Advances in High Energy and Spatial Resolution STEM EELS

Engineering the structural or chemical architecture of functional materials at the nano or even atomic level enables emergent properties that rely on the interplay between fundamental properties of matter such as charge, spin and local atomic-scale chemistry. Thanks to advances in monochromators, state-of-the-art electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM), offers nowadays the ability to map materials and atomic structures with an angstrom size electron beam and an energy resolution for EELS under 5meV. (Krivanek <i>et al.</i>, 2014) These capabilities have allowed to probe the spectroscopic signature of phonons down to the single atom level. (Hage <i>et al.</i>, 2020) Here, we present strategies for high spatial and energy resolution STEM-EELS experiments to interrogate proximity effects of heterostructure materials at the atomic scale and present opportunities for new experiments using monochromated electron probes.<br/>Bi₂Se₃ is a topological insulator (TI) with topologically protected helical two-dimensional surface states and one-dimensional bulk states associated with crystal defects, in proximity with graphene. The strong spin-orbit interaction and proximity effects result in subtle and controllable electronic band structure changes at and near the interface, with exciting potential for spintronic applications. Here we probe at high energy resolution the interfaces in a system consisting of Bi₂Se₃ films grown by chemical vapor deposition on epitaxial graphene/SiC(0001), where the number of carbon layers can be carefully controlled to tune possible proximity effects between the film and the substrate. In addition to a direct interrogation of the chemical bonds (Bugnet <i>et al.</i>, 2022) between the layers via their vibrational response, these observations are linked to the interplay between the various phonon modes and the Dirac plasmons in the TI layers, whose dispersion is mapped in momentum space with nm spatial sensitivity using a recently developed methodology for nanoscale momentum-resolved spectroscopy.<br/>Magnonics is an emergent field within spintronics utilizing the ability to generate and propagate controllably a spin-wave in nm-sized magnetic structures, with a view to build a new generation of devices for data processing and storage. It was also recently demonstrated that magnons can be utilized to convert spin to charge (or charge to spin) currents, a critical step for integration of spin and charge devices. STEM EELS is proposed to be a leading candidate technique to attempt the detection of magnons at the nanoscale, perhaps down to at the atomic scale, thanks to a recently developed theoretical calculation framework and preliminary experimental investigations (Lyon <i>et al.</i>, 2021). Here, we report on progress in designing experiments building on these initial steps using in-situ STEM-EELS. The prototypical spin-to-charge conversion system considered for this proof of principle attempt consists of Yttrium Iron Garnet (YIG)/platinum (Pt) bilayer, a widely used materials combination where a magnon created by a thermal gradient creates spin accumulation at the YIG/Pt interface, which subsequently diffuses into the nonmagnetic Pt, and via inverse spin Hall effect (ISHE) creates a voltage signal in the Pt layer.<br/><br/><b>References</b><br/>[1] O. L. Krivanek et- al, J. Phys. Conf. Ser. <b>522</b>, 012023 (2014).<br/>[2] F. S. Hage et- al, Science <b>367</b>, 1124 LP (2020).<br/>[3] M. Bugnet et- al, Phys. Rev. Lett. <b>128</b>, 116401 (2022).<br/>[4] K. Lyon, et- al, Phys. Rev. B <b>104</b>, 214418 (2021).