The authors review the results of a wide variety of experiments on materials such as La2CuO4 and Nd2CuO4 that contain weakly coupled CuO2 layers. These materials are antiferromagnetic insulators with very large Heisenberg exchange energies, which become high-temperature superconductors when charge carriers are added to the CuO2 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 CuO2 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. S0034-6861(98)00403-6. References: 172
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