Positron Theoretical Calculation And Its Application To The Analysis Of The Material Microstructures

As the antiparticle of electron, positron is same to electron except with the opposite charge, it can also be used as a probe to characterize the electronic structure of materials. In past decades, this method has been widely applied in the field of condensed physics and medical science and so on due to its unique advantages, and becomes a new independent discipline which is named Positron Annihilation Spectroscopy (PAS).Analysis of the direct experimental data plays a very important role in PAS. For example, the positron lifetime spectroscopy gives the data of the positron annihilation lifetime which is the time interval from the positron production to its annihilation with the electron, and the lifetime is related to the type and size of defects. We can obtain the two or three types of positron lifetime and their corresponding concentration using the common spectrum analysis software, but we can not get the intuitive information that we concern most, namely, which kind of defect exists in material. At this point, compared with the calculated positron lifetime for all possible kinds of defects, we can conclude the possible types of defect in material, and this is crucial for the analysis of the property of material. In the same way, Doppler broadening spectra and angular correlation technique can be used to study the electronic structural information around the defect, and we can realize that the positron mainly annihilates with the electron with the specified element and shell compared with theoretical calculation. In addition, mono-energetic slow positron beam with adjustable energies can used to investigate the defect property of surface and interface of materials, and also can used to study the defect distribution versus different depth. It measures the Doppler broadening spectra versus the positron energy. Though we can easily reach the trend of the change of the defect concentration versus the positron energy, the "layer" is not clear, the implantation depths are not fixed for mono-energetic positrons, while they obey the specified distribution and positrons appear with a maximum probability in a certain depth which is the so called mean positron implantation depth. For further analysis using VEPFIT software, we need the information of the implantation profiles which directly affect the accuracy of fitting results.The aim of this work is to analyze the experimental data measured by PAS in a simple and feasible way, then apply these methods into explaining the relationship between the microstructure and the physical property of materials. The major research achievements of this doctoral dissertation are as follows:(1) The positron implantation properties are studied carefully using the latest Geant4 code. The simulated backscattering coefficients, the implantation profiles and the median implantation depths for mono-energetic positrons with energy range from 1 keV to 50 keV normally incident on different crystals are reported. Compared with the previous experimental results, our simulation backscattering coefficients are in reasonable agreement with them, and we think that the accuracy may be related to the structures of the host materials in Geant4 code. Based on the reasonable simulated backscattering coefficients, the adjustable parameters of the implantation profiles which are dependent on materials and implantation energies are obtained. The most important point is that we calculate the positron backscattering coefficients and median implantation depths in amorphous polymers for the first time and our simulations are in fairly well agreement with the previous experimental results. So we think it is simple and feasible to simulate the implantation profiles using the latest Geant4 code, and this has significant consequences for the convenient calculation of positron implantation especially in elemental and multilayer systems.(2) The superposed neutral atom model, the pseudo-potential model, and the full-potential model are used to deal with the positron local potential. While the positron wave function is solved self-consistently by the finite difference method, the positron-electron correlation potential and its enhancement factor are handled within in the frame work of the local density approximation and the generalized gradient approximation. We have respectively calculated the positron bulk lifetime of three kinds of single crystal solid:the alpha iron of a body-centered cubic structure, the aluminum of a face-centered cubic structure, and the silicon of a double face-centered cubic structure. Calculation results agree well with the published experimental data. At the same time, the impact on positron bulk lifetime due to electron density grid point accuracy, positron-electron correlation potential and enhancement factor is analyzed carefully. Finally, we discuss the advantages and disadvantages of the three methods for calculating the positron bulk lifetime. In summary, an effective and reasonable calculation for the positron bulk lifetime should take into account the electron density, positron-electron correlation potential, and enhancement factor, etc, especially the enhancement factor. Moreover, the calculation of the bulk and defect positron lifetime is performed using MATLAB code, it isn’t related to the other first-principles calculation software at all and the interface is very kind, this is very convenient for theoretical analysis in our lab.(3) The first-principles calculations of positron lifetimes of mono-vacancies in crystals were investigated. We use the two-component density functional theory to respectively compute positron lifetimes of neutral charge state of VAl defect in aluminium, Vsi defect in silicon, Vc, VSi and Vc+Csi defects in 3C silicon carbide, VGa and VAs defects in gallium arsenide, taking into account atomic relaxation due to vacancy and electronic structural relaxation due to the presence of the positron. Three different calculation schemes are used. We find that the electron density inside the vacancy more or less increases due to the presence of the positron if the ionic positions are kept fixed, and the positron becomes more localized after the electronic structural relaxation for the case of VAl defect in aluminium and Vsi defect in 3C silicon carbide, but it is opposite for the case of VGa defect in gallium arsenide and Vc defect in 3C silicon carbide. The results with no consideration of the relaxation are even much closer to the experimental ones, therefore the atomic relaxation due to the positron play an important role in calculating the positron lifetime of mono-vacancies in crystals.(4) Combined with the theoretical calculation of the positron lifetime and the common used XRD and SEM, the positron lifetime spectroscopy and Coincidence Doppler broadening spectrum are used to investigate the structure and defect of half-doped ceramics with perovskite structure Sm0.5Ca0.5MnO3 which are prepared by conventional solid-state reaction route in different sintering temperatures. We find that there exits much manganese mono-vacancy in the bulk, and manganese ions gather towards grain boundary, so A-site vacancy and Oxygen related defects are induced. Holes with large size gradually decompose into holes with small size as the sintering temperature increases. Moreover, we analyze the effect of the microstructural defect on the magnetic and magnetoresistance of Smo.sCao.sMnO3. In the low temperature region, the change trend of saturation magnetization is consistent with the PAS result. The transition temperature of the charge-order state is highest in the sample S3, while lowest in sample S4, this indicates that the presence of VMn goes against the appearance of the charge-order state. The transition temperature of ferromagnetic and anti-ferromagnetic increases as the sintering temperature increases, this shows that the big grains and small holes are favor to the appearance of ferromagnetic. Over the measuring temperature range, the appearance of Metal-Insulator transition can not be observed, this may be caused by the presence of large number of VMn in the bulk and Oxygen related defects in the boundary. On the one hand, the double exchange interaction can always get promoted and the transition probability of eg electron is increased by the magnetic field, on the other hand, the double exchange interaction is suppressed due to the presence of VMn and Oxygen defects, therefore the metal conductive behavior doesn’t appear in high magnetic field and low temperature. Moreover, combined with the theoretical calculation of positron lifetime, we investigate the feasibility of studying the microstructure of abnormal high temperature superconductor KxFe2-ySe2 using PAS.In summary, the conjoint analysis of positron theoretical calculation and experimental data enhances the reliability and intuitive of PAS applied in functional materials.