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

Abstract

In this study, we characterized Fe 3 O 4 (001) films using scanning tunneling microscopy/spectroscopy (STM/STS). On the film surfaces, individual iron atoms and (√ 2× √ 2)R45 • reconstructed structures were observed by STM. The STS results showed the higher local density of states just below the Fermi level of narrow sections than wide ones of the surface reconstruction perpendicular to the iron rows. Periodical density of states modulations reproducing this electronic structure were clearly observed by the differential tunneling conductance map. These experimental results revealed the presence of subsurface charge ordering of Fe 2+-Fe 2+ and Fe 3+-Fe 3+ dimers, as proposed in previous density functional theory studies.

Electron-transfer-induced metallic electronic states in a H/Fe3O4(001) film subsurface

“…The averaged distances between neighboring rows and between neighboring atoms within each row are ∼0.6 and ∼0.3 nm, respectively. 20,21) Because these atom periodicities correspond to those of the Fe atoms in the B-terminated Fe 3 O 4 (001) surface [denoted as Fe B (S) in Fig. 1(d)], the observed atoms should represent single Fe B (S) atoms.…”

“…Recent STM/STS studies of a UHV-prepared Fe 3 O 4 (001) film revealed charge ordering of Fe 2+ -Fe 2+ and Fe 3+ -Fe 3+ dimers in the subsurface layer. 20) In this study, we investigated the local electronic properties of subsurface Fe atoms located beneath an OH group [denoted as Fe H (S-1) in Fig. 1(d)] to reveal the effect of H atoms on Fe electronic states in the subsurface layer where the abovementioned charge ordering occurs.…”