  S=1/2 quantum spin chains and ladders with random exchange coupling are
studied by using an effective low-energy field theory and transfer matrix
methods. Effects of the nonlocal correlations of exchange couplings are
investigated numerically. In particular we calculate localization length of
magnons, density of states, correlation functions and multifractal exponents as
a function of the correlation length of the exchange couplings. As the
correlation length increases, there occurs a "phase transition" and the above
quantities exhibit different behaviors in two phases. This suggests that the
strong-randomness fixed point of the random spin chains and random-singlet
state get unstable by the long-range correlations of the random exchange
couplings.
