REFLECTIONLESS SPIN-WAVES IN HEISENBERG-CHAINS WITH SPIN IMPURITY SEGMENTS

We study, via a scattering transfer-matrix approach, the spin waves of a Heisenberg chain with randomly inserted impurity segments having sp in and coupling strength different from the host ones. Numerical resul ts show the existence of completely unscattered reflectionless modes, whose number is sensitive to the impurity parameters. If both the spin and the coupling strength of the impurities are different from those of the host we find a novel type of resonant propagating. wave, for wh ich the reflection by a single segment is minimum but not vanishing. T he relevance of the present results to other systems and to possible a pplications are discussed.