Strontium Titanate

Polar surfaces are known to be unstable due to the divergence of the surface electrostatic energy. Here we report on the experimental determination, by grazing incidence x-ray diffraction, of the surface structure of polar Ti-terminated (111) SrTiO 3 single crystals. We find that the polar instability of the 1 × 1 surface is solved by a pure electronic reconstruction mechanism, which induces out-of-plane ionic displacements typical of the polar response of SrTiO 3 layers to an electron confining potential. On the other hand, the surface instability can be also eliminated by a structural reconstruction driven by a change in the surface stoichiometry, which induces a variety of 3 × 3 (111) SrTiO 3 surfaces consisting in an incomplete Ti (surface)-O 2 (subsurface) layer covering the 1 × 1 Ti-terminated (111) SrTiO 3 truncated crystal. In both cases, the TiO 6 octahedra are characterized by trigonal distortions affecting the structural and the electronic symmetry of several unit cells from the surface. These findings show that the stabilization of the polar (111) SrTiO 3 surface can lead to the formation of quasi-two-dimensional electron systems characterized by radically different ground states which depend on the surface reconstructions.

"The quantum paraelectric strontium titanate can be made ferroelectric through replacing oxygen atoms with their heavy isotopes. Although suppressed quantum fluctuations have been widely believed to be the origin, the details remain unsettled. The controveries are often framed using Barrett formula, which involves two quantities termed T0 and T1, respectively. The obervations can be equally explained by assuming either a decrease in T1 or an increase in T0 upon isotope replacement. The conventional view holds a direct connection between quantum fluctuations and the T1 and hence adopts the decreasing T1 picture. In this paper, we offer a different opinion, in which the T1 bears a different meaning and quantum
fluctuations are attached to another quantity to be denoted ω0. We show that a decrease in ω0 could diminish quantum fluctuations and simultaneously enhance T0. A vibronic mechanism is presented as a possible route to the ω0 change. The isotope effects are argued to be rather non-local. The dynamics of the system could be highly quasi-one-dimensional and this is then employed in discussing the relation between
a soft mode and a recently observed central mode."

Complex cluster TiO5·VO z  and SrO11·VO z 
where VO z =VO X, VO • , VO •• vacancies were identified in disordered SrTiO3 powders prepared by the polymeric precursor method, based on experimental measurements by x-ray absorption near edge structure spectroscopy. The paramagnetic complex states of TiO5·VO •  and SrO11·VO •  with unpaired electrons were confirmed by electron paramagnetic resonance spectroscopy.The disordered powders showed strong photoluminescence at room temperature. Structural defects of disordered powders, in terms of band diagram, density of states, and electronic charges, were interpreted using high-level quantum mechanical calculations in the density functional framework. The four periodic models used here were consistent with the experimental data and explained the presence of photoluminescence.

Quantum confinement in a thin-film geometry offers viable routes for tuning the critical properties of superconductors through modification of both density of states and pairing interaction. Low-density systems like doped strontium titanate are especially susceptible to these confinement-induced effects. In this paper, we show that the superconducting critical temperature $T_c$ is enhanced through quantum confinement in SrTiO$_3$/SrTi$_{1-x}$Nb$_x$O$_3$/SrTiO$_3$ heterostructures at $x=1\%$ concentration, by measuring resistivity transitions and the Hall carrier density for different thicknesses of the doped layer. We observe a nonmonotonic raise of $T_c$ with decreasing layer thickness at constant carrier density as estimated from the Hall effect. We analyze the results by solving a two-band model with a pairing interaction reproducing the density-dependent $T_c$ of doped SrTiO$_3$ in the bulk, that we confine to a potential well established self-consistently by the charged Nb dopa...

Surface x-ray diffraction has been employed to elucidate the structure of the interface between a well characterized (001) surface of 0.1 wt% Nb-SrTiO3 and liquid H2O. Results are reported for the clean surface, the surface in contact with a drop of liquid water and the surface after the water droplet has been removed with a flow of nitrogen. The investigation revealed the clean surface, prepared via annealing in 1 x 10-2 mbar O2 partial pressure, is unreconstructed and rough on a short length scale. The surface is covered with large terraces, the topmost layer of which is either TiO2 or SrO with an area ratio of about 7/3. For the surface in contact with water our results reveal that associative H2O adsorption is favoured for the TiO2 terminated terrace whereas adsorption is dissociative for the SrO terminated terrace, which validates recent first principles calculations. After removal of the water droplet, the surface largely resembles the water-covered surface but now with a disordered overlayer of water present on the surface.