铁中空位的正电子湮没特性

从第一性原理和二分量密度泛函理论出发．应用离散变办法和嵌入集团模型完全自洽地计算了不同相状态的铁单空位的正电子湮没特性．同时讨论了晶格膨胀和晶格弛豫对正电子湮没特性的影响。     

Diffusion in mercury cadmium telluride—an update

The various diffusion coefficients (self-diffusivity, chemical self-diffusivity compositional diffusivity, impurity diffusivity) are defined. The conditions required for the observation of a Kirkendall effect are described. There is good agreement in the results for Hg and for Cd self-diffusion. The evidence suggests that the self-diffusivity is largely independent of P_Hg above ~300°C but shows an increase as Hg saturation is approached (for both Hg and Cd). Good agreement is also found in the Arrhenius parameters describing the movement of the p to n conversion boundary. Modeling of this diffusion boundary does, however, raise problems which are discussed. The situation in compositional interdiffusion is more complex: above ~400°C, reasonable agreement exists between various workers for large x_Hg but not for small values; below 400°C, substantial disagreement is evident. In impurity diffusion exhibits both erfc and non-erfc profiles for reasons which are unclear. Good agreement is found between In diffusivity measurements at high In concentrations: at low concentrations, the diffusivity decreases dramatically. As diffusion yields erfc profiles with good agreement again being found: a notable feature is the P_Hg dependence of the As diffusivity. Where appropriate, diffusion models are discussed.     

Non-equilibrium Green’s function (NEGF) simulation of metallic carbon nanotubes including vacancy defects

The electronic behavior of metallic carbon nanotubes under the influence of externally applied electric fields is investigated using the Non-Equilibrium Green’s function method self consistently coupled with three-dimensional (3D) electrostatics. A nearest neighbor tight binding model based on a single pz orbital for constructing the device Hamiltonian is used. The 3D Poisson equation is solved using the Finite Element Method. Carbon nanotubes exhibit a very weak metallic behavior, and external electric fields can alter the electrostatic potential of the tubes significantly. A single vacancy defect in the channel of a metallic carbon nanotube can decrease its conductance by a factor of two. More than one vacancy can further decrease the conductance.     

Study of defects and thermal stability of ultrathin Cu films on Ta(110) and Ta(100) by thermal helium desorption spectrometry

Thermal helium desorption spectrometry has been used to study the interaction of helium with defects in Cu films (5-300 Å) deposited on Ta(110) and Ta(100) single crystals by ultrahigh vacuum electron beam evaporation. The thermal stability of the Cu films was also investigated. Cu films on Ta(110) and Ta(100) at room temperature are metastable and on heating, the films transform into islands. The temperature at which this takes place is strongly dependent on the Cu film thickness and for a given thickness (> 40 Å) occurs at a lower temperature on Ta(100) than on Ta(110). The activation energy for island formation is 1.6 ± 0.4 eV for 50 Å Cu/Ta(110) and 0.8 ± 0.1 eV for 100 Å Cu/Ta(100) obtained by Kissinger analysis. The geometry of the Cu islands resulting from annealing 50 Å Cu films at 1000 K for 10 s depends strongly on the Ta substrate orientation. There is evidence for the stressed states of both the Cu films and the Ta substrates. Helium release from monovacancies and vacancy clusters in Cu films (> 75 Å) on Ta(110) and Ta(100) was detected at ~ 750 K and ~ 800-1000 K respectively. The sublimation of the Cu films from the Ta substrates could be observed by the release of retained helium at ~ 1300 K.     

The effects of hydrogen and vacancy on the tensile deformation behavior of Σ3 symmetric tilt grain boundaries in pure fe

The interplay of hydrogen, vacancy and grain boundary plays an important role in hydrogen induced premature fracture in the metallic materials. In this work, two models with the representative high angle grain boundaries with the low and high grain boundary energy have been built according to the coincidence site lattice theory. The effects of hydrogen and hydrogen-vacancy combination on the deformation behaviors of the two models were investigated by means of molecular dynamics simulation with the straining direction vertical to the grain boundary. It is found that in both cases hydrogen tends to segregate and maintain a high local stress state around the grain boundary, and promote the premature fracture compared with the hydrogen-free model. Vacancy enhanced the effects of hydrogen in the model with grain boundary of the lower energy. However, vacancy promoted the dislocation evolution behavior in the model with grain boundary of the higher energy. The simulation results were further explained by considering the effects of hydrogen on the generalized stacking fault energy and the work of separation of the grain boundary.     

Tuning the electronic structure of the transparent conducting oxide Cu2O

The electronic structure of Cu₂O is important for its application as a p-type transparent conducting oxide (TCO). To be useful as a TCO, a material needs to show enhanced transparency in the visible range (band gap > 3 eV) as well as good conduction properties. While Cu₂O has too small a band gap, alloys of Cu₂O and Al₂O₃ or Cu₂O and alkaline earth oxides are known to display enhanced transparency, with little degradation of electrical properties. It is of interest to consider how to dope Cu₂O p-type, e.g. Cu vacancies (oxidation) or cationic dopants. We present a study of the electronic structure and effective hole masses of stoichiometric and oxidised Cu₂O and study metal cation doping, using density functional theory (DFT), to analyse p-type doping scenarios. We show that formation of a Cu vacancy is relatively facile, introducing delocalised hole states, with a light hole present. Substitutional cation doping with Al and Au/Ag is found to decrease the band gap but maintains a light hole effective mass necessary for p-type conduction.     

Critical Assessment 5: Thermodynamic data for vacancies

Abstract

The thermodynamic properties of crystalline phases are generally represented well in terms of the compound energy formalism and the model has been implemented in all the major software packages used to calculated phase equilibria. The formalism is particularly successful in modelling defects in crystals, both the formation of holes or vacancies on lattice sites, and the dissolution of atoms on interstitial sublattices. In the former case the formalism may require the definition of data for pure vacancy itself. In this assessment we investigate the possible values for the Gibbs energy of vacancy to be used in the compound energy formalism and show that a value of zero or negative will always lead to unwanted catastrophic stability of the phase.     

Growth of extrusions in localized cyclic plastic straining

Cyclic strain localization into persistent slip bands produces on the surface of cyclically deformed materials persistent slip markings in the form of extrusions and intrusions. The localized cyclic plastic straining in persistent slip bands leads to the production and annihilation of dislocations as well as point defects. Production, annihilation and migration of point defects, preferably vacancies, is quantitatively characterized. The effect of vacancy production and migration on the extrusion growth is treated under simplified conditions. At low temperatures when vacancies are immobile, a static extrusion is produced. At elevated temperature the diffusion equation is solved, a general expression for stabilized rate of extrusion growth is obtained and temperature dependence of the growth rate can be predicted. The predictions of the model are compared with experimental observations.