The electronic structure and the energetics of oxygen deficient interfaces, constituted by MgO(100) substrates that are capped by epitaxial TiO2 or SrTiO3 layers, are studied within first principles. Various configurations with one O vacancy per (1x1) surface unit cell, which is located either in the MgO or in the SrTiO3 interfacial layers, are compared. The excess electrons either fill Ti states at the bottom of the conduction band or form an F-center as in MgO(100), depending not only upon the side of the interface in which the vacancy is created, but also on the actual vacancy environment. In most cases that we consider, the interface has a metallic character. Our main result regards the order of stability of the various configurations; the oxygen vacancies can be formed more easily in the SrTiO3 overlayer than in the MgO substrate. Moreover, we show that the formation energies of O vacancies can be related to the nature and energy level of the excess electron state. Such a behavior is primarily determined by the interfacial electrostatic potential, which plays a prominent role as in other oxide/oxide interfaces.

The in-plane infrared response of the high-T-c cuprate superconductors was studied using the spin-fermion model, where charged quasiparticles of the copper-oxygen planes are coupled to spin fluctuations. First, we analyzed structures of the superconducting-state conductivity reflecting the coupling of the quasiparticles to the resonance mode observed by neutron scattering. The conductivity sigma computed with the input spin susceptibility in the simple form of the mode exhibits two prominent features: an onset of the real part of sigma starting around the frequency omega(0) of the mode and a maximum of a related function W(omega), roughly proportional to the second derivative of the scattering rate [1/tau](omega), centered approximately at omega=omega(0)+Delta(0)/h, where Delta(0) is the maximum value of the superconducting gap. The two structures are well-known from earlier studies. Their physical meaning, however, has not been sufficiently elucidated thus far. Our analysis involving quasiparticle spectral functions provides a clear interpretation. Second, we explored the role played by the spin-fluctuation continuum, whose spectral weight is known to be much larger than the one of the mode. We have shown that the experimental spectra of 1/tau can be approximately reproduced by augmenting the resonant-mode component of the spin susceptibility by a suitable continuum component with a considerably higher spectral weight and with a characteristic width of several hundreds meV. The computed spectra of 1/tau display a new structure in the midinfrared which is related to the finite width of the occupied part of the conduction band. Third, we investigated the temperature dependence (TD) of sigma assuming that the normal state spin susceptibility consists of an overdamped low energy mode and the continuum component. The differences between the experimental normal-state spectra and those of the superconducting state, including some interesting effects at higher frequencies, are reasonably well-reproduced. Motivated by recent experimental (ellipsometric) works by Molegraaf and co-workers [H. J. A. Molegraaf , Science 295, 2239 (2002)] and Boris and co-workers [A. V. Boris , Science 304, 708 (2004)], we further studied the TDs of the effective kinetic energy (KE) and of the intraband spectral weight I-O. Calculations for the trivial case of noninteracting quasiparticles in the normal state and a BCS-like superconducting state reveal a strong sensitivity of the TD of I-O to details of the dispersion relation. The TDs of KE and I-O in the interacting case, for the set of the values of the input parameters used throughout this work, are similar to those of the trivial case. The physics beyond the changes occurring when going from the normal to the superconducting state, however, is shown to be more complex, involving, besides the formation of the gap, also a feedback effect of the spin fluctuations on the quasiparticles and a significant shift of the chemical potential.

The electronic structure and the energetics of oxygen deficient interfaces, constituted by MgO(100) substrates that are capped by epitaxial $\mathrm{Ti}{\mathrm{O}}_{2}$ or $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ layers, are studied within first principles. Various configurations with one O vacancy per $(1\ifmmode\times\else\texttimes\fi{}1)$ surface unit cell, which is located either in the MgO or in the $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ interfacial layers, are compared. The excess electrons either fill Ti states at the bottom of the conduction band or form an $F$-center as in MgO(100), depending not only upon the side of the interface in which the vacancy is created, but also on the actual vacancy environment. In most cases that we consider, the interface has a metallic character. Our main result regards the order of stability of the various configurations; the oxygen vacancies can be formed more easily in the $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ overlayer than in the MgO substrate. Moreover, we show that the formation energies of O vacancies can be related to the nature and energy level of the excess electron state. Such a behavior is primarily determined by the interfacial electrostatic potential, which plays a prominent role as in other oxide/oxide interfaces.

The epitaxy of ultrathin SrTiO3 films on the stoichiometric MgO(001) surface is studied through first-principles calculations. The detailed atomic structure, the elastic properties, and the bonding characteristics of various interfaces are discussed and synthesized in a coherent picture. In particular, we show that (1) only the TiO2/MgO contact is thermodynamically stable and makes possible the growth of thicker strontium titanate layers on magnesia; (2) many characteristics of the thick SrTiO3(001)/MgO(001) epitaxial interface are recovered already when a SrTiO3 bilayer is adsorbed. The evolution of the valence band offset at the interface is also studied as a function of the nature and the thickness of the deposited films. For both epitaxial, TiO2 terminated, adsorbed bilayer and thick SrTiO3(001)/MgO(001) interface, the upper edges of the valence band are almost aligned.

We discuss several aspects of electron redistributions in low-dimensional oxides: surfaces, ultra-thin films or clusters. We rely upon a combination of first principles density functional calculations, Bader analysis and an analytical model, the bond electron transfer model, that we derive and which is valid whatever the covalent strength of the anion–cation bonding. We show that, despite the fact that nearly bulk-like charges are found on low-coordinated sites, there usually exists a strong increase of the covalent character of the oxygen–cation bonds, which is reflected in experimentally measurable quantities. On polar orientations, we discuss the efficiency of various processes yielding a cancellation of the macroscopic dipole moment.