Anomalous Zeeman shift of the defect bound states in epitaxial FeSn films on SrTiO₃(111)

The crystal structure of Kagome magnet FeSn consists of alternating planes of Fe₃Sn Kagome and Sn honeycomb lattices, which provides a versatile platform for correlated topological phases hosting symmetry-protected electronic excitations and magnetic states.  To date most work on FeSn has been carried out on bulk materials. In this work, we synthesize high quality FeSn thin films on SrTiO₃(111) substrates by molecular beam epitaxy. The growth mode is three-dimensional, and x-ray diffraction confirms single crystalline FeSn structure. Using in situ low temperature scanning tunneling microscopy/spectroscopy, we observe honeycomb lattice on the Sn-terminated films, and close-packed lattice for the Fe₃Sn termination. In addition, we also directly visualize single Sn vacancy in Fe₃Sn layer and di-vacancy in Sn layer, as well as their response to applied magnetic fields. The bound states induced by these defects exhibit anomalous Zeeman shift under out-of-plane magnetic fields, where the energy of the bound states shifts linearly towards higher energy independent of the direction of the magnetic field. These findings provide critical insights to the understanding of magnetism in Kagome magnetic materials.