Longitudinal magnon in the tetrahedral spin system Cu2Te2O5Br2 near quantum criticality -: art. no. 174405

We present a comprehensive study of the coupled tetrahedra compound Cu2Te2O5Br2 by theory and experiments in external magnetic fields. We report the observation of a longitudinal magnon in Raman scattering in the ordered state close to quantum criticality. We show that the excited tetrahedral-singlet sets the energy scale for the magnetic ordering temperature T-N. This energy is determined experimentally. The ordering temperature T-N has an inverse-log dependence on the coupling parameters near quantum criticality.