  Even at room temperature, quantum mechanics plays a major role in determining
the quantitative behaviour of light nuclei, changing significantly the values
of physical properties such as the heat capacity. However, other observables
appear to be only weakly affected by nuclear quantum effects (NQEs): for
instance, the melting temperatures of light and heavy water differ by less than
4 K. Recent theoretical work has attributed this to a competition between intra
and inter molecular NQEs, which can be separated by computing the anisotropy of
the quantum kinetic energy tensor. The principal values of this tensor change
in opposite directions when ice melts, leading to a very small net quantum
mechanical effect on the melting point. This paper presents the first direct
experimental observation of this phenomenon, achieved by measuring the
deuterium momentum distributions n(p) in heavy water and ice using Deep
Inelastic Neutron Scattering (DINS), and resolving their anisotropy. Results
from the experiments, supplemented by a theoretical analysis, show that the
anisotropy of the quantum kinetic energy tensor can also be captured for
heavier atoms such as oxygen.
