Formation of the strain-induced electronic superstructure on the magnetite (111) surface

– We present direct experimental evidence of the formation of a superstructure on the (111) surface of a magnetite, Fe3O4 single crystal. The superstructure, which has a periodicity of 42A and three-fold symmetry, has been observed by means of STM and LEED. Under the correct conditions of oxygen pressure and sample anneal temperature, the superstructure is reproducibly formed throughout most of the sample surface. Clear atomic resolution within the superstructure has been achieved. The characteristics of the superstructure, including its dependency on the tunnel bias voltage and its atomic-scale periodicity, suggest that it is an electronic effect rather than a mosaic of several iron oxide phases. We explain the results in terms of the formation of giant static polarons, although we notice that other types of electronlattice instabilities such as charge density wave may offer possible explanations. Polarons with dimensions of many interatomic distances in three-dimensional systems are unlikely to exist but the situation for twoand one-dimensional cases is predicted to be different. We suggest three possible scenarios of instability linking the electron band structure and lattice distortions in magnetite: either resulting from reallocation of Fe and Fe valence states between octahedral sites or, alternatively, from reallocation between octahedral and tetrahedral sites. Introduction. – Magnetite (Fe3O4) is the oldest known magnetic material and is also the origin of the word magnetism. In some respects it is unique, while in many others it is a typical example of a system illustrating a range of important phenomena in materials science, geology [1], medicine [2] and even extra-terrestrial exploration [3]. Magnetite has the inverse spinel crystal structure, which is based on an f.c.c. lattice of oxygen (O2−) anions, containing mixed valence Fe cations in tetrahedrally and octahedrally coordinated interstices. The unit cell contains 32 anions, 8 cations placed at sites with tetrahedral coordination (so-called A-sites) and 16 cations placed at sites with octahedral coordination (B-sites) (fig. 1). The A-sites are occupied by Fe ions and the B-sites are filled with an equal number of Fe and Fe ions. Magnetite is one of the very few oxides with a relatively high electrical conductivity, ∼ 200Ω−1cm−1 at 300K, due to electron hopping between the B-sites [4]. Magnetite is also a well-known example of a material that undergoes a metal-insulator transition, one of the most important phenomena in solid-state physics. At the Verwey temperature TV = 115–124K, electrons freeze at the B-sites forming a charge ordered state. The exact arrangement of the Fe ions within the lattice below TV, and the presence of short-range or long-range order is the subject of a continuing debate that has