An Impurity Driven Phase Transition in the Antiferromagnetic Spin-1   Chain
  Using an asymptotically exact real space renormalization procedure, we find that the Heisenberg antiferromagnetic spin-1 chain undergoes an impurity driven second order phase transition from the Haldane phase to the random singlet phase, as the bond distribution is broadened. In the Haldane phase and near the critical point, there is a Griffiths region in which the gap is filled and the susceptibility diverges in a non-universal manner. The correlation length critical exponent is approximately 2.3. 
R. A. Hyman (Georgia Tech), Kun Yang (Princeton)