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Quantum Heisenberg spin chains with random couplings and spin sizes are studied using a real-space renormalization group technique. These systems belong to a new universality class of disordered quantum spin systems realized in, e. g. , Sr₃CuPt₁-ₗIrₗO₆. The low-energy fixed point is characterized by the formation of weakly coupled large effective spins. At low temperature T the entropy obeys a power law T^ (0. 44), and the susceptibility follows a Curie-like behavior.
Westerberg et al. (Mon,) studied this question.