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Low Energy Singlets in the Excitation Spectrum of the Spin Tetrahedra System Cu_2Te_2O_5Br_2

P. Lemmens·K. Y. Choi·A. Ionescu·J. Pommer·G. Guentherodt·R. Valenti·C. Gros·W. Brenig·M. Johnsson·P. Millet·F. Mila
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Abstract

Low energy Raman scattering of the s=1/2 spin tetrahedra system Cu_2Te_2O_5Br_2 is dominated by an excitation at 18 cm^{-1} corresponding to an energy E_S=0.6Δ, with Δthe 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_2Te_2O_5Cl_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.

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P. Lemmens, K. Y. Choi, A. Ionescu, J. Pommer, G. Guentherodt, R. Valenti, C. Gros, W. Brenig, M. Johnsson, P. Millet, F. Mila. 2001-11-05. Low Energy Singlets in the Excitation Spectrum of the Spin Tetrahedra System Cu_2Te_2O_5Br_2. https://doi.org/10.1016/s0022-3697(02)00056-2

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