Cuprates, copper-based materials discovered in the late 1980s, superconduct at temperatures reachable with liquid nitrogen, but there is still no agreement on why. At the heart of the puzzle is a state called the pseudogap, in which some electrons seem to "vanish". One explanation is that they freeze into patterns; the other is that they already form pairs (Cooper pairs) before the material becomes superconducting.
The team, led by Milan Allan of Ludwig Maximilian University of Munich and Ingmar Swart of Utrecht University, used a scanning tunneling microscope to measure "shot noise": the fluctuations caused by individual charges arriving in a current. If electrons arrive in pairs, the noise increases. Building a sensitive enough instrument took about four years. In measurements on the cuprate BSCCO, the pairing energy was well above the transition temperature.
According to the researchers, the picture is that pairs form first and only at lower temperatures start moving together. In other words, the bottleneck is not pairing but coherence between the pairs. The finding rules out the idea that the pseudogap comes from frozen patterns alone, but does not settle the whole question, and the technique cannot yet measure above the transition temperature. The researchers caution that this is not a direct route to a room-temperature superconductor.