Experimental studies of nuclear quantum effects in condensed matter: the case of water

Water continues to be the focus of a considerable number of experimental and theoretical investigations because of the important role it plays in many phenomena in fundamental and applied science, as well as its biological and technological importance. The ability of water to create a three-dimensional network of hydrogen bonds makes this molecule uniquely versatile. Atoms in water display characteristic mean kinetic energies well above the thermodynamic temperature of the system, as opposed to the celebrated case of a perfect gas of non-interacting particles. Nuclear quantum effects have a profound influence on the structure and dynamics of hydrogen-bonded systems, such as water. These effects are reflected in the underlying nuclear momentum distributions, experimentally accessible using electron-volt neutron spectroscopy at pulsed spallation neutron sources, with a technique known as deep inelastic neutron scattering. In this topical review, the technique is discussed along with the conceptual framework and several case studies that bring to the fore the role of nuclear quantum effects in describing water. These state-of-the-art measurements are also benchmarked against theoretical predictions using path-integral molecular dynamics. We close by discussing and highlighting current challenges and future opportunities in this thriving area of condensed-matter research.