Quantum phase transitions probed by EPR spectra in dimeric spin arrays with supramolecular couplings

Dimeric compounds with nearly isolated molecular units (d-units) having pairs of spins s1 and s2 coupled by antiferromagnetic (AFM) exchange H-0 = -J(0)s(1) center dot s(2) are non-trivial quantum spin systems having pri-mary roles in magnetism. Weakly coupled infinite arrays of AFM d-units in crystal structures have an appealing spin dynamic arising from their energy-gapped level structure and display magnetic properties with important roles in materials science. They received great additional attention when it was discov-ered that the spin entanglement introduced by interdimeric couplings with magnitude J1 (with IJ(1) I << IJ(0)I) gives rise to bosonic systems with novel properties and quantum phase transitions at high temperature (T). In this work, we collect recent advances in the interpretation of EPR spectral changes in terms of quantum phase transitions of arrays of d-units in the presence of weak interdimeric couplings. We review previous investigations of the problem and focused new experiments on the paradigmatic compound copper acetate monohydrate (CAH), collecting a detailed set of spectra in single-crystal and powder samples. The spectral features arising from this coupling are merging and narrowing of the peaks of the spectra of single crystals for specific magnetic field (B-0) orientations, and an extraordinary "U -peak" in the powder samples associated with the quantum phase transition of the dimeric spin array. Our historical overview collects studies of similar compounds with the same dimeric feature in which the U-peaks were generally misinterpreted as a double-quantum transition, or ignored. We describe pro-cedures to identify and quantify the U-peak and the merging and narrowing phenomena, with a protocol to extract the interdimeric coupling magnitude. As a novel contribution, we explain the experimental results by proposing a spin model with a microscopic flip-flop mechanism involving the absorption and emission of two simultaneous spin-one exc