Random Quantum Spin Chains: A Real-Space Renormalization Group Study

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., Sr₃CuPt_1-xIr_xO₆. 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^δ (δ~=0.44), and the susceptibility follows a Curie-like behavior.