HEAT-CAPACITIES IN HALDANE-GAP ANTIFERROMAGNET NENP AND TMNIN UNDER MAGNETIC-FIELD

Heat capacity measurements are performed in Haldane-gap antiferromagne t NENP and TMNIN under magnetic field up to 13T. For NENP, the exponen tial temperature dependence of heat capacity gives strong evidence of existence of the energy gap. The field dependence of the gap energy sh ows characteristic increase above the critical field, which is conside red to correspond to appearance of the staggered moment observed in H- 1-NMR measurement. We obtain similar results qualitatively for TMNIN. Moreover, the density of state in the lowest excited state is also dis cussed.