SPIN DYNAMICS IN 1D ANTIFERROMAGNETIC SYSTEM Y2BANI1-XZNXO5

The spin dynamics of the 1D antiferromagnetic Heisenberg system Y2BaNi 1-xZnxO5 has been investigated by thermal (for x = 0) and cold (for x = 0.01 and 0.03) neutron scattering. The former set of measurements en ables us to study the temperature dependence of the transitions from t he ground state to states above the Haldane gap. The latter, low incid ent energy measurements, reveal a quasielastic component in the spectr a which shows an activated temperature behaviour and a strong dependen ce on the chain length. We suggest a mechanism which could be the orig in of the present observations.