In this work, focusing on Tc < T < T0 regime, we show that the short range correlations below T0 come along with non-conventional excitations, and that this dynamics does not evolve down to Tc. Excitations spectra are characterized by a very broad and almost flat dynamical response which extends up to 1-1.5 meV, coexisting with a quasi-elastic response or not, depending on the wave-vector [4]. Although classical mechanics qualitatively captures some of the experimental observations [4,5], we show that it fails to reproduce our excitation spectra. In particular, classical spin dynamics simulations predict an energy scale twice smaller than the experimental observations. These results suggest that the spin dynamics of Yb2Ti2O7 is governed by quantum fluctuations, and that the observed spectra rather correspond to a continuum of deconfined spinons as expected in quantum spin liquids, than simple paramagnons. The careful determination of the exchange tensors, which places Yb2Ti2O7 close to a FM/AFM phase boundary [4,5], further suggests that this quantum dynamics arise from competing FM and AFM correlations.

[1] Bonville et al. arXiv:0306470 (2003) ; Thompson et al. PRL 106, 187202 (2011) ; Ross et al. PRB 84, 174442 (2011) ; Chang et al. Nature Comm. 3, 992 (2012) ; [2] D’Ortenzio PRB 88, 134428 (2013) ; Lhotel et al. PRB 89, 224419 (2014) ; Chang et al. PRB 89, 184416 (2014) ; [3] Applegate et al. PRL 109, 097205 (2012) ; [4] J. Robert, E. Lhotel, S. Guitteny, C. Decorse, B. Canals, I. Mirebeau, S. Petit, in preparation. [5] Yan et al., arXiv:1311.3501 (2013)

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