Princeton University Department of Physics Colloquium

The pairing of electrons underlies the formation of a superconducting state with zero electrical resistance. After twenty years of work, the mechanism of pairing and the temperature at which pairs first form in high-temperature copper-oxide superconductors are still hotly debated. Do pairs form at the critical temperature like conventional superconductors? Is pairing mediated by a bosonic excitation, as in conventional BCS superconductors, or is pairing with d-wave symmetry an unavoidable consequence of strong Coulomb repulsion in these compounds? In search of experimental answers to these important questions, we have develop several new techniques, based on the scanning tunneling microscope (STM), to visualize the process of pair formation on the atomic scale and to probe what controls the strength of pairing in these compounds with high precision. We show that pairing in the cuprates is strongly local, with pairs forming in nanoscale regions of samples over a range of temperatures above the critical temperature. These findings are consistent with observations that cuprates can act like fluctuating superconductors above their transition temperature. The new experimental techniques also allows us to probe bosonic excitation that strongly couple to electrons to demonstrate that unlike conventional superconductors, such electron-boson coupling do not control the pairing strength in these materials. In contrast, our experiments show that pairing appears to be controlled by high energy excitations that may be better understood (not as bosonic excitations but) in the context of cuprates as doped Mott insulators.