We consider quantum spin systems with dimerization, which at strong coupling have singlet
ground states. To account for strong correlations, the S= 1 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 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.

We consider quantum spin systems with dimerization, which at strong coupling have singlet
ground states. To account for strong correlations, the S" 1 elementary excitations are
described as dilute Bose gas with infinite on-site repulsion. This approach is applied to the
two-layer Heisenberg antiferromagnet. 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 states of the elementary …