Superconductors transport electrical current without any resistance, but the majority of them only function at temperatures just above absolute zero. During the late 1980s, researchers discovered that copper-based materials known as cuprates achieve superconductivity at much higher temperatures that can be reached using affordable liquid nitrogen. However, nearly 40 years later, physicists still lack consensus regarding the underlying mechanism.
A research team led by Milan Allan from Ludwig Maximilian University of Munich and Ingmar Swart from Utrecht University in the Netherlands published a study in Science revealing answers obtained by listening to the faint electrical crackle generated as current flows into a cuprate.
The Missing Electrons
A peculiar state designated as the pseudogap lies at the core of the mystery surrounding high-temperature superconductivity in cuprates. Allan stated that the pseudogap is a phenomenon of missing electrons. When temperatures exceed the threshold where cuprates turn superconducting, a portion of their electrons appear to disappear, with measurements detecting significantly fewer particles at expected energy levels. Allan noted that while various probes confirm these vanishing electrons, the exact cause remains unknown.
Throughout the decades, researchers proposed two primary competing theories to explain the observation. The first hypothesis suggests that electrons become trapped within a specific structure, freezing into patterns such as charge density waves while superconductivity emerges independently. The alternative theory connects the phenomenon directly to superconductivity. Allan explained that other researchers believe the electrons are missing because they form pairs. These electron pairs, known as Cooper pairs, serve as the current carriers in a superconductor. If such pairs emerge prior to the material becoming superconducting as it cools, they would act as a prerequisite for superconductivity, functioning as a waiting room where pairs exist without yet moving together in unison.
Listening to Current Flow
Resolving this debate proved exceptionally difficult because numerous experimental signatures overlap within the phenomenon. Different scientific instruments yield readings, but Allan pointed out that these measurements never provide conclusive results, remaining incoherent and difficult to interpret. The team required a direct method to observe pairs, prompting Allan and his colleagues to develop a new approach.
