Direct observation of the Haldane gap in NENP by far-infrared spectroscopy in high magnetic fields

Abstract

We report the first direct and comprehensive determination of the energy levels involved in the Haldane gap occurrence: the singlet ground state and the first triplet excited state. Precise values of the parameters are obtained from the analysis of the magneto-optical resonance transitions observed in the far infrared for frequencies ranging from 70 to 1000 GHz and magnetic fields up to 18 T.