Bound states in weakly disordered spin ladders
  We study the appearance of bound states in the spin gap of spin-1/2 ladders induced by weak bond disorder. Starting from the strong-coupling limit, i.e., the limit of weakly coupled dimers, we perform a projection on the single-triplet subspace and derive the position of bound states for the single impurity problem of one modified coupling as well as for small impurity clusters. The case of a finite concentration of impurities is treated with the coherent-potential approximation in the strong-coupling limit and compared with numerical results. Furthermore, we analyze the details in the structure of the density of states and relate their origin to the influence of impurity clusters. 
M. Arlego, W. Brenig, D.C. Cabra, F. Heidrich-Meisner, A. Honecker, G.   Rossini