Superconductors carry electricity without resistance, a feat that most of them only manage a few degrees above absolute zero - which is to say, at temperatures where "practical application" remains a distant dream. In the late 1980s, copper-based materials called cuprates were found to superconduct at higher temperatures, reachable with affordable liquid nitrogen. Nearly 40 years later, physicists still don't agree on why. This is what happens when you discover something works before you understand it - a situation familiar to anyone who has ever fixed a computer by restarting it.
In a recent Science study, a team 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. Yes, they literally listened to the noise. It's not quite "turn it off and on again," but it's close.
Central to the mystery 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. This is the physics equivalent of losing your keys and finding them in the last place you look, except the keys are fundamental particles and the last place is a quantum state.
Over the decades, physicists have put forward two main competing explanations. The first hypothesizes that electrons get stuck in some sort of structure - freezing into patterns like charge density waves - and superconductivity develops separately. The alternative tightly links the 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 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. Imagine a dance floor where everyone has a partner but no one can agree on the choreography.
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." So the next time someone complains about noisy electronics, tell them it might just be electrons pairing up.
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. It's like trying to hear a duet in a crowded room where most people are singing solo.
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." Four years of graduate student labor - the unsung hero of experimental physics.
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 superconductivity. Instead, pairing showed up right at the pseudogap energy. In other words, the electrons weren't stuck; they were pairing up. The pseudogap is not a traffic jam; it's a couples' retreat.
"The fact that there is more noise indicates that pairing is associated with this large gap," Allan said. The pseudogap's energy varies from spot to spot on the surface, but wherever the team listened, the extra noise always started rising at the local pseudogap energy. The same pattern held in samples with different chemical compositions. This is the experimental equivalent of checking every room in the house and finding the same thing in each one.
Ruling out other sources of noise, such as atoms shifting on the surface, took a lot of effort. "Noise is super hard to interpret, and that was basically the thing that we had the most sleepless nights about," Allan said. The decisive check involved placing the samples in magnetic fields of up to six tesla, which suppresses pairing while leaving the overall pattern of missing electrons largely intact. As the magnetic field grew, the noise dropped back toward the single-electron level, exactly as expected if pairs were behind it. Magnetic fields: the party poopers of the quantum world.
The strength of electron pairing can be expressed as the energy required to pull a pair apart. Lowering the temperature helps boost pairing because the warmer a material becomes, the more violently its particles jostle and the more pairs get broken. In conventional superconductors, two ingredients needed for superconductivity - pair formation and phase coherence - occur simultaneously at the critical temperature. (Phase coherence is where the electrons start moving together as a fluid.) In cuprates, Allan and Swart's study points to a different sequence. It's not a simultaneous dance; it's a two-step.
The pairing energy the team measured is well above the temperature at which the material superconducts. "So the picture that emerges is that you first have pre-formed pairs, and then only at lower temperatures, phase coherence sets in," Swart said. That suggests the superconductivity bottleneck is not getting electrons to pair, but getting the pairs to move together. The electrons are ready to dance; they just need a choreographer.
"If you have pairing at such high energies, that means that you might be able to push Tc [superconducting temperature] higher," Allan said. "This is something people hoped for in the very early days of cuprates, and now everybody's afraid to even mention that. But to me it gives hope." Hope: the thing that springs eternal, even in physics labs.
An optimistic reading of the results is that cuprate electrons could theoretically pair at room temperature, and we only need to make them coherent to achieve a room-temperature superconductor. The team, though, thinks that's a bit naive. "I think Ingmar and I both believe that it's not going to be that simple, because you have all kinds of other effects that are coupled there," Allan said. So no, we're not getting room-temperature superconductors tomorrow. But maybe the day after.
The researchers also stress that their experiment does not fully answer the central question. It rules out the idea that the pseudogap comes from frozen patterns alone, but not that such patterns are intertwined with pairing. "Whatever the detailed explanation of the pseudogap is, pairing should be part of it," Swart said. In science, as in life, the answer is often "yes, and."
Because the technique used in the study loses sensitivity as temperature rises, the team couldn't yet listen above the superconducting temperature, where the pseudogap is usually defined. "Hopefully other groups will do that," Allan said. "What limits Tc, especially if the energy of pairing is high? What happens at Tc? That's for me the most interesting question." Translation: we've opened the door; someone else can walk through it.
Swart wants to make noise maps of the surface with higher resolution and, eventually, an instrument that can detect coherence directly, which he hopes should give us separate markers for the onset of phase coherence and electron pairing. Allan, meanwhile, increasingly thinks the route to better superconductors runs through chemistry, aided by AI, rather than pure physics. "When it comes to trying to make the superconducting material better and increase Tc, I have more hope in direct trial and error," he said. Ah, trial and error - the scientific method's most reliable, if least glamorous, tool. Some things never change, even in quantum physics.