Scanning tunneling spectroscopy study of the electronic structure of Fe3O4 surfaces

Scanning tunneling spectroscopy STS experiments were performed on the 001 and 111 surfaces of single crystalline magnetite. Room temperature spectra exhibit a 0.2 eV gap around Ef. 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 K, just above and below the Verwey transition temperature TV 120 K , respectively. It is confirmed that above TV a 0.2 eV gap exists in the surface density of states DOS around Ef. Furthermore, broad bands are resolved on both sides of Ef, with peaks centered on +0.5 eV and −0.45 eV. Below TV 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 Ef. The similarity of the gap value before and after the transition points away from an ionic charge ordering occurring at the magnetite surface below TV. 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.