Physicists have started listening to the faint electrical noise produced by current flowing through a cuprate superconductor, a method they hope will reveal the electron pairs that may lie behind one of the field's most stubborn puzzles. The work, published in the journal Science, was led by Milan Allan of LMU Munich and Ingmar Swart of Utrecht University.
Superconductors carry electric current with zero resistance, but most of them only do so a few degrees above absolute zero. Cuprates are an exception: they superconduct at considerably higher temperatures, reachable with liquid nitrogen. Since their discovery in the late 1980s, however, the physical mechanism that makes them work has remained disputed.
A central piece of the puzzle is the so-called pseudogap. Above the temperature at which the material becomes superconducting, fewer electrons than expected are available at certain energies. Despite many experimental probes, scientists have not identified what causes it. Two main explanations compete. One holds that electrons become trapped and arrange themselves into charge density wave patterns, with superconductivity developing separately from that pattern. The other proposes that the missing electrons are already paired as Cooper pairs, forming before the material becomes superconducting, in a state that would act as a precursor to full superconductivity.
The new study aims to tell these possibilities apart by looking for the pairs directly rather than inferring them from indirect measurements. Allan has said that existing instrument readings are indirect and inconclusive, and the question of which hypothesis is correct is not yet settled. The listening approach is intended to provide a more direct window into the electron behavior at the heart of the effect.