Beyond Vibrational Spectroscopy: Hunting the Signature of Elusive Quasiparticles with Monochromated STEM-EELS
Year: 2024
Type: article
Abstract: State-of-the-art monochromated electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) now offers angstrom-size electron beams 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. Provided the scattering geometry is carefully chosen to maximize signal [2], it is now possible to study quasiparticles such as phonons in materials at atomic resolution, including at single atom defects [3,4]. However, going beyond vibrational spectroscopy, there are many other excitations, including magnons, whose nanoscale signature may be detected with similar experimental strategies, possibly even down to the atomic scale. A first system of interest in this search for ‘elusive quasiparticles’ is obtained by placing Bi2Se3, a topological insulator (TI) with topologically-protected helical two-dimensional surface states and one-dimensional bulk states associated with crystal defects, in close proximity with graphene. This promotes strong spin-orbit interaction and proximity effects result in subtle and controllable electronic band structure changes near the interface, including the emergence of so-called Dirac plasmon modes, with exciting potential for spintronic applications. The use of a dark-field (DF) EELS geometry, whereby the spectrometer entrance aperture is displaced away from the direct beam to promote the relative contribution of (localised) impact scattering at the expense of (delocalised) dipole contributions has already provided a demonstration of chemically and atomically sensitive phonon mapping at the SiC/graphene interface [5]. Further distinct spectral signals observed at the interface between graphene and Bi2Se3, resolved either spatially or in momentum space, point to the detection of the Dirac plasmons in the TI layers through their interaction with the phonon modes in the epitaxial graphene. Furthermore, phonon vibrational modes occupy an energy range from zero to a few hundred meV in solid state materials, a range also occupied by energy losses due to magnons, a phenomenon arising from the collective excitation of the electrons’ spin in a lattice. It is therefore expected that in direct analogy with phonons the signature and momentum dispersion of magnons should also be accessible within the remit of STEM-EELS. Magnonics is an exciting 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 to a lack of fundamental understanding of these processes at the nanoscale in the vicinity of interfaces. Momentum-resolved (MR-), in combination with DF-EELS, was proposed to be a candidate technique to attempt the detection of magnons, thanks to a recently developed calculation framework [6,7]. Here, MR- and DF-EELS experiments on a number of prototypical systems, including an yttrium iron garnet (YIG)/platinum (Pt) bilayer, offer preliminary practical tests of the feasibility of the detection of magnons in the STEM. [8]
Source: Microscopy and Microanalysis
Institutions: University of Leeds, SuperSTEM, University of York, Uppsala University
Language: English
Subfield: Structural Biology
Domain: Life Sciences
Open Access status: bronze
OpenAlex ID: W4401004768