The boson peak of deeply cooled confined water reveals the existence of a low temperature liquid-liquid crossover

The boson peak of deeply cooled water confined in the pores of a silica xerogel is studied by inelastic neutron scattering at different hydration levels to separate the contributions from matrix, water on the pore surfaces and internal water. Our results reveal thatat high hydration level, where the contribution from internal water is dominant, the temperature dependence of the boson peak intensity shows an inflection point at about 225 K. The complementary use of differential scanning calorimetry to describe the thermodynamics of the system allows identifying the inflection point as the signature of a water liquid-liquid crossover