Specific Heat of defects in the Haldane System Y₂BaNiO₅

We calculate the specific heat of the antiferromagnetic spin-1 chain compound Y_2BaNi_{1-x}Zn_xO_5 in the presence of a magnetic field. The low-energy spectrum of a Heisenberg Hamiltonian, which includes realistic anisotropies, has been solved using the density matrix renormalization group. The observed Schottky anomaly is very well described by this theory. For large chains ($N>50$), contrary to previous interpretations of the specific heat data, we find S=1/2 states. These results are thus also consistent with electron- spin-resonance data for NENP.