Electron-volt neutron spectroscopy: beyond fundamental systems

Abstract

This work provides an up-to-date account of the use of electron-volt neutron spectroscopy in materials research. This is a growing area of neutron science, capitalising upon the unique insights provided by epithermal neutrons on the behaviour and properties of an increasing number of complex materials. As such, the present work builds upon the aims and scope of a previous contribution to this journal back in 2005, whose primary focus was on a detailed description of the theoretical foundations of the technique and their application to fundamental systems [see Andreani et al., Adv. Phys. 54 (2005) p.377] A lot has happened since then, and this review intends to capture such progress in the field. With both expert and novice in mind, we start by presenting the general principles underpinning the technique and discuss recent conceptual and methodological developments. We emphasise the increasing use of the technique as a non-invasive spectroscopic probe with intrinsic mass selectivity, as well as the concurrent use of neutron diffraction and first-principles computational materials modelling to guide and interpret experiments. To illustrate the state of the art, we discuss in detail a number of recent exemplars, chosen to highlight the use of electron-volt neutron spectroscopy across physics, chemistry, biology, and materials science. These include: hydrides and proton conductors for energy applications; protons, deuterons, and oxygen atoms in bulk water; aqueous protons confined in nanoporous silicas, carbon nanotubes, and graphene-related materials; hydrated water in proteins and DNA; and the uptake of molecular hydrogen by soft nanostructured media, promising materials for energy-storage applications. For the primary benefit of the novice, this last case study is presented in a pedagogical and question-driven fashion, in the hope that it will stimulate further work into uncharted territory by newcomers to the field. All along, we emphasise the increasing (and much-needed) synergy between experiments using electron-volt neutrons and contemporary condensed matter theory and materials modelling to compute and ultimately understand neutron-scattering observables, as well as their relation to materials properties not amenable to scrutiny using other experimental probes

Hydrogen dynamics in solid formic acid: insights from simulations with quantum colored-noise thermostats

“…where σ 2 is the second moment of the NMD and y is the so-called West-scaling variable, defined as the projection of an associated radial nuclear momentum p onto the direction of the momentum transferQ [69]. For a given nucleus, σ 2 can be, within the harmonic approximation, further associated with the atom-projected density of vibrational states (AVDOS), g(ω), according to the following relation:…”

“…In brief, NCS operates within the limits of the so-called impulse approximation (IA) [69], with hundreds of electronVolt energy transferhω from incident neutrons and up to tens ofÅ −1 of momentum transfer, Q. The consequence of the IA is that a time-of-flight (TOF) spectrum measured on VESUVIO consists of separate recoil peaks, each stemming from the longitudinal nuclear momentum distribution (NMD) of each respective nucleus present in the target system, J IA y,Q [69].…”

“…In brief, NCS operates within the limits of the so-called impulse approximation (IA) [69], with hundreds of electronVolt energy transferhω from incident neutrons and up to tens ofÅ −1 of momentum transfer, Q. The consequence of the IA is that a time-of-flight (TOF) spectrum measured on VESUVIO consists of separate recoil peaks, each stemming from the longitudinal nuclear momentum distribution (NMD) of each respective nucleus present in the target system, J IA y,Q [69]. Moreover, the intensity of each NCS recoil peak is proportional to the product of the total bound scattering cross-section and the number of atoms under the beam, thus rendering NCS, unlike INS, completely free from the neutron weighting problem, which greatly facilitates the theoretical interpretation of the spectra.…”

The road to a station for epithermal and thermal neutron analysis

“…Looking at these proceedings and at a recent review [4], the scientific community using eV neutron spectroscopy on the VESUVIO spectrometer has broadened the area of investigated subjects towards, for example, energy materials [35] and real-life systems [36]. Such an evolution in the scientific programme has been catalysed, amongst other reasons, by an increased attention to the several techniques simultaneously available during a DINS experiment.…”

“…Looking at the proceedings of the VI (2014) [1] and VII (2017) international workshops on eV neutron spectroscopy [2], one can notice how the use of Deep Inelastic Neutron Scattering (DINS) to characterise Nuclear Quantum Effects (NQEs) mostly on hydrogen dynamics [3] has lead to MAss-selective Neutron SpEctroscopy (MANSE), whereby the Nuclear Momentum Distributions (NMDs) of several elements in a given sample are accessed at the same time [4,5,6]. Furthermore, a number of recent investigations carried out on the VESUVIO spectrometer [7], the flagship instrument for eV neutron spectroscopy at the ISIS Facility [8], successfully combined DINS with concurrent Neutron Diffraction (ND) measurements [9,10], or employed broad-range Neutron Transmission (NT) [11,12].…”