Scanning tunneling spectroscopy (STS) experiments were performed on the (001) and (111) surfaces of single crystalline magnetite. Room temperature spectra exhibit a $\sim 0.2\mathrm{eV}$ 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 $95\mathrm{K}$, just above and below the Verwey transition temperature $(T_V\sim 120\mathrm{K})$, respectively. It is confirmed that above $T_V$ a $\sim 0.2\mathrm{eV}$ 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 $\sim +0.5\mathrm{eV}$ and $\sim -0.45\mathrm{eV}$. 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.

• Received 11 May 2006

DOI:https://doi.org/10.1103/PhysRevB.74.085416