Longitudinal magnon in the tetrahedral spin system Cu 2 Te 2 O 5 Br 2 near quantum criticality

We present a comprehensive study of the coupled tetrahedra compound ${\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}{\mathrm{Br}}_{2}$ 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.