LOW-ENERGY FIXED-POINTS OF RANDOM QUANTUM SPIN CHAINS

The one-dimensional isotropic quantum Heisenberg spin systems with ran dom couplings and random spin sizes are investigated using a real-spac e renormalization-group scheme. It is demonstrated that these systems belong to a universality class of disordered spin systems, characteriz ed by weakly coupled large effective spins. In this large-spin phase t he uniform magnetic susceptibility diverges as T-1 with a nonuniversal Curie constant at low temperatures T, while the specific heat vanishe s as T-/alpha//lnT/ for T-->0. For a broad range of initial distributi ons of couplings and spin sizes the distribution functions approach a single fixed-point form, where alpha(approximate to) -0.44. For some s ingular initial distributions, however, the fixed-point form of distri butions becomes nonuniversal, suggesting that there is a line of fixed points.