For nearly four decades, physicists have argued about what makes copper-oxide materials superconduct at temperatures far above absolute zero. A new experiment has now detected electron pairs in a cuprate superconductor at the very energy where electrons appear to go missing, offering direct evidence that pairing is tied to a long-debated feature called the pseudogap.
The work, led by Milan Allan of Ludwig Maximilian University of Munich and Ingmar Swart of Utrecht University, used a scanning tunneling microscope to measure tiny current fluctuations—shot noise—at different locations and energies on a cuprate crystal. Their findings appear in Science.
The pseudogap problem
Cuprates are copper-based materials that superconduct at temperatures reachable with liquid nitrogen, a practical advantage over conventional superconductors that need extreme cooling. But the mechanism behind their high transition temperatures remains unresolved.
Above the superconducting temperature, cuprates enter the pseudogap state. Measurements show fewer electrons than expected at certain energies, as if some have disappeared. Two broad explanations have competed for years: one holds that electrons freeze into static patterns such as charge density waves, while the other proposes that they form pairs before superconductivity begins.
In a superconductor, electrons travel as Cooper pairs. If those pairs exist at higher temperatures but cannot yet move in unison, they could be a prerequisite for the superconducting state rather than a side effect.
How noise reveals pairing
Shot noise arises from the arrival of individual charges in a current. Allan compared it to rain hitting a roof: make the drops heavier and the sound grows louder. If electrons consistently arrive in pairs, the noise increases in a measurable way.
Using a needle-sharp tip, the team passed current into BSCCO crystals cooled to a few degrees above absolute zero. By adjusting the voltage, they tuned the energy of the electrons being probed, similar to turning a radio dial. The challenge was sensitivity: in cuprates, single electrons continue to flow alongside pairs, weakening the pairing signal. It took years of electronics work by persistent students to make the measurement possible, Allan said.
In conventional superconductors, pairing produces a clean doubling of the noise. In the cuprate samples, the extra noise appeared right at the local pseudogap energy—not only at the energies associated with superconductivity. The same pattern held across different chemical compositions and despite the pseudogap energy varying from spot to spot.
Ruling out alternatives
Separating the pairing signal from other noise sources, such as atoms shifting on the surface, was difficult. The decisive test involved magnetic fields up to six tesla, which suppress pairing while leaving the missing-electron pattern largely intact. As the field increased, the noise fell back toward the single-electron level, consistent with pairs being its source.
The team then compared the energy needed to break a pair with the temperature at which the material superconducts. In conventional superconductors, pairing and phase coherence—the onset of coordinated motion—occur together at the critical temperature. In this cuprate, the measured pairing energy sits well above the superconducting temperature. The that emerges, Swart said, is of pre-formed pairs that only later become coherent.
That shifts attention from pair formation to pair synchronization. Allan suggested the result offers hope that higher transition temperatures might be possible, though he cautioned that other effects are likely to complicate any simple path to a room-temperature superconductor.
Open questions
The experiment rules out the idea that frozen patterns alone explain the pseudogap, but it does not show that such patterns are unrelated to pairing. As Swart put it, whatever the detailed explanation, pairing should be part of it.
The technique loses sensitivity as temperature rises, so the team could not yet probe above the superconducting temperature where the pseudogap is usually defined. Allan hopes other groups will take on that measurement and investigate what limits the transition temperature. Swart wants higher-resolution noise maps and, eventually, an instrument that detects coherence directly, which could separate the onset of pairing from the onset of coherent motion.
Allan, meanwhile, increasingly sees chemistry—aided by artificial intelligence—as a more promising route to better superconductors than pure physics. For improving materials and raising transition temperatures, he said, direct trial and error may offer more hope.
