Narrow Results

We extend our analysis of the gauging of rigid symmetries in bosonic two-dimensional sigma models with Wess–Zumino terms in the action to the case of world-sheets with defects. A structure that permits a non-anomalous coupling of such sigma models to world-sheet gauge fields of arbitrary topology is analyzed, together with obstructions to its existence, and the classification of its inequivalent choices.

The appropriate procedure for analyzing experimental data of defects in metals is discussed. The following two diverse procedures have been proposed earlier: either in terms of single vacancy formation with temperature dependent enthalpy and entropy, or by assuming coexistence of vacancies and divacancies with temperature-independent parameters. Using aspects of thermodynamics of the defect formation processes in solids, we show that the former procedure leads to self-consistent parameters.

We investigate the effect of vacancy defect reconstruction on electron transport properties in a (4, 0) zigzag and (5, 5) armchair silicon-carbide nanotubes (SiCNTs) by applying self consistent non-equilibrium Green's function formalism in combination with the density-functional theory to a two probe molecular junction constructed from SiCNTs. The geometry optimization results show that single vacancies and di-vacancies in SiCNTs have different reconstructions. A single vacancy when optimized, reconstructs into a 5-1DB configuration in both zigzag and armchair SiCNTs, and a di-vacancy reconstructs into a 5-8-5 configuration in zigzag and into a 5-2DB configuration in armchair SiCNTs. Analysis of frontier molecular orbitals (FMO) and transmission spectrum show that the vacancy defect increases the band gap of (4, 0) metallic SiCNT and decreases the band gap of (5, 5) semiconducting SiCNT. Bias voltage dependent current characteristic show reduction in overall current in metallic SiCNT and an increase in overall current in semiconducting SiCNT.

The defect centers in TlGaSSe single crystals have been investigated by performing thermoluminescence (TL) measurements with various heating rates between 0.5 K/s and 1.0 K/s in the temperature range of 10–180 K. The TL spectra, with peak maximum temperatures at 39 K and 131 K, revealed the existences of two defect levels. Curve fitting, initial rise and peak shape methods were used to determine the activation energies of two defect centers. The experimental results also showed that the trapping process was dominated by second-order kinetics for the trap related with low temperature peak while the general order (mixed order) kinetics was dominant for the trap donated to high temperature peak. Furthermore, heating rate dependences and traps distributions were studied for two defect centers separately. Thermal quenching effect dominates the behavior of these defects as the heating rate is increased. Also, quasi-continuous distributions were established with the increase of the activation energies from 16 meV to 27 meV and from 97 meV to 146 meV for the traps associated with the peaks observed at low and high temperatures, respectively.

In this paper, α-Mn₂O₃ thin films were fabricated by plasma-assisted molecular beam epitaxy on SrTiO₃ and Nb:SrTiO₃, respectively. The grown samples showed room temperature ferromagnetism (RFM) properties. All the experimental results manifested that the RFM properties in undoped thin films were induced by oxygen vacancies formed during the growth process. Even more, the ferromagnetism of thin films grown on Nb:SrTiO₃ were enhanced, and these results confirmed the fact that oxygen vacancies induced ferromagnetism. That is to say, more oxygen vacancies result the more unpaired electrons induced prominent abnormal spin causing ferromagnetism.

Here, we investigate the following key prediction of a thermodynamical model that interrelates the defect parameters with the bulk elastic and expansivity data: for various defect processes in a given matrix material, a proportionality exists between defect entropies and enthalpies. The investigation is focused on BaF₂ for which ab initio calculations within density functional theory and the generalized-gradient approximation have been recently made as far as the formation and migration of intrinsic defects are concerned, as well as for the elastic constants. Four defect processes have been studied in BaF₂: anion Frenkel formation, fluorine vacancy migration, fluorine interstitial motion and electrical relaxation associated with a single tetravalent uranium. For these processes, the entropies and enthalpies vary by almost two orders of magnitude and reveal a proportionality between them. We find that this proportionality is solely governed by the bulk elasticity and expansivity data, which conforms to the aforementioned thermodynamical model.

We, at the ab initio level, simulated the rearrangement magnitudes of the adjacent neighboring ions, surrounding the (100) surface F-center in ABO₃ perovskite matrixes. They are noticeably greater than the respective ionic shift magnitudes of the adjacent neighboring ions surrounding the bulk F-center. In ABO₃ perovskites, the electron charge is noticeably better bounded on the inside of the bulk oxygen vacancy, as interior the respective (100) surface vacancy. The oxygen vacancy formation energy, located on the (100) surface of ABO₃ perovskites, as a rule, is smaller as in the bulk. This slight energy distinction encourages the oxygen vacancy segregation from the ABO₃ perovskite bulk to their (100) surfaces. The ABO₃ complex oxide (100) surface F-center generated defect levels are positioned nearer to the (100) surface CB bottom than the bulk F-center generated respective defect levels. In contrary, the BaF₂, SrF₂ and CaF₂, both, surface and bulk F-center charges are well localized inside the fluorine vacancy. The ionic rearrangement magnitudes of the adjacent neighboring ions, surrounding the surface and bulk F-centers in BaF₂, SrF₂ and CaF₂ matrixes, are much smaller regarding the respective situation in ABO₃ perovskites.

Nitrogen (N) is an important impurity in silicon (Si), which associates with impurities as well as with other defects to form defect complexes. The knowledge of the properties and behavior of defect structures containing carbon (C), N and oxygen (O) is important for the Si–based electronic technology. Here, we employ density functional theory (DFT) calculations to investigate the association of nitrogen with carbon and oxygen defects to form the C_iN and C_iNO_i defects. We provide evidence of the formation of these defects and additional details of their structure, their density of states (DOS) and Bader charges. Therefore, C_iN and C_iNO_i defects are now well characterized.