Phase Diagram of the 2-Leg Heisenberg Ladder with Alternating   Dimerization
  Using the Lanczos method we determine the phase diagram of the 2-leg AF-Heisenberg ladder with alternating dimerization. It consists of a resonating valence bond phase and a dimer phase separated by a critical line. Our results are in good agreement with previous conjectures obtained with the non linear sigma model. 
M.A. Martin-Delgado, J.Dukelsky, G. Sierra