Low Energy Singlets in the Excitation Spectrum of the Spin Tetrahedra System Cu2Te2O5Br2

Low energy Raman scattering of the s=1/2 spin tetrahedra system Cu2Te2O5Br2 is dominated by an excitation at 18 cm corresponding to an energy ES = 0.6∆, with ∆ the spin gap of the compound. For elevated temperatures this mode shows a soft mode-like decrease in energy pointing to an instability of the system. The isostructural reference system Cu2Te2O5Cl2 with a presumably larger inter-tetrahedra coupling does not show such a low energy mode. Instead its excitation spectrum and thermodynamic properties are compatible with long range Néel-ordering. We discuss the observed effects in the context of quantum fluctuations and competing ground states.