Numerical renormalization-group study of spin correlations in one-dimensional random spin chains

We calculate the ground-state two-spin correlation functions of spin-1/2 qu antum Heisenberg chains with random exchange couplings using the real-space renormalization-group scheme. We extend the conventional scheme to take ac count of the contribution of local higher multiplet excitations in each dec imation step. This extended scheme can provide highly accurate numerical da ta for large systems. The random average of staggered spin correlations of the chains with random antiferromagnetic (AF) couplings shows algebraic dec ay like 1/r(2), which verifies the Fisher's analytic results. For chains wi th random ferromagnetic (FM) and AF couplings, the random average of genera lized staggered correlations is found to decay more slowly than a power law , in the form close to 1/ln(r). The difference between the distribution fun ctions of the spin correlations of the random AF chains and of the random F M-AF chains is also discussed. [S0163-1829(99)04941-3].