Low energy singlets in the excitation spectrum of the spin tetrahedra system Cu2Te2O5Br2

Abstract Low energy Raman scattering of the s =1/2 spin tetrahedra system Cu 2 Te 2 O 5 Br 2 is dominated by an excitation at 18 cm −1 corresponding to an energy E S =0.6 Δ , with Δ 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 Cu 2 Te 2 O 5 Cl 2 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 Neel-ordering. We discuss the observed effects in the context of quantum fluctuations and competing ground states.