Visualizing pair formation on the atomic scale and the search for the mechanism of superconductivity inhigh-Tc cuprates

Abstract

We have developed several new experimental techniques, based on the scanningtunneling microscope, to visualize the process of pair formation on the atomicscale and to probe with high precision what controls the strength of pairing inhigh-Tc cuprate superconductor compounds. These new experiments provide evidence that pairingin these exotic superconductors occurs above the bulk transition temperature and innanoscale regions with sizes of 1–3 nm. The high temperature nucleation and proliferationof these nanoscale puddles have a strong connection to the temperature–doping phasediagram of these superconductors. On average we have found that the pairing gapΔ and the temperature at which they first nucleateTp follow thesimple relation: 2Δ/kBTp∼8. Moreover, the variations of the pairing strength on the nanoscale can beexamined to find microscopic clues to the mechanism of pairing. Specifically, wehave found evidence that suggests that strong electronic correlation, as opposedto coupling of electrons to bosons, is responsible for the pairing mechanism inthe cuprates. Surprisingly, we have found that nanoscale measurements ofelectronic correlations in the normal state (at temperatures as high as twiceTc) can be used to predict the strength of the local pairing interaction at low temperatures.