(001)*

The surface of SrTiO₃ (001) is considered to be weakly polar and in this work, we study the validity of this notion. Using low energy electron diffraction at room temperature, a surface structural distortion was found. Structural analysis shows the presence of strong surface rumpling in the TiO₂ terminated surface with the Ti and O atoms move in and outward. Density functional calculations confirm the measured rumpling, and experimental data show the distortion is localized at the surface. Angle-dependent core-level X-ray photoemission spectroscopy (XPS) shows that the surface rumpling strongly impacts the electronic structure of the surface. Density functional theory demonstrates that the valence state of Ti at the surface is reduced while O is enhanced. In addition, we found the Ti-O bonds are more covalent near the surface. Changes in the XPS satellite structures at the surface are consistent with this picture of the change in bonding, indicating that the (001) surface of SrTiO₃ is not polar and charge rearrangement is a consequence of surface rumpling. We will also present our latest finding on the effect of cubic-tetragonal transition on the surface structure.