Imaging the evolution of d states at a strontium titanate surface

Oxide electronics is a promising alternative to the conventional silicon-based semiconductor technology, owing to the rich functionalities of oxide thin films and heterostructures. In contrast to the silicon surface, however, the electronic structure of the SrTiO₃ surface, the most important substrate for oxide thin films growth, is not yet completely understood. Here we report on the electronic states of a reconstructed (001) surface of SrTiO₃ determined in real space, with scanning tunneling microscopy/spectroscopy and density functional theory calculations. We found a remarkable energy dependence of the spectroscopic image: Theoretical analysis reveals that symmetry breaking at the surface lifts the degeneracy in the t_2g state (d_xy, d_yz, and d_zx) of Ti 3d orbitals, whose anisotropic spatial distribution leads to a sharp transition in the spectroscopic image as a function of energy. The knowledge obtained here could be used to gain further insights into emergent phenomena at the surfaces and interfaces with SrTiO₃.