The parents of the copper oxide high-temperature superconductors are antiferromagnetic insulators. It’s thought that they would conduct electricity were it not for the very strong interactions among their electrons. Just dope those insulators to a level of 5% or more by chemically adding or removing electrons, and you get the cuprate compounds that superconduct at critical temperatures T c up to 140 K. When T c is plotted as a function of doping, it encloses a dome-shaped superconducting region, as seen in figure 1(a). Abutting the dome on the low-doping side is a mysterious “pseudogap” region, in which materials exhibit a gap in the density of states near the Fermi energy even though they are not superconducting. Theorists seeking the mechanism underlying high-T c superconductors must account for the strongly interacting parents and the pseudogap neighbors.
The plot in figure 1(a) has two temperature scales: T c,...
