Coexistence of antiferromagnetism and dimerization in a disordered spin-Peierls model: Exact results

A model of disordered spin-Peierls system is considered, where domain walls are randomly distributed as a telegraph noise. For this realization of the disorder in an XY spin chain, we calculate exactly the density of stales as well as several thermodynamic quantities. The resulting physical behavior should be qualitatively unchanged even for an XXZ chain, up to the isotropic XXX point. For weak disorder, besides a high energy regime where the behavior of a pure spin-Peierls system is recovered, there is a crossover to a low energy regime with singular thermodynamic properties and enhanced antiferromagnetic fluctuations. These-regimes are analyzed with the help of exact results, and the relevant energy scales determined. We discuss the possible relevance of such a disorder realization to the doped inorganic spin-Peierls compound CuGeO3.