Valence Bond Glass Phase in Dilute Kagome Antiferromagnets

We present a theory for site dilution in the kagome lattice Heisenberg model. The presence of an empty site leads to strong singlet bonds opposite to the impurity. It also creates a free spin which delocalizes near the impurity. Finite impurity concentration leads to a valence bond glass phase with no spin gap, large spin susceptibilities, linear specific heat due to two-level systems, as well as singlet and triplet excitations that decompose into kink-antikink pairs. It provides a framework for a comprehensive understanding of thermodynamic, neutron, and Raman measurements in the herbertsmithite material ZnCu3(OH)6Cl2, including recently reported H/T and omega/T scaling.