Monte Carlo study of the separation of energy scales in quantum spin 1/2 chains with bond disorder

One-dimensional Heisenberg spin 1/2 chains with random ferro- and antiferro magnetic bonds are realized in systems such as Sr3CuPt1-xIrxO6. We have inv estigated numerically the thermodynamic properties of a generic random bond model and of a realist ic model of Sr3CuPt1-xIrxO6 by the quantum Monte Ca rlo loop algorithm. For the first time we demonstrate the separation into t hree different temperature regimes for the original Hamiltonian based on an exact treatment, especially we show that the intermediate temperature regi me is well-defined and observable in both the specific heat and the magneti c susceptibility The crossover between the regimes is indicated by peaks in the specific heat. The uniform magnetic susceptibility shows Curie-like be havior in the high-, intermediate- and low-temperature regime, with differe nt values of the Curie constant in each regime. We show that these regimes are overlapping in the realistic model and give numerical data for the anal ysis of experimental tests.