Electron Compton scattering and its application to condensed matter phenomena

Abstract

Investigation of electron Compton scattering using electron energy loss spectroscopy has been carried out to study projected electron momentum density distributions in amorphous carbon films, as well as two technologically important materials: few-layer WS films, which undergo indirect-to-direct band gap transition in the monolayer limit, and VO flakes which exhibit a metal-to-insulator transition upon cooling to below ≈67 °C.

An important concept underlying the Compton scattering measurements is the so-called impulse approximation, the validity of which is crucial for the interpretation of the Compton scattering data in terms of electron momenta in the sample. Hence, a simple material – amorphous carbon film – has been selected to test the impulse approximation at low electron scattering angles, which provides the possibility for a faster, more efficient data collection and eliminates the need for the core electron background subtraction from the acquired electron momentum density profiles via complex computational methods. Further, plasmon background subtraction has been investigated in the context of electron beam damage-resistant materials, as well as options to carry out high spatial resolution (sub-nm) Compton measurements using scanning-transmission electron microscopy.

As for electron Compton measurements in monolayer and bilayer WS, suitable experimental conditions have been first established in order to avoid any sample damage associated with the electron beam irradiation. Here, Compton scattering experiments were carried out in order to study the differences in the projected electron momentum densities in the two structures resulting from the changes in the band gap type. The results were also verified via CASTEP density functional theory calculations. Furthermore, considering that the WS samples contained quite significant amorphous surface carbon contamination, theoretical calculations were appropriately adjusted for an improved match to the experimental measurements.

Finally, Compton scattering measurements were carried out on VO flakes to study the properties of electrons in the metallic and insulating phases. Just as in the measurements of WS films, optimisation of experimental conditions which minimise electron beam damage and maximise electron count rate has been of high importance. The measured data was then compared to the theoretical predictions found in the literature, which combine both density functional theory and quantum Monte Carlo calculations to obtain the difference Compton profiles.