RANDOM-EXCHANGE QUANTUM HEISENBERG-CHAINS

The one-dimensional quantum Heisenberg model with random +/-J bonds is studied for S=1/2 and S=1. The specific heat and the zero-held suscep tibility are calculated by using high-temperature series expansions an d the quantum transfer matrix method. The susceptibility shows a Curie -like temperature dependence at low temperatures as well as at high te mperatures. The numerical results for the specific heat suggest that t here are anomalously many low-lying excitations. The qualitative natur e of these excitations is discussed based on the exact diagonalization of finite-size systems.