Electronic structure of non-magnetic impurities in dilute alloys of Al

The electronic structure of substitutional non-magnetic impurities Cu, Ag, Cd, Mg, Zn, Ga, In, Ge, Si and Sn in Al is studied using density functional theory. A simple physical model is proposed to calculate the effective charges on impurities in trivalent metal Al. A linear relation is found between the effective charges on impurities and impurity vacancy capture radii. The spherical solid model (SSM) is used to account for discrete nature of the host. The impurity-induced change in charge density, scattering phase shifts, host-impurity potential, residual resistivity and impurity self-energy are calculated. Higher order scattering phase shifts are found significant and the host-impurity potential is found proportional to effective charge on impurity in its vicinity. The self-consistently calculated potential is used to calculate the electric field gradients (EFGs) at the first and second nearest neighbours (1NNs, 2NNs) of impurity. The calculated values are in agreement with the experimental results.     

A scenario for the electronic state in the manganese perovskites: the orbital correlated metal

We propose a novel scenario for the electronic state in the manganese perovskites. We argue that, at low temperatures and within the ferromagnetic state, the physics of these colossal magnetoresistance compounds may be characterized by a correlated metallic state near a metal insulator transition where the orbital degrees of freedom play the main role. This follows from the observation that a two-band degenerate Hubbard model under a strong magnetic field can be mapped onto a para-orbital single band model. We solve the model numerically using the quantum Monte-Carlo technique within a dynamical mean field theory which is exact in the limit of large lattice connectivity. We argue that the proposed scenario may allow for the qualitative interpretation of a variety of experiments which were also observed in other (early) transition metal oxides. Received: 3 October 1997 / Revised: 9 December 1997 / Accepted: 12 January 1998     

Non-destructive testing of tubes using a time reverse numerical simulation (TRNS) method

A method for the detection of defects in cylindrical structures and the determination of their positions and orientations is presented in this paper. The scattered field, which is generated by the interaction of excited guided waves with a defect, is evaluated with an approach named time reverse numerical simulation method (TRNS). Since the excited waves and the scattered field propagate along the sample, the time-consuming scanning of the whole tube can be eliminated. The scattered displacement field is measured in three dimensions over time with a laser vibrometer at different locations distributed equally around the circumference at a fixed axial coordinate far away from the defect. Instead of analyzing the complicated time signals directly, they are played back in time. If the recorded displacement histories of the scattered field are reversed in time and played back in an identical structure, the waves travel back the same path and interfere to a maximum at their origin. The result is an amplitude increase at the position of the defect where the scattered field was generated. Instead of playing back the recorded time signals in an experiment, this step is replaced by a numerical simulation. Only this enables the visualization and detection of the amplitude increase. As long as the simulation is of high accuracy, the position of the maximum interference corresponds exactly to the location of the defect in the experiment, although no defect is implemented in the simulation.     

N2分压对ZrN/WN纳米多层膜缺陷性质与力学性能的影响

利用射频磁控溅射系统在不同N₂分压的条件下，制备了一系列ZrN/WN纳米多层膜.借助慢正电子湮没技术分析了样品的缺陷性质，采用纳米压痕仪研究了多层膜的力学性能.结果发现：N₂分压为0.4Pa的多层膜具有最小的空位型缺陷浓度，其中心层和膜基结合层的平均S参数分别为0.4402和0.4641，而较低或较高的N₂分压都可能导致空位型缺陷浓度的增加.随着空位型缺陷浓度的减小，多层膜的硬度和临界载荷增大.对于空位型缺陷浓度最小的多层膜，其硬度和临界载荷达到最大值，分别为34.8GPa和100mN，说明较低的缺陷浓度有利于提高多层膜的力学性能. 关键词： ZrN/WN纳米多层膜 缺陷性质 力学性能 慢正电子湮没     

On the flow of thermal defects from the bulk to surfaces

This paper treats flow of defects between bulk and surface sites, as a crystal passes towards equilibrium, for some practical cases. These include the realistic but quite elaborate example in which vacancy flow from the bulk is coupled to surface step edges, acting as sinks, by reaction with adatoms that are believed to dominate transport on metal surfaces. It is shown how surface processes modify the defect flow from the bulk only at short times. Lacking accurate parameters (such as concentrations) for surface defects, a crude modeling of the theoretical results is offered in order to explore likely generic behavior. The model employs a recently described approximate universality of behavior, scaled to the melting temperature, relevant mainly to fcc (1 1 1) surfaces. Under a range of conditions it is the reaction of advacancies with adatoms that provides the important channel for bulk vacancy flow. Adatom flow onto the terraces from surface step edge sinks is the bottleneck to flow above a crossover temperature (depending on step spacing) and equilibrium recombination is the bottleneck below the crossover.     

Field electron emission images of multi-walled carbon nanotubes

Field electron emission microscope images from multi-walled carbon nanotubes can typically be characterized by the presence of five pentagons surrounding a sixth central pentagon. The observations of bright line centered interference patterns between adjacent pentagons in the field electron emission microscope images of multi-walled carbon nanotubes have been reported in the literature. We have observed a shift from bright to dark line centered interference patterns and associated this with the presence of surface adsorption. In order to identify the origin of the contaminant, multi-walled carbon nanotubes were dosed with H₂, H₂O, CO and O₂ and then imaged in the field electron emission microscope. Only the samples exposed to O₂ showed a shift from a bright line centered pattern between adjacent pentagons of a clean surface to a dark line centered pattern when one pentagon was contaminated or a bright line centered pattern when both adjacent pentagons become contaminated. The results of the experimental studies and the modeling of the changes in the field emission pattern as phase shifts in the wave function of the tunneling electrons due to modifications in the surface work function are presented.     

STM image visualization of Si(1 1 1) 7 × 7 surface (graphic simulation and implementation)

The possibilities of graphic STM image simulation of a clean Si(1 1 1) 7 × 7 surface at atomic level are indicated. The presented procedure takes into account various types of deformation on the surface near the Fermi level in order to classify them and explain their origin. It also gives a clear hint to insert relevant physical phenomena in a suggested analysis. This goal is achieved exploiting the results of DAS (dimmer adatom stacing fault) model by means of standard mathematical programmes. A clean Si(1 1 1) 7 × 7 surface is considered as the representative example, but similar evaluation is possible for another non-metal and metal surfaces.     

TEM investigation of the formation mechanism of deformation twins in Fe–Mn–Si–Al TWIP steels

The microstructure of a Fe–Mn–Si–Al twinning-induced plasticity (TWIP) steel exhibiting remarkable work hardening rate under uniaxial tensile deformation was investigated using transmission electron microscopy to uncover the mechanism(s) controlling the nucleation and growth of the mechanically induced twins. The results show that the stair-rod cross-slip deviation mechanism is necessary for the formation of the twins, while large extrinsic stacking faults homogenously distributed within the grains could act as preferential sources for the activation of the deviation process. The influence of such features on the thickness and strength of the twins and the resulting mechanical behaviour is discussed and compared to similar works recently performed on Fe–Mn–C TWIP steels.     

Coded nanoscale self-assembly

We demonstrate coded self-assembly in nanostructures using the code seeded at the component level through computer simulations. Defects or cavities occur in all natural assembly processes including crystallization and our simulations capture this essential aspect under surface minimization constraints for self-assembly. Our bottom-up approach to nanostructures would provide a new dimension towards nanofabrication and better understanding of defects and crystallization process.