Far Infrared Resonance Transition Study of the Chain-End S = 1/2 Modes in an S = 1 Antiferromagnetic Chain

The unpaired spin S = 1/2 states which occur at the ends of an S = 1 Heisenberg-like antiferromagnetic chain, have been observed recently in low-temperature electron spin-resonance measurements of Ni(C₂H₈N₂)₂ NO₂ ClO₄containing selected impurities. We present here a further study on this topic by far infrared transmission spectra under the magnetic field. The splitting of S = 1/2 modes was clearly observed and explained by the interaction of S = 1/2 modes at two nearby chains, which can be separated by vacancy-like defect.