Random Bonds and Topological Stability in Gapped Quantum Spin Chains

Using an asymptotically exact real space renormalization procedure, we find that the dimerized spin-1/2 chain is extremely stable against bond randomness. For weak dimerization or, equivalently, strong randomness, it is in a Griffiths phase with short-range spin-spin correlations and a divergent susceptibility. The string topological order persists. We conjecture that random integer spin chains in the Haldane phase exhibit similar thermodynamic and topological properties.