Physicists Use Electrical Noise to Unravel High-Temperature Superconductor Puzzle

Electricity flows through superconductors without any resistance, though most varieties require temperatures just a few degrees above absolute zero to function. During the late 1980s, scientists discovered that cuprates—materials based on copper—could achieve superconductivity at substantially higher temperatures, making them practical to cool with liquid nitrogen rather than more exotic refrigerants. Despite the passage of nearly four decades, the scientific community remains divided on the fundamental mechanism behind this phenomenon.

Researchers at Ludwig Maximilian University of Munich and Utrecht University in the Netherlands, led by Milan Allan and Ingmar Swart, have published findings in Science that shed light on this longstanding question. Their approach involved analyzing the faint electrical noise generated as current passes through a cuprate sample.

The missing electrons

At the heart of the cuprate superconductivity puzzle sits an enigmatic phenomenon known as the pseudogap. "The pseudogap is a phenomenon of missing electrons," Allan explained. When cuprates are heated above their superconducting transition temperature, a puzzling disappearance occurs: measurements reveal far fewer electrons than theory predicts at the energy levels where they should appear. "Everybody knows that from different probes, you see that there's these vanishing electrons, but nobody knows why," Allan said.

Two competing theoretical frameworks have emerged over the years to explain this mystery. One school of thought proposes that electrons become trapped within some kind of structural arrangement. According to this view, electrons crystallize into patterns such as charge density waves, with superconductivity arising as a separate phenomenon. The competing perspective draws a direct connection between the missing electrons and superconductivity itself. "Other people think they're missing because they form pairs," Allan said. In superconductors, pairs of electrons known as Cooper pairs are responsible for carrying electrical current. If these pairs materialize as the material cools but before true superconductivity emerges, they could represent a preparatory state—a kind of waiting room where pairs exist but cannot yet move in perfect synchronization.

Distinguishing between these theories has proven extraordinarily challenging. "Lots of different experimental signatures are kind of lumped inside this," Allan noted. While various measurement techniques each detect something, their results remain ambiguous. "It's always something kind of incoherent, hard to grasp," he said. The researchers needed a method to observe pairs directly, and they devised one.

Heavier raindrops

The team's strategy centered on measuring shot noise—the minute variations produced when individual charges arrive as part of an electrical current stream. Allan drew an analogy to precipitation. "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."

Using a scanning tunneling microscope, the team passed current between an extremely sharp needle tip and a cuprate sample, enabling them to detect noise at particular energy levels and various points across the surface. In traditional superconductors, pairing produces a straightforward doubling of the noise signal. In cuprates, however, the effect is considerably fainter because individual electrons continue to flow alongside the paired electrons.

Achieving sufficient instrument sensitivity to resolve the pairing signal required years of development. "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 step at the gap

The researchers studied crystals of BSCCO, a well-known cuprate compound, maintained at temperatures just above absolute zero, and correlated the noise patterns with the energies where electrons vanish. By varying the voltage applied between the needle tip and sample, they could select which electron energies to examine—similar to tuning a radio frequency. If frozen patterns alone caused the pseudogap, the team reasoned, pairing-related noise should emerge only at energies tied to superconductivity. Instead, the pairing signal appeared precisely at the pseudogap energy.

"The fact that there is more noise indicates that pairing is associated with this large gap," Allan stated. Though the pseudogap energy fluctuates from location to location on the crystal surface, the extra noise consistently began rising at the local pseudogap energy wherever measurements were taken. This pattern remained consistent across samples with varying chemical formulations.

Eliminating alternative noise sources, including surface atomic motion, demanded considerable experimental rigor. "Noise is super hard to interpret, and that was basically the thing that we had the most sleepless nights about," Allan said. The critical verification involved subjecting samples to magnetic fields reaching six tesla, which disrupts pairing while preserving the overall electron-absence pattern. As magnetic field strength increased, the noise diminished back toward single-electron levels, precisely matching predictions if pairs were responsible.

The strength of electron pairing can be quantified as the energy needed to separate a pair. Temperature reduction enhances pairing because warmer materials experience more vigorous particle motion, which breaks apart more pairs. In conventional superconductors, the two requirements for superconductivity—pair formation and phase coherence—occur simultaneously at the critical temperature. (Phase coherence represents the point where electrons begin moving together as a unified fluid.) The Allan and Swart study indicates cuprates follow a different sequence.

The pairing energy measured by the team substantially exceeds the temperature at which the material becomes superconducting. "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. This suggests the true limitation on superconductivity is not achieving electron pairing, but rather synchronizing paired electrons to move together.

"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."

Tempering hopes

One optimistic interpretation of these results suggests that cuprate electrons might theoretically pair at room temperature, requiring only coherence to achieve a room-temperature superconductor. The research team, however, considers this perspective overly simplistic. "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.

The researchers emphasize that their findings do not completely resolve the central question. The work eliminates the possibility that frozen patterns alone explain the pseudogap, but does not rule out that such patterns interact with pairing. "Whatever the detailed explanation of the pseudogap is, pairing should be part of it," Swart said.

The measurement technique employed in the study becomes less effective at higher temperatures, preventing the team from listening above the superconducting transition point, where the pseudogap is conventionally 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."

Swart intends to create higher-resolution noise maps of crystal surfaces and eventually develop an instrument capable of directly measuring coherence, which could provide separate indicators for when phase coherence and electron pairing each begin. Allan, for his part, increasingly believes that advancing superconductor performance depends more on chemical experimentation, potentially guided by artificial intelligence, than on theoretical physics alone. "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.