Critical dynamics of singlet excitations in a frustrated spin system

We construct and analyze a frustrated quantum spin model with plaquette order, in which the low-energy dynamics is controlled by spin singlets. At a critical value of frustration the singlet spectrum becomes gapless, indicating a quantum transition to a phase with dimer order. The magnetic susceptibility has an activated form throughout the phase diagram, whereas the specific heat exhibits a rich structure and a power-law dependence on temperature at the quantum critical point. This generic behavior can be relevant to quantum antiferromag