Propagating rotational jump events drive liquid-liquid transition in supercooled liquid water

Abstract

We observe, at low temperature, the appearance of propagating events that originate from the inter-conversion between adjacent four and five coordinated water molecules in supercooled liquid water, resulting in the migration of the coordination number five in a string-like fashion, creating rotational jumps along the way. The length of the connected events increases with lowering temperature. Each propagation event is terminated by a 3-coordinated species, present in a small number at large supercooling, which interacts cooperatively with 5-coordinated molecules to annihilate both the species. We find that these growing dynamical correlations manifest in a divergent-like growth of a non-linear density response function, \times4(t), which is given by a four point time correlation function (FPTCF). The locus of the maximum of \times4(t), when plotted against the time t* of maximum, exhibits a sharp peak precisely at the temperature where the static response functions (specific heat, isothermal compressibility) also show similar sharp, divergent-like, peak. While the decay of population fluctuation time correlation function of both 4- and 5- coordinated species slows down dramatically and a step-like feature of the relaxation becomes evident, the lifetime itself of 5-corrdinated species remains short. These results suggest a new molecular mechanism of low temperature anomalies and of the liquid-liquid transition in terms of initiation, growth and termination of propagating jump and inter-conversion events.