ESR STUDY OF THE HALDANE-GAP SYSTEM Y2BANIO5 IN HIGH MAGNETIC-FIELDS

We performed the ESR experiments in Y2BaNiO5, which has been proposed as a good candidate for the Haldane gap system, over a wide magnetic f ield range up to 100 T. We observed a relatively broad EPR signal at t emperatures above the gap energy and another resonance peak which pred ominates the spectra below 100 K. We also found some more resonant pea ks, which exhibit an infinite energy when extrapolated to zero magneti c field. The gap energy is estimated to be 5.22 meV. This resonance se ems to correspond to the transition between the singlet state and the triplet state, because the intensity of this peak becomes stronger as the temperature is lowered. However, the gap energy obtained in the pr esent experiment is smaller than the Haldane gap obtained from the neu tron inelastic scattering. (C) 1998 Elsevier Science B.V. All rights r eserved.