Physicists have resolved a nearly 40-year-old debate surrounding high-temperature superconductors by listening to the faint electrical crackle of current flowing through copper-based materials.
According to a Science study, researchers tracked tiny electrical fluctuations known as shot noise to discover that electron pairing begins at the energy level of the pseudogap, placing pair formation before the material actually achieves zero-resistance superconductivity.
While traditional superconductors carry electricity without resistance only a few degrees above absolute zero, copper-based materials called cuprates were discovered in the late 1980s to superconduct at higher temperatures that can be reached using affordable liquid nitrogen.
Despite nearly four decades of research, the underlying mechanism has remained contested, according to UA.NEWS. Central to the mystery is the pseudogap, a strange state where measurements show that electrons appear to vanish at specific energy levels above the temperature where superconductivity takes hold.
Over the decades, scientists debated two primary explanations. One hypothesis suggested that electrons become trapped and freeze into structured patterns, such as charge density waves, while superconductivity develops independently.
The alternative theory linked the missing electrons directly to superconductivity, proposing that electrons form pairs—known as Cooper pairs—that act as a precondition or waiting room for superconductivity before moving together in lockstep. Previous experiments yielded inconsistent results because various instruments detected overlapping and inconclusive signatures.
To cut through the ambiguous data, a team of researchers led by Milan Allan of Ludwig Maximilian University of Munich and Ingmar Swart of Utrecht University in the Netherlands utilized a scanning tunneling microscope.
As detailed by Artiverse.ca, the instrument passed current between a needle-sharp tip and crystals of the cuprate BSCCO cooled to a few degrees above absolute zero, allowing the team to measure shot noise—weak current fluctuations created by the arrival of individual electric charges.
While conventional superconductors experience a clean doubling of noise when electrons pair up, cuprates present a harder signal to detect because single electrons continue flowing alongside pairs. Isolating this pairing signal required four years of intensive work.
Instead of extra pairing noise appearing only at energy levels strictly associated with superconductivity, the researchers discovered that additional noise began rising precisely at the energy level of the local pseudogap. This relationship held true across samples with different chemical composition, even as the pseudogap energy varied from spot to spot across the material’s surface.
To verify these findings, the team placed the crystals in a magnetic field of up to six tesla. According to coverage from Bozokmedia and Newsbeep, the magnetic fields suppressed pair formation while leaving the overall pattern of electron depletion largely intact. As the field strength increased, the electrical noise dropped back toward baseline single-electron levels, firmly tying the extra noise to electron pairing and connecting the pseudogap to Cooper pairs.