Maxima in the thermodynamic response and correlation functions of deeply supercooled water

Femtosecond x-ray laser pulses were used to probe micrometer-sized water droplets

that were cooled down to 227 kelvin in vacuum. Isothermal compressibility and correlation

length were extracted from x-ray scattering at the low–momentum transfer region. The

temperature dependence of these thermodynamic response and correlation functions

shows maxima at 229 kelvin for water and 233 kelvin for heavy water. In addition, we

observed that the liquids undergo the fastest growth of tetrahedral structures at similar

temperatures. These observations point to the existence of a Widom line, defined as

the locus of maximum correlation length emanating from a critical point at positive

pressures in the deeply supercooled regime. The difference in the maximum value of the

isothermal compressibility between the two isotopes shows the importance of

nuclear quantum effects.