High-Energy-Resolution Dark-Field EELS for Quantum Materials: From Mapping Localised Vibrational Modes to Magnonics in The STEM

State-of-the-art monochromated electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) now offers angstrom size electron beam and an energy resolution for EELS under 5meV [1]. These capabilities enable studies of the interplay between fundamental properties of matter such as charge, spin and local chemistry, at the atomic scale. A striking example is presented by epitaxial graphene grown on SiC(0001) by thermal methods, which is known to present a so-called ‘buffer’ layer, whereby the last layer of carbon in contact with the SiC substrate possesses a strikingly different electronic structure from the free-standing epitaxial graphene, impacting possible applications in quantum devices [2]. The use of a dark-field EELS (DF-EELS) geometry to probe the vibrational response of the system [3], atomic plane by atomic plane, reveals the true nature of the chemical bonds formed at this interface, a characterisation that would not be possible with any other technique. Further growing Bi₂Se₃, a topological insulator (TI) with topologically-protected helical two-dimensional surface states and one-dimensional bulk states associated with crystal defects, on top of the graphene layers, takes advantage of strong spin-orbit interaction and proximity effects and results in subtle and controllable electronic band structure changes. The same experimental geometry allows the observation of the Dirac plasmons in the TI layers, and the study of their dispersion in momentum space. DF-EELS was also proposed to be a leading candidate technique to attempt the detection of magnons, even down to at the atomic scale, thanks to a recently developed theoretical calculation framework and preliminary experimental investigations [4,5]. Magnonics is an emergent field within spintronics research whereby a spin-wave (or magnon) is propagated controllably in nano-dimensional magnetic structures allowing to build a new generation of devices for data processing and storage. In addition, it was recently demonstrated that magnons can be utilised to convert spin to charge currents and vice versa, a critical step for integration of spin and charge devices. Despite recent progress, many challenges hinder practical applications due a lack of fundamental understanding of these processes at the nanoscale in the vicinity of interfaces. DF-EELS experiments can be used to study the spin-to-charge conversion in a system consisting of Yttrium Iron Garnet (YIG)/platinum (Pt) bilayer, a widely and intensively used materials combination and a prototypical system to demonstrate the detection of magnons and magnon-phonon polarons in the STEM.<br/>[1] O. L. Krivanek <i>et al.</i>, Ultramicroscopy <b>203</b>, 60 (2019)<br/>[2] G. Nicotra <i>et al.</i>, ACS Nano <b>7</b>, 3045 (2013)<br/>[3] F.S. Hage <i>et al.</i>, Science <b>367</b>, 1124 (2020)<br/>[4] K. Lyon <i>et al.</i>, Phys. Rev. B <b>104</b>, 214418 (2021).<br/>[5] J.A. Castellanos-Reyes <i>et al.</i>, Phys. Rev. B, accepted (2023)