Novel approach to description of spin liquid phases in low-dimensional quantum antiferromagnets

We consider quantum spin systems with dimerization, which at strong couplin g have singlet ground states. To account for strong correlations, the S = 1 elementary excitations are described as dilute Bose gas with infinite on-s ite repulsion. This approach is applied to the two-layer Heisenberg antifer romagnet. Our analytic results for the triplet gap, the excitation spectrum and the location of the quantum critical point are in excellent agreement with numerical calculations. We also discuss singlet and triplet bound stat es of the elementary triplets. It has been demonstrated that the existence of such bound slates is an intrinsic property of the dimerized spin liquid. (C) 1999 Elsevier Science B.V. All rights reserved.