Probing collective oscillation of $d$-orbital electrons at the nanoscale

TL;DR

This study uses high-energy electrons to investigate collective oscillations of $s$, $p$, and $d$ electrons at the nanoscale in quantum wells, revealing new spectral features linked to $d$ electrons and their interaction with electron-hole pairs.

Contribution

It demonstrates the ability to probe $d$-orbital electron oscillations at the nanoscale using electron energy loss spectroscopy and ab-initio theory, highlighting the role of $d$ electrons in plasmon modes.

Findings

Identification of $d$-electron contributions to plasmon spectra.

Spectral features vary with Ga content in quantum wells.

Distinct energy modes for $d$-electron plasmons compared to valence plasmons.

Abstract

Here we demonstrate that high energy electrons can be used to explore the collective oscillation of $s$, $p$, and $d$ orbital electrons at the nanometer length scale. Using epitaxial AlGaN/AlN quantum wells as a test system, we observe the emergence of additional features in the loss spectrum with increasing Ga content. A comparison of the observed spectra with ab--initio theory reveals the origin of these spectral features is attributed to 3$d$--electrons contributed by Ga. We find that these modes differ in energy from the valence electron plasmons in Al$_{1-x}$Ga$_x$N due to the different polarizability of the $d$ electrons. Finally, we study the dependence of observed plasmon modes on Ga content, lending insight into plasmon coupling with electron--hole excitations.