2DEG at the surface of STO

Reduced dimension electron systems boast of very interesting physics from both the fundamental and practical point of view. For example, 2DEG houses quantum hall effects, which even today constitutes a rich arena of study [Nature, 469:185–188]. Such 2DEG exists in a diversity of systems, such as semiconductor or transition metal oxide hetero structures. More recently, it defines topological insulators. Now a work in Nature shows that, 2DEG exists also in simple surfaces of strontium titanate, and more interestingly, this 2DEG has similar characteristics with those found at the interfaces of STO with the second compound [doi:10.1038/nature09720]. This offers an easy way to fabricate 2DEG systems. More work is needed in elucidating the mechanism.

As silicon is the basis of conventional electronics, so strontium titanate (SrTiO3) is the foundation of the emerging field of oxide electronics1,2. SrTiO3 is the preferred template for the creation of exotic, two-dimensional (2D) phases of electronmatter at oxide interfaces3–5 that havemetal–insulator transitions6,7, superconductivity8,9 or large negative magnetoresistance10. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs), which is crucial to understanding their remarkable properties11,12, remains elusive. Here we show, using angle-resolved photoemission spectroscopy, that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO3 (including the non-doped insulating material) independently of bulk carrier densities over more than seven decades. This 2DEG is confined within a region of about five unit cells and has a sheet carrier density of 0.33 electrons per square lattice parameter. The electronic structure consists of multiple subbands of heavy and light electrons. The similarity of this 2DEG to those reported in SrTiO3-based heterostructures6,8,13 and field-effect transistors9,14 suggests that different forms of electron confinement at the surface of SrTiO3 lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO3-based devices and a novel means of generating 2DEGs at the surfaces of transition-metal oxides.