ESR ABSORPTION FROM THE GROUND-STATE IN HALDANE SYSTEMS - THEORY AND EXPERIMENT

The ESR experiment provides us with much information on the ground and excited states of Haldane systems in a magnetic field. In this paper recent experimental results are reported on two typical Haldane materi als, NENP and NINO. Then a theoretical interpretation is given on the spectrum; it is shown in particular how the ESR spectrum is related to the nature of the ground and excited states, which change with the ap plied magnetic field.