Physicists still do not agree on why copper-based materials superconduct at temperatures reachable with affordable liquid nitrogen.
Nearly 40 years after the discovery of cuprates, the underlying mechanism remains debated. A recent study published in Science provides new data on the phenomenon by listening to the faint electrical crackle of current flowing into a cuprate.
Milan Allan of Ludwig Maximilian University of Munich and Ingmar Swart of Utrecht University in the Netherlands led the team of physicists behind the new research. Central to the ongoing mystery of high-temperature superconductivity is a strange state known as the pseudogap. “The pseudogap is a phenomenon of missing electrons,” Allan said.
Above the temperature at which cuprates become superconducting, measurements show that some of their electrons seem to vanish. Far fewer electrons appear at the energies where they should normally be found. “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 for the missing electrons in cuprates. The first hypothesis suggests that electrons get stuck in some sort of structure by freezing into patterns, such as charge density waves, while superconductivity develops separately. The alternative explanation links the missing electrons directly to superconductivity itself. “Other people think they’re missing because they form pairs,” Allan said.
These pairs of electrons, known as Cooper pairs, carry current in a superconductor. If these pairs appear before the material becomes superconducting as it cools, they would act as a precondition for superconductivity. In this scenario, the pairs exist but cannot yet move together in lockstep.
Settling this scientific debate proved extremely difficult because many different experimental signatures are lumped inside the pseudogap. Different instruments record signals, but their readings are never conclusive. “It’s always something kind of incoherent, hard to grasp,” Allan said. The researchers needed a direct way to see pairs, which led Allan and his colleagues to their new experimental approach.
Experimental Insight
Researchers listened to the faint electrical crackle of current flowing into a cuprate to study the pseudogap directly, moving past decades of inconclusive instrument readings.
The research team figured out a way to observe pairs directly by listening to the faint electrical crackle of current entering the copper-based material. This allowed the physicists to probe the incoherent readings that historically clouded cuprate research. By analyzing the current flow, the team gathered fresh data on whether electron pairing precedes full superconductivity.
The findings published in the Science study offer some answers to a puzzle that has challenged physicists for nearly four decades.
Cuprates are copper-based materials discovered in the late 1980s that display superconductivity at temperatures significantly higher than traditional materials, reaching levels accessible with liquid nitrogen.
The pseudogap is a state where measurements detect far fewer electrons than expected at specific energy levels above the temperature where the material achieves zero electrical resistance.
The research was led by Milan Allan of Ludwig Maximilian University of Munich and Ingmar Swart of Utrecht University in the Netherlands.
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