Observation of Flat Bands and Dirac Cones in Epitaxial CoSn Thin Films

Materials with dispersionless flat bands in momentum space are ideal for studying strongly correlated electronic states. Kagome-structured CoSn has been recognized as one of the materials hosting flat bands several hundred meV below Fermi level. A promising future direction is to grow the thin films and fabricate the devices which allow the tuning of flat bands towards Fermi level. However, most of the reported epitaxial CoSn thin films were grown on metallic buffer layers so far, which may not allow the tuning of flat bands through voltage gating. In this work, we synthesized CoSn thin films on insulating substrates by molecular beam epitaxy. The structure of the sample was confirmed by reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD). The flat bands and Dirac cones were observed using angle-resolved photoemission spectroscopy (ARPES). The transport properties of the CoSn thin films were also studied. This work paves the way towards device fabrication and fine tuning of band filling to promote flat band related phenomena for future studies.