S = 1 excitations in heat capacity of the Haldane compound TMNIN doped with a nonmagnetic impurity

We have studied the field dependence of the heat capacity of a typical Hald ane antiferromagnet (CH3)(4)NNi(NO)(3) (TMNIN) doped with the nonmagnetic i mpurity Zn2+, as well as of a pure TMNIN. The overall features of the magne tic heat capacity, including the characteristic broad maximum, are almost r eproduced by the theoretical curve with J/k(B) = -12 K for both systems in zero field, The field dependence in the impure systems is described well by assuming the Schottky heat capacity with S = 1, not S = 1/2 expected from the VBS model. A possible reason why the S = 1 model is suitable is discuss ed.