The authors review the results of a wide variety of experiments on materials such as and that contain weakly coupled layers. These materials are antiferromagnetic insulators with very large Heisenberg exchange energies, which become high-temperature superconductors when charge carriers are added to the layers. The growth of large single crystals has made it possible to carry out neutron scattering, as well as anisotropic optical, transport, and magnetization measurements. The properties of the undoped layer are reviewed, and the evolution of magnetic, optical, and transport properties with the addition of charge carriers is discussed. The emphasis is on the pure and lightly doped materials, although the magnetism in the superconductors is discussed.
© 1998 The American Physical Society