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 susceptibility are calculated by using high-temperature series expansions and the quantum transfer matrix method. The susceptibility shows a Curie-like temperature dependence at low temperatures as well as at high temperatures. The numerical results for the specific heat suggest that there are anomalously many low-lying excitations. The qualitative nature of these excitations is discussed based on the exact diagonalization of finite-size systems.