Novel Approach to Description of Spin-Liquid Phases in Low-Dimensional Quantum Antiferromagnets

We consider quantum spin systems with dimerization, which at strong coupling have singlet ground states. To account for strong correlations, the $S=1\mathrm{}$ elementary excitations are described as a dilute Bose gas with infinite on-site repulsion. This approach is applied to the two-layer Heisenberg antiferromagnet at $T=0\mathrm{}$ with general couplings. Our analytic results for the triplet gap, the excitation spectrum, and the location of the quantum critical point are in excellent agreement with numerical results obtained by dimer series expansions.