Scanning tunneling spectroscopy study of the electronic structure ofFe3O4surfaces

Scanning tunneling spectroscopy (STS) experiments were performed on the (001) and (111) surfaces of single crystalline magnetite. Room temperature spectra exhibit a $\ensuremath{\sim}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ gap around ${E}_{f}$. The importance of perfect surface order to the existence of this gap is illustrated. STS is also carried out on the (111) surface, at 140 and $95\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, just above and below the Verwey transition temperature $({T}_{V}\ensuremath{\sim}120\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, respectively. It is confirmed that above ${T}_{V}$ a $\ensuremath{\sim}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ gap exists in the surface density of states (DOS) around ${E}_{f}$. Furthermore, broad bands are resolved on both sides of ${E}_{f}$, with peaks centered on $\ensuremath{\sim}+0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $\ensuremath{\sim}\ensuremath{-}0.45\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Below ${T}_{V}$ it is shown that the value of the gap in the surface DOS remains similar, however, the peaks resolved in the conduction and valence bands shift markedly away from ${E}_{f}$. The similarity of the gap value before and after the transition points away from an ionic charge ordering occurring at the magnetite surface below ${T}_{V}$. However, the shifting of the bands points to a certain degree of electronic ordering or charge disproportionation playing an integral part in the Verwey transition, at the magnetite surface.