We investigate the source of the variation of the pairing strength in high-Tc superconductors. We have developed a new technique that allows us to study the same nanoscale region of the material through a broad range of temperatures. Using this technique, we can probe the temperature evolution of metallic electronic states into superconducting Cooper pairs. We demonstrate a striking connection between the metallic electronic states and pairing strength: electrons showing signs of stronger repulsion at very high temperature formed the strongest bonded pairs at low temperatures. This observation runs contrary to the behavior of electrons in traditional superconducting materials, in which electron-electron repulsion is not conducive to superconductivity.

We have also found that the traditional signatures of what some might call the pair binding 'glue' are present. We can measure these signatures with high accuracy on the atomic scale. Unlike conventional superconductors, they don't seem to control the formation of the superconducting pairs: they are more like spectators.

Published in Science Magazine: Report, Supplementary Information