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Direct observation of subsurface charge ordering in Fe3O4(001) by scanning tunneling microscopy/spectroscopy

In this study, we characterized Fe3O4(001) films using scanning tunneling microscopy/spectroscopy (STM/STS). On the film surfaces, individual iron atoms and ()R45° reconstructed structures were observed by STM. The STS results showed that the local density of states just below the Fermi level was higher in narrow sections than in wide sections of the surface reconstruction perpendicular to the iron rows. Periodic density of states modulations reproducing this electronic structure were clearly observed in the differential tunneling conductance map. These experimental results revealed the presence of subsurface charge ordering of Fe2+–Fe2+ and Fe3+–Fe3+ dimers, as proposed in previous density functional theory studies.