Dzyaloshinskii-Moriya interactions in valence-bond systems

We investigate the effect of Dzyaloshinskii-Moriya interactions on the low-temperature magnetic susceptibility for a system whose low energy physics is dominated by short-range valence bonds (singlets). Our general perturbative approach is applied to specific models expected to be in this class, including the Shastry-Sutherland model of the spin-dimer compound SrCu2(BO3)(2) and the antiferromagnetic Heisenberg model of the recently discovered S=1/2 kagome compound ZnCu3(OH)(6)Cl-2. The central result is that a short-ranged valence-bond phase, when perturbed with Dzyaloshinskii-Moriya interactions, will remain time-reversal symmetric in the absence of a magnetic field but the susceptibility will be nonzero in the T -> 0 limit. Applied to ZnCu3(OH)(6)Cl-2, this model provides an avenue for reconciling experimental results, such as the lack of magnetic order and lack of any sign of a spin gap, with known theoretical facts about the kagome Heisenberg antiferromagnet.