Manifestations of Broken Symmetry: The Surfaces Phases of Ca_2−xSr_xRuO₄

The discovery of superconductivity in Sr₂RuO₄ has renewed vigor in the study of correlated electron systems. The evolution of a p-wave superconducting state from a para- magnetic 2-dimensional Fermi liquid shows the ruthenate superconductivity is anything but conventional. Sr₂RuO₄ is isostructural with La₂CuO₄, the parent compound for the high temperature superconducting family La2_−xSr_xRuO₄. The substitution of Ca²⁺ for Sr²⁺ generates a different structure involving a static rotation and tilt of the RuO₆ octahedral, however, the antiferromagnetic insulating ground state of Ca₂RuO₄ is more akin to the cuprate parent. The generation of Ca_2−xSr_xRuO₄ has offered a new family of com- pounds where the evolution from an antiferromagnetic insulator to a superconductor can be studied. Bulk studies have demonstrated how the intricate couplings between structural, orbital, electronic, and magnetic degrees of freedom are responsible for the exotic phases of the system. The layered perovskite structure which plays a key role in the properties observed also makes the crystals amenable to cleaving. Breaking symmetry by the creation of a surface on a quasi 2-dimensional system offers an opportunity to gain insight into the role of structure and symmetry on the properties of the system and offers a new avenue to discover new physics. Inelastic neutron scattering has been utilized to reveal the structural instability against the RuO₆ tilt. While the Σ₄ phonon mode involving the octahedral tilt shows classic soft phonon mode behavior across a tetragonal to orthorhombic phase transition, a new anomalous mode is discovered and its origin is explored. Surface phonon dynamics have been investigated across a Mott metal-to-insulator transition utilizing High Resolution Electron Energy Loss Spectroscopy where it is revealed the surface electronic transition temperature is significantly lower than the bulk. Low Energy Electron Diffraction has been employed to investigate the surface structure and structural transitions on the surface. Results show surface relaxations inhibit the RuO₆ tilt dramatically altering the ensuing orthorhombic phase transition near a quantum critical point at x_c = 0.5. It is also revealed that structural distortions accompanying the bulk metal-to-insulator transition are simply nonexistent on the surface. Physical manifestations from breaking symmetry in a correlated electron system are revealed.