Visualizing pair formation on the atomic scale and the search for the mechanism of superconductivity in high-T_c cuprates

We have developed several new experimental techniques, based on the scanning tunneling microscope, to visualize the process of pair formation on the atomic scale and to probe with high precision what controls the strength of pairing in high-T_c cuprate superconductor compounds. These new experiments provide evidence that pairing in these exotic superconductors occurs above the bulk transition temperature and in nanoscale regions with sizes of 1-3nm. The high temperature nucleation and proliferation of these nanoscale puddles have a strong connection to the temperature-doping phase diagram of these superconductors. On average we have found that the pairing gap Δ and the temperature at which they first nucleate T_p follow the simple relation: 2Δ/k_BT_p∼8. Moreover, the variations of the pairing strength on the nanoscale can be examined to find microscopic clues to the mechanism of pairing. Specifically, we have found evidence that suggests that strong electronic correlation, as opposed to coupling of electrons to bosons, is responsible for the pairing mechanism in the cuprates. Surprisingly, we have found that nanoscale measurements of electronic correlations in the normal state (at temperatures as high as twice T_c) can be used to predict the strength of the local pairing interaction at low temperatures.