Superconductors: the 40-year argument about missing electrons just became testable

Superconductors carry current with no electrical resistance at all. A family of copper-based compounds called cuprates reaches those temperatures using a cheap, widely available coolant, liquid nitrogen, and since the late 1980s those materials have been the focus of a stubborn argument about what the electrons do just before a material starts conducting perfectly.

The state at the center of that argument is called the pseudogap. Above the temperature at which a cuprate becomes superconducting, measurements keep finding a shortage of electrons at the energies where they should be. Electrons do not vanish; fewer of them carry the energy they otherwise would. Two explanations have survived scrutiny. In one, the electrons freeze into a repeating pattern of charge, a charge density wave, and superconductivity arrives later as something separate. In the other, the missing electrons are Cooper pairs, two electrons bound together that carry current once they move in lockstep, and the pseudogap is a waiting room where pairs exist but cannot yet move as one. "Everybody knows that from different probes, you see that there's these vanishing electrons, but nobody knows why," said Milan Allan of Ludwig Maximilian University of Munich, who led the work with Ingmar Swart of Utrecht University.

The instrument the team built listens to the faint electrical crackle that individual charges make when they arrive, a signal physicists call shot noise. A scanning tunneling microscope runs current between a needle-sharp tip and a cooled crystal; turning the voltage on the tip, like a radio dial, selects which electron energies it hears. Allan compares it to rain on a 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." When electrons arrive as a pair rather than singly, the charge per arrival doubles, and the noise with it. In conventional superconductors, pairing produces a clean doubling of the noise; in cuprates, single electrons keep flowing alongside the pairs, so the signal is much weaker and the amplifier had to be sensitive enough to separate it from the background.

Shot noise gives the two explanations different predictions. If the pseudogap is only a frozen pattern, the pairing signal should switch on at the smaller energy associated with superconductivity. If the pseudogap is a pairing phenomenon, it should switch on at the pseudogap energy itself. The team's open preprint of a paper now published in Science on 5 November 2026 (journal record) frames the test as a step in the effective charge at the pseudogap energy or at the lower kink energy, and reports the answer on the side of pairing. The team measured crystals of Bi2Sr2CaCu2O8+delta, the compound usually written BSCCO, on microscopes running between about 0.3 and 4.2 kelvin, across samples with different doping levels. Wherever the tip listened, the extra noise always started rising at the local pseudogap energy, and the measured pairing energy, the strength it takes to pull a pair apart, reached up to 70 milli-electron volts, well above the temperature at which the material superconducts.

The decisive check came from a magnetic field of up to six tesla, which suppresses pairing while leaving the overall pattern of missing electrons largely intact. If the extra noise came from real pairs, it should fade as the field grew. It did: the noise dropped back toward the single-electron level while the missing-electron pattern stayed where it was. Separating that signal from everything else took substantial effort, the wire story reports, including checking that atoms moving around on the sample surface were not the source of the extra noise (surface-atom check). "Noise is super hard to interpret, and that was basically the thing that we had the most sleepless nights about," Allan said.

The paper rules out a pseudogap caused by local order alone, but says it does not rule out a pseudogap in which pairing and local order are intertwined. "Whatever the detailed explanation of the pseudogap is, pairing should be part of it," Swart said. The result is therefore a test that the field could not previously run, rather than a verdict on which explanation is correct.

The Utrecht release frames the result as a route toward room-temperature superconductivity. "We show that one of the two key conditions for superconductivity can be met at room temperature. If that weren't possible, room-temperature superconductivity would be out of reach altogether," Swart said, adding that phase coherence is the bottleneck. That is the authors' reading of a measured energy scale rather than a measurement at room temperature, and the preprint is candid that the region above the superconducting transition, where the pseudogap is normally defined, was out of reach because the amplifier loses sensitivity as it warms. Allan was careful too: "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." The published version is slightly more cautious than the preprint, whose abstract says the results show phase coherence as the limit where the Science abstract says they suggest it.

Until now, the two explanations could not be told apart, because every probe saw a jumble of signatures that supported both readings. Allan's summary of that history: readings that were "always something kind of incoherent, hard to grasp." Swart's next step is an instrument that detects phase coherence directly, so the onset of pairing and the onset of coherent motion get separate markers. "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."