R.G. Moore

Surface stability of epitaxial SrRuO 3 thin films in vacuum

Surface stability of nearly defect-free epitaxial SrRuO3 thin films grown by pulsed laser deposition was studied using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and electron spectroscopies. Even after exposure to atmosphere, surfaces exhibited distinct LEED patterns providing evidence of unusual chemical stability. Surface order disappeared after heating to 200 °C in vacuum. To investigate, SrRuO3 thin films were annealed up to 800 °C in high vacuum and examined for chemical state and topography. Formation of unit-cell deep pits and the Ru-rich particles begins at low temperatures. Hydrocarbon contaminatio no n the surface contributes to this process.

Manifestations of Broken Symmetry: The Surface Phases of Ca2-xSrxRuO4

The surface structural phases of Ca(2-x)SrxRuO4 are investigated using quantitative low energy electron diffraction. The broken symmetry at the surface enhances the structural instability against the RuO6 rotational distortion while diminishing the instability against the RuO6 tilt distortion occurring within the bulk crystal. As a result, suppressed structural and electronic surface phase transition temperatures are observed, including the appearance of an inherent Mott metal-to-insulator transition for x=0.1 and possible modifications of the surface quantum critical point near x(c) approximately 0.5.

Phonon softening and anomalous mode near the xc=0.5 quantum critical point in Ca2-xSrxRuO4

Inelastic neutron scattering is used to measure the temperature-dependent phonon dispersion in Ca{sub 2-x}Sr{sub x}RuO{sub 4} (x = 0.4,0.6). The in-plane {Sigma}{sub 4} octahedral tilt mode softens significantly at the zone boundary of the high-temperature tetragonal (HTT) I4{sub 1}/acd structure as the temperature approaches the transition to a low-temperature orthorhombic (LTO) Pbca phase. This behavior is similar to that in La{sub 2}CuO{sub 4}, but an inelastic feature that is not found in the cuprate is present. An anomalous phonon mode is observed at energy transfers greater than the {Sigma}{sub 4}, albeit with similar dispersion. This anomalous phonon mode never softens below {approx} 5 meV, even for temperatures below the HTT-LTO transition. This mode is attributed to the presence of intrinsic structural disorder within the I4{sub 1}/acd tetragonal structure of the doped ruthenate.