Abstract
Under pressure the spin tetrahedron system Cu${}_{2}$Te${}_{2}$O${}_{5}$Br${}_{2}$ shows a softening of the collective magnetic mode whereas the energy of local spin-exchange processes increases. This apparent contradiction is due to the system's proximity to a quantum critical point induced by competing interactions that effectively decouple the tetrahedra. The high-pressure state is therefore nonmagnetic and dominated by valence bond excitations. Anomalies in the frequency and the intensity of magnetic Raman scattering are evidence for a weakly discontinuous evolution of the quantum critical state. In contrast, structural parameters as well as optical phonon modes evolve continuously with pressure without any sign for discontinuous criticality. No new low-energy excitations are observed. In contrast, the expected fingerprints of the spin-liquid phase are broadened due to Dzyaloshinskii-Moriya interactions.
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