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

Abstract

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., ${\mathrm{Sr}}_3\mathrm{Cu}{\mathrm{Pt}}_{1-x}{\mathrm{Ir}}_x{\mathrm{O}}_6$. 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 $T^{\delta }$ ($\delta \approx 0.44$), and the susceptibility follows a Curie-like behavior.