Atomic Quantum Dynamics in Materials Research

The motion of atoms in materials is generally treated within the classical framework of Newton's laws and molecular dynamics. In this limit, atoms move with mean kinetic energy proportional to the temperature of the system, as in the case of a perfect gas of noninteracting particles. For systems with more quantized degrees of freedom, such as intramolecular motions, or in the descriptions of the influence of a finite kinetic energy of the atoms on the properties of solids, atomic dynamics may be described in terms of a normal-mode, harmonic oscillator picture, or treated with rigid constraints. However, there are a number of important examples where atoms should be treated as fully quantum objects, with their dynamics derived from a Schrödinger equation based on a well-defined interatomic interaction potential. Such an approach is mandatory in order to describe nuclear quantum effects, such as zero-point energies. The quantum nature of atoms can be quantitatively described by nuclear momentum distributions and mean kinetic energies of atoms. These observables are accessible using deep inelastic neutron scattering, a technique based on the use of epithermal neutrons available at instruments at spallation neutron sources. VESUVIO is the most successful example of this type of instrument and it is based at the ISIS Facility, UK. A description of deep inelastic neutron scattering and the VESUVIO spectrometer is presented here, together with the conceptual framework and a number of case studies to emphasize the role of nuclear quantum effects in condensed matter systems.