Specific heat of defects in the S=1 chain system Y2BaNiO5

The specific heat of the antiferromagnetic spin-1 chain compound Y2BaNi1-xZ nxO5 in the presence of a magnetic field is calculated. Solving the low-ene rgy spectrum of an anisotropic Heisenberg Hamiltonian with DMRG and conside ring the experimental Values for the parameters and an approximate model fo r interchain interactions, the observed C(B,T) is quantitatively reproduced . For large chains (N > 50), contrary to previous interpretations of the sp ecific heat data we find S = 1/2 states. These results are thus also consis tent with electron-spin-resonance data for NENP. (C) 1999 Elsevier Science B.V. All rights reserved.