THERMAL EXCITATION IN THE HALDANE-GAP SYSTEM (CH3)4NNI(NO2)3

The characteristic temperature dependence of heat capacity C has been observed in a S = 1 Haldane-gap system (CH3)4NNi(NO2)3 (TMNIN) at low temperatures. In zero external field, the data of C below 2 K fit the equation C = AT-2 exp (-DELTA/k(B)T), where the gap energy is DELTA/k(B) = 4.2 K. As the external field increases to 30 kOe, the value of the gap energy DELTA becomes smaller and the heat capacity C comes to depend nearly linearly on the temperature T. A cooling phenomenon has been found in the process of adiabatic magnetization at low temperatures, which is most enhanced for the applied field just above 33 kOe. These results are consistent with the existence of the Haldane gap.