spread over a wide frequency range and remaining nearly temperature independent over a wide range of temper- ature is strongly suggestive of a Valence Bond Liquid phase, with short-range Valence Bond Order exceeding a couple of lattice constants. These excitations can be re- garded as a kink-antikink pair or two spinons, which are confined in a very small spatial region. They extend up to fairly high temperatures of order J , where such excita- tions are necessarily broadened by the underlying classi- cal liquid environment. Whether at low enough tempera- tures, the development of long-range quantum coherence leads to truly delocalized spinons as in algebraic quan- tum spin-liquid phases or to the development of much sharper localized and extended triplet excitations as in a Valence Bond Crystal remains to be seen. 

We would like to thank Mark de Vries for valuable discussions. 

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