented and persistent students who worked for, I don’t know, four years on this to get it working,” Allan said. “There was not a single trick. It was electronics craftsmanship.”
The team examined crystals of a cuprate known as BSCCO, cooled to a few degrees above absolute zero, and compared the noise to the energies at which electrons go missing. By adjusting the voltage between the tip of the needle and the sample, the team could choose the energy of the electrons they were listening to, a bit like turning a radio dial. If the pseudogap came purely from frozen patterns, the team reasoned, the extra noise of pairing should only kick in at energy levels associated with superco