MAGNETISM IN THE INSULATING PARENTS OF THE HIGH-TC SUPERCONDUCTORS - WELL OR ILL UNDERSTOOD

We discuss appreciable differences between the results for the covalen t reduction of the ordered moment in antiferromagnetic CuO2 planes obt ained by various groups, including the present authors. We explain the differences as due to differences in definition, the definition that we used being closely based on experiment (magnetic neutron Bragg scat tering), in contrast to the others. We also discuss the large differen ce between the zero-point spin fluctuations as calculated by one of th e groups and the well-known result of spin wave theory.