9320892 Klein Technical abstract: Cuprate superconductors evolve from a strongly-correlated state which has one carrier per active site and which is an antiferromagnetic insulator. When carriers are added via doping, correlation effects are believed to cause high temperature superconductivity. Photons can excite carrier pairs - one doubly-occupied copper site plus one empty site - or a carrier from a copper orbital to another orbital on the same site. These excitations are produced by photon absorption, Raman scattering, or as the intermediate state of a spin-exchange Raman process. To better understand their behavior the excitations are probed as functions of temperature, doping and magnetic field. Non-technical abstract: High temperature superconductors owe their properties to strong repulsive interactions among the carriers of electric current. They move among copper atoms, and when each copper atom has precisely one carrier, repulsion causes the carriers to become localized on the copper sites. Electrical conduction then ceases; the material is an insulator. In order to move the carriers a finite "quantum" of energy has to be supplied. This is done via adsorption of light or by means of scattered light. The properties of the excited carriers is studied as functions of temperature, doping and magnetic field. The goal is to understand the insulating phase in detail and thereby better understand the superconducting state that results from it. ***
