RANDOM EXCHANGE HEISENBERG CHAIN FOR CLASSICAL AND QUANTUM SPINS

The Heisenberg chain with random IJ bonds is studied for the quantum s pin s = 1/2 and in the classical limit. The former is treated by high- temperature expansion and transfer matrix calculation while the latter can be analyzed exactly. The disorder leads to a 1/T behavior of the low-temperature susceptibility in the classical system. For s = 1/2 ou r analysis reveals a significant residual entropy at low temperature. From this we conclude that for quantum spins the susceptibility exhibi ts three different regimes in temperature and that the specific heat h as a peak in the very low-temperature regime.