  When a liquid is rapidly cooled below its melting point without inducing
crystallization, its dynamics slow down significantly without noticeable
structural changes. Elucidating the origin of this slowdown has been a
long-standing challenge. Here, we report a theoretical investigation into the
mechanism of the dynamic slowdown in supercooled water, a ubiquitous yet
extraordinary substance characterized by various anomalous properties arising
from local density fluctuations. Using molecular dynamics simulations, we found
that the jump dynamics, which are elementary structural change processes,
deviate from Poisson statistics with decreasing temperature. This deviation is
attributed to slow variables competing with the jump motions, i.e., dynamic
disorder. The present analysis of the dynamic disorder showed that the primary
slow variable is the displacement of the fourth nearest oxygen atom of a
jumping molecule, which occurs in an environment created by the fluctuations of
molecules outside the first hydration shell. As the temperature decreases, the
jump dynamics become slow and intermittent. These intermittent dynamics are
attributed to the prolonged trapping of jumping molecules within extended and
stable low-density domains. As the temperature continues to decrease, the
number of slow variables increases due to the increased cooperative motions.
Consequently, the jump dynamics proceed in a higher-dimensional space
consisting of multiple slow variables, becoming slower and more intermittent.
It is then conceivable that with further decreasing temperature, the slowing
and intermittency of the jump dynamics intensify, eventually culminating in a
glass transition.
