Surface Structural Phases of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$

Surface structural phases of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ (CSRO) are investigated using Low Energy Electron Diffraction (LEED-IV). The surface structure and phases are compared to the bulk phases in this layered material. Normally at a crystal surface the reduced atomic coordination enhances electron-electron correlations, if there is not a reconstruction, and stabilizes the Mott-insulating phase resulting in higher MIT temperatures than in the bulk. Surprisingly, Ca$_{1.9}$Sr$_{0.1}$RuO$_{4}$ exhibits an unusual surface MIT 20K lower than the bulk. The properties of CSRO are extremely sensitive to the RuO$_{6}$ octahedral orientation. LEED-IV shows no symmetry change associated with the MIT, but small distortions of the RuO$_{6}$ octahedral that drive the surface MIT. In addition, the tetragonal-to-orthorhombic surface phase transition near the Quantum Critical Point at x=0.5 is investigated and contrasted with the bulk phase transition.