The pseudogap in high-temperature superconductors: an experimental survey

We present an experimental review of the nature of the pseudogap in the\ncuprate superconductors. Evidence from various experimental techniques points\nto a common phenomenology. The pseudogap is seen in all high temperature\nsuperconductors and there is general agreement on the temperature and doping\nrange where it exists. It is also becoming clear that the superconducting gap\nemerges from the normal state pseudogap. The d-wave nature of the order\nparameter holds for both the superconducting gap and the pseudogap. Although an\nextensive body of evidence is reviewed, a consensus on the origin of the\npseudogap is as lacking as it is for the mechanism underlying high temperature\nsuperconductivity.\n