Oxide Chemistry Below the Surface: Although metal oxides, such as titanium dioxide (TiO ₂ ), are used for catalytic oxidation reactions and photocatalysis, the O ₂ does not react directly with substrates. Vacancies in the surface region of the TiO ₂ rutile phase can transfer a negative charge to adsorbed O ₂ to create more reactive species. By contrast, in anatase—the phase associated with nanoscale TiO ₂ particles—subsurface vacancies form. Setvin et al. (p. 988 ) used a scanning tunneling microscopy tip to pull these vacancies to the surface in a niobiumdoped anatase crystal and followed the transformation of adsorbed O ₂ ⁻ into a peroxo species and a bridging O ₂ dimer.

We use first-principles calculations to investigate the stability of biaxially strained Pnma perovskite CaMnO${}_3$ towards the formation of oxygen vacancies. Our motivation is provided by promising indications that novel material properties can be engineered by application of strain through coherent heteroepitaxy in thin films. While it is usually assumed that such epitaxial strain is accommodated primarily by changes in intrinsic lattice constants, point defect formation is also a likely strain-relaxation mechanism. Our first-principles calculations of oxygen vacancy defect formation energy indeed show a strong strain dependence: We find that tensile strain lowers the formation energy, consistent with the established chemical expansion concept that oxygen deficiency increases the molar volume in oxides. In addition, we find that strain differentiates the formation energy for different lattice sites, suggesting its use as a route to engineering vacancy ordering in epitaxial thin films.

We expand the well-known notion that quantum criticality can induce superconductivity by proposing a concrete mechanism for superconductivity due to quantum ferroelectric fluctuations. To this end, we investigate the origin of superconductivity in doped ${\mathrm{SrTiO}}_3$ using a combination of density functional and strong coupling theories within the framework of quantum criticality. Our density functional calculations of the ferroelectric soft mode frequency as a function of doping reveal a crossover related to quantum paraelectricity at a doping level coincident with the experimentally observed top of the superconducting dome. Thus, we suggest a model in which the soft mode fluctuations provide the pairing interaction for superconductivity carriers. Within our model, the low doping limit of the superconducting dome is explained by the emergence of the Fermi surface, and the high doping limit by departure from the quantum critical regime. We predict that the highest critical temperature will increase and shift to lower carrier doping with increasing ${}^{18}\mathrm{O}$ isotope substitution, a scenario that is experimentally verifiable. Our model is applicable to other quantum paraelectrics, such as ${\mathrm{KTaO}}_3$.

Anatase is a metastable polymorph of ${\mathrm{TiO}}_2$. In contrast to the more widely studied ${\mathrm{TiO}}_2$ rutile, O vacancies ($V_{\mathrm{O}}$’s) are not stable at the anatase (101) surface. Low-temperature STM shows that surface $V_{\mathrm{O}}$’s, created by electron bombardment at 105 K, start migrating to subsurface sites at temperatures $\ge 200\mathrm{K}$. After an initial decrease of the $V_{\mathrm{O}}$ density, a temperature-dependent dynamic equilibrium is established where $V_{\mathrm{O}}$’s move to subsurface sites and back again, as seen in time-lapse STM images. We estimate that activation energies for subsurface migration lie between 0.6 and 1.2 eV; in comparison, density functional theory calculations predict a barrier of ca. 0.75 eV. The wide scatter of the experimental values might be attributed to inhomogeneously distributed subsurface defects in the reduced sample.

Interplay of spin, charge, orbital and lattice degrees of freedom in oxide heterostructures results in a plethora of fascinating properties, which can be exploited in new generations of electronic devices with enhanced functionalities. The paradigm example is the interface between the two band insulators LaAlO₃ and SrTiO₃ that hosts a two-dimensional electron system. Apart from the mobile charge carriers, this system exhibits a range of intriguing properties such as field effect, superconductivity and ferromagnetism, whose fundamental origins are still debated. Here we use soft-X-ray angle-resolved photoelectron spectroscopy to penetrate through the LaAlO₃ overlayer and access charge carriers at the buried interface. The experimental spectral function directly identifies the interface charge carriers as large polarons, emerging from coupling of charge and lattice degrees of freedom, and involving two phonons of different energy and thermal activity. This phenomenon fundamentally limits the carrier mobility and explains its puzzling drop at high temperatures.