Anomalous Temperature Dependence and Isotope Effect in the Structural Dynamics of Deeply Supercooled Water

Despite simple chemical structure and its importance in our life, water remains one of the most puzzling liquids.\footnote{Angell, C. A. \textit{Science} 319, 582-587 (2008).} Combining neutron scattering and dielectric spectroscopy we show that quantum fluctuations have a pronounced effect on dynamics in deeply supercooled water. Dielectric measurements revealed that water has an anomalously weak temperature dependence of structural dynamics close to $T_{g} \approx $ 136K with unphysical low fragility index $m \approx $ 14. Additionally, we observed an anomalously large isotope shift of $T_{g}$ between H$_{2}$O and D$_{2}$O, $\Delta T_{g}$ $\sim$ 8-10K, in a strong contrast to the isotope effect on $T_{g}$ observed in other hydrogen bonding liquids. The observed anomalous behavior is consistent with the recently suggested idea of quantum zero-point vibrations affecting dynamics of supercooled water.\footnote{Novikov, V. N. {\&} Sokolov, A. P. \textit{Phys. Rev. Lett.} 110, 065701 (2013).} We speculate that the apparent fragile-to-strong crossover in dynamics of water can be ascribed to quantum effects dominating structural relaxation at low temperatures. These results have significant implications for our understanding of water dynamics and its peculiar behavior at low temperatures.