Critical dynamics of singlet excitations in a frustrated spin system - art. no. 064412

We construct and analyze a frustrated quantum spin model with plaquette ord er, in which the low-energy dynamics is controlled by spin singlets. At a c ritical value of frustration the singlet spectrum becomes gapless, indicati ng a quantum transition to a phase with dimer order. The magnetic susceptib ility has an activated form throughout the phase diagram, whereas the speci fic heat exhibits a rich structure and a power-law dependence on temperatur e at the quantum critical point. This generic behavior can be relevant to q uantum antiferromagnets on Kagome and pyrochlore lattices where (almost) al l low-energy excitations are spin singlets, as well as to the CaV4O9 lattic e and the strongly frustrated antiferromagnets Cu2Te2O5(Cl,Br)(2) and Li2VO (Si,Ge)O-4.