In this study, we present a combined density functional theory and many-body perturbation theory study on the electronic and optical properties of TiO(2) brookite as well as the tetragonal phases rutile and anatase. The electronic structure and linear optical response have been calculated from the Kohn-Sham band structure applying (semi)local as well as nonlocal screened hybrid exchange-correlation density functionals. Single-particle excitations are treated within the GW approximation for independent quasiparticles. For optical response calculations, two-particle excitations have been included by solving the Bethe-Salpeter equation for Coulomb correlated electron-hole pairs. On this methodological basis, gap data and optical spectra for the three major phases of TiO(2) are provided. The common characteristics of brookite with the rutile and anatase phases, which have been discussed more comprehensively in the literature, are highlighted. Furthermore, the comparison of the present calculations with measured optical response data of rutile indicate that discrepancies discussed in numerous earlier studies are due to the measurements rather than related to an insufficient theoretical description.

The electronic and optical properties of zinc-blende (zb) Al x Ga 1−x N over the whole alloy composition range are presented in a joint theoretical and experimental study. Because zb-GaN is a direct ( v → c ) semiconductor and zb-AlN shows an indirect ( v → X c ) fundamental band gap, the ternary alloy exhibits a concentration-dependent direct-indirect band gap crossing point the position of which is highly controversial. The dielectric functions of zb-Al x Ga 1−x N alloys are measured employing synchrotron-based ellipsometry in an energy range between 1 and 20 eV. The experimentally determined fundamental energy transitions originating from the , X, and L points are identified by comparison to theoretical band-to-band transition energies. In order to determine the direct-indirect band gap crossing point, the measured transition energies at the X point have to be aligned by the calculated position of the highest valence state. Thereby density-functional theory (DFT) based approaches to the electronic structure, ranging from the standard (semi)local generalized gradient approximation (GGA), self-energy corrected local density approximation (LDA-1/2), and meta-GGA DFT (TB-mBJLDA) to hybrid functional DFT and many-body perturbation theory in the GW approximation, are applied to random and special quasirandom structure models of zb-Al x Ga 1−x N. This study provides interesting insights into the accuracy of the various numerical approaches and contains reliable ab initio data on the electronic structure and fundamental alloy band gaps of zb-Al x Ga 1−x N. Nonlocal Heyd-Scuseria-Ernzerhof-type hybrid-functional DFT calculations or, alternatively, GW quasiparticle calculations are required to reproduce prominent features of the electronic structure. The direct-indirect band gap crossing point of zb-Al x Ga 1−x N is found in the Al rich composition range at an Al content between x = 0.64 and 0.69 in hybrid functional DFT, which is in good agreement with x = 0.71 determined from the aligned experimental transition energies. Thus our study solves the long-standing debate on the nature of the fundamental zb-Al x Ga 1−x N alloy band gap.

Combining electron paramagnetic resonance, density functional theory, and positron annihilation spectroscopy (PAS), we identify the nitrogen interstitial defect in GaN. The isolated interstitial is unstable and transforms into a split interstitial configuration (N-N)(N). It is generated by particle irradiation with an introduction rate of a primary defect, pins the Fermi level at E(C)-1.0 eV for high fluences, and anneals out at 400 °C. The associated defect, the nitrogen vacancy, is observed by PAS only in the initial stage of irradiation.

We present detailed density-functional theory studies of the structures of clean Au͑110͒ surfaces and the energetics of various atomic oxygen coverages. Various ͑1 ϫ r͒ "missing row" reconstructions of the clean Au͑110͒ surface have been investigated. The surface energies of the clean reconstructed surfaces are found to be very close with an energy minimum for Au͑110͒-͑1 ϫ 3͒. The ͑111͒ microfacets formed at Au͑110͒-͑1 ϫ r͒ reconstructed surfaces with r Ͼ 1 allow further adsorption sites. Adsorption in pseudothreefoldcoordinated sites, along close-packed rows in the topmost gold layer, is favored over other adsorption sites. On Au͑110͒-͑1 ϫ r͒, adsorption energies are generally higher for these sites.

The presently unknown band offset in nonpolar cubic GaN/AlN superlattices is investigated by inter-sub-band and interband spectroscopies as well as ab initio calculations. On one hand, the conduction-band offset (CBO) has been determined from the comparison of the measured transition energies with model calculations within the effective mass approximation. On the other hand, the valence-band offset (VBO) and the CBO are accurately simulated by calculating many-body corrections within the GW approximation on top of hybrid-functional density functional theory calculations. Thus, a CBO of (1.4 ± 0.1) eV and a VBO of (0.5 ± 0.1) eV is obtained as a result of both approaches.

We have investigated the structural and electronic properties as well as the linear optical response of amorphous TiO 2 within density functional theory and a numerically efficient density functional based tight-binding approach as well as many-body perturbation theory. The disordered TiO 2 phase is modeled by molecular dynamics. The equivalence to experimentally characterized amorphous phases is demonstrated by atomic structure factors and radial pair-distribution functions. By density functional theory calculations, using both the semilocal Perdew-Burke-Ernzerhof functional and the nonlocal Heyd-Scuseria-Ernzerhof screened hybrid functional, the electronic energy gap is found to be larger than in the crystalline TiO 2 phases rutile and brookite but close to the anatase band gap. The quasiparticle energy gap of amorphous TiO 2 is determined to be 3.7 eV, while the optical gap is estimated to 3.5 eV. The disorder-induced formation of localized electronic states has been analyzed by the information entropy of the charge density distributions. The frequency-dependent optical constants, calculated from the complex dielectric function, have been determined in independent particle approximation. Besides similar absorption characteristics between the most common crystalline phases and amorphous TiO 2 , we find distinct differences in the optical spectra in the energy region between 5 eV and 8 eV. These differences can be assigned to the loss of symmetry in the local atomic structure of the disordered material. While the composition of the crystalline phases rutile, anatase, and brookite is well described by periodic arrangements of distorted TiO 6 octahedra building blocks, the amorphous phase is characterized by partial loss of this octahedral coordination and the disorder-induced formation of under-and over-coordinated Ti ions. This leads to the absence of the characteristic crystal-field splitting of unoccupied Ti 3d states into e g and t 2g like subbands. The optical characteristics of the amorphous phase are interpreted as a superposition of optical transitions that reflect the various local symmetries of the manifold of synthesizable crystalline TiO 2 phases. The linear optical properties, calculated within the independent-particle approximation, are found to be in good agreement with the available experimental data.

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We perform a comprehensive theoretical study of the structural and electronic properties of potassium niobate (KNbO3) in the cubic, tetragonal, orthorhombic, monoclinic, and rhombohedral phase, based on density-functional theory. The influence of different parametrizations of the exchange-correlation functional on the investigated properties is analyzed in detail, and the results are compared to available experimental data. We argue that the PBEsol and AM05 generalized gradient approximations as well as the RTPSS meta-generalized gradient approximation yield consistently accurate structural data for both the external and internal degrees of freedom and are overall superior to the local-density approximation or other conventional generalized gradient approximations for the structural characterization ofKNbO3. Band-structure calculations using a HSE-type hybrid functional further indicate significant near degeneracies of band-edge states in all phases which are expected to be relevant for the optical response of the material.