Determining Chirality of Non-Centrosymmetric FeGe and MnGe Thin Films via STM

Recent interest in the ‘B20’ phase of FeGe and MnGe stems from noncollinear magnetic states, such as magnetic Skyrmions. Here we present a joint STM and DFT study of FeGe and MnGe films grown by molecular beam epitaxy. An average surface lattice constant of ~6.8 Å (FeGe) and of ~6.9 Å (MnGe), in agreement with the bulk values, was observed via LEED, as well as in situ RHEED during the MBE growth. Atomic resolution images of each of the four possible chemical terminations in the FeGe films were characterized by distinct image contrast and corrugation, as well as local density of states in tunneling spectra. Likewise, three of the four possible chemical terminations of MnGe were identified in atomic resolution STM images. These assignments were confirmed by the good agreement between the STM images and DFT-simulated images using the Tersoff-Haman approximation. Having determined the surface terminations, STM images of the atomic layering order and registry across step edges allows us to uniquely determine the stacking order, and hence chirality of these films, which is difficult with conventional crystallography techniques.