Magnetic resonances in the Haldane-gap materials NENP and NINO

Abstract We present selected results of a detailed study of the magnetic resonances in the Haldane-gap materials NENP and NINO as a function of radiation energy and temperature for controlled orientation of the external magnetic field with respect to the crystal axes. The experiments were performed using monochromatic submillimeter radiation produced either by an optically pumped laser system or a continuously tunable backward wave oscillator spectrometer for both Faraday and Voigt configurations. The transmission spectra include both allowed and forbidden transitions and give the possibility to detect the transitions between the lowest lying states in the energy-level scheme. The experimental data indicate a hybridization at the anticrossing points, especially near the phase transition at the critical magnetic field B c .