Heat Capacities of Haldane-Gap Antiferromagnet NENP in Magnetic Field

Heat capacity measurements of the quasi one-dimensional S= 1 antiferromagnet Ni(C2H8N2)2NO2(ClO4) are performed in static magnetic fields up to 13 T. Applying a magnetic field, a broad anomaly appears below 5 K, which reflects the gap energy between the nonmagnetic ground state singlet and the excited states. The field-dependence of the energy gap estimated from the measured heat capacities gives the critical field values close to those obtained in high-field-magnetization measurements and non-zero gap remaining at the critical field. Within our experimental range of temperature and magnetic field, we have not observed any sharp peaks corresponding to the expected three dimensional magnetic long range ordering.