Ye Bang-Jiao

Simulation of positron backscattering and implantation profiles using Geant4 code

For the proper interpretation of the experimental data produced in slow positron beam technique, 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, and we think that the accuracy may be related to the structures of the host materials in the 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 good agreement with the previous experimental results.

Dependence of intrinsic defects in ZnO films on oxygen fraction studied by positron annihilation

Defects in ZnO films grown by radio-frequency reactive magnetron sputtering under variable ratios between oxygen and argon gas have been investigated by using the monoenergetic positron beam technique. The dominate intrinsic defects in these ZnO samples are O vacancies (V-O) and Zn interstitials (Zn-i) when the oxygen fraction in the O-2/Ar feed gas does not exceed 70% in the processing chamber. On the other hand, zinc vacancies are preponderant in the ZnO Elms fabricated in richer oxygen environment. The concentration of zinc vacancies increases with the increasing (2) fraction. For the oxygen fraction 85%, the number of zinc vacancies that could trap positrons will be smaller. It is speculated that some unknown defects could shield zinc vacancies. The concentration of zinc vacancies in the ZnO films varies with the oxygen fraction in the growth chamber, which is in agreement with the results of photoluminescence spectra.

A new positron annihilation lifetime spectrometer based on DRS4 waveform digitizing board

A new simple digital positron lifetime spectrometer has been developed. It includes a DRS4 waveform digitizing board and two scintillation detectors based on the XP2020Q photomultiplier tubes and LaBr3 scintillators. The DRS4 waveform digitizing can handle small pulses, down to few tens of millivolts, and its time scale linearity and stability are very good. The new system has reached a 206 ps time resolution, which is better than the conventional analog apparatus using the same detectors. These improvements make this spectrometer more simple and convenient in comparison with other spectrometers, and it can be applied to the other scintillation timing measurements with picosecond accuracy.

Theoretical study on the positron annihilation in Rocksalt structured magnesium oxide

Based on the atomic superposition approximation (ATSUP) and first-principles pseudopotential plane-wave methods, the bulk and Mg mono-vacancy positron lifetime of magnesium oxide were calculated using Arponen—Pajamme and Boronski—Nieminen positron-annihilation-rate interpolation formula respectively. The calculated values are in good agreement with experimental values and the first-principles method gives more convincing results. The positron annihilation density spectra analysis reveals that positrons mainly annihilate with valence electrons of oxygen atoms when the magnesium-vacancy appears within magnesium oxide.

Realization of the Three-Qubit Toffoli Gate in Molecules *

We present the experimental realization of this gate with a solution of chlorostyrene molecules. Our method does not depend heavily on the two-qubit controlled operation, which used to serve as the basic quantum operation in quantum computing. At present, we use transition operator that can be applied to all qubits in one operation. It appears that no experimental realization has yet been reported up to now regarding the implementation of quantum Toffoli gate using transition pulse on three-qubit nuclear magnetic resonance quantum computers. In addition, our method is experimentally convenient to be extended to more qubits.

Dielectric properties and structural defects in BaTi1?xSnxO3 ceramics

As a function of Sn doping concentration x, dielectric properties and X-ray diffraction measurements were carried out on BaTi1-xSnxO3 (BTSx) ceramics fabricated by the solid-state reaction route. Positron annihilation lifetime spectroscopy and coincident Doppler-broadening spectroscopy were also measured for the evaluation of defects in the BTSx ceramics. Dielectric properties measurement reveals that the permittivity of BTSx ceramic gradually increases with increasing Sn dopant content for x≤3%, and then decreases. This change of permittivity is found to agree well with the relative defect concentration estimated using two positron annihilation techniques. The S-W plot indicates that the defect species do not change with Sn doping. The variation correlations between defects and dielectric properties further proves that BTSx ceramics with the higher relative defect concentration present a lower permittivity.

The application of artificial neural networks to the inversion of the positron lifetime spectrum

A new method of processing positron annihilation lifetime spectra is proposed. It is based on an artificial neural network (ANN)-back propagation network (BPN). By using data from simulated positron lifetime spectra which are generated by a simulation program and tested by other analysis programs, the BPN can be trained to extract lifetime and intensity from a positron annihilation lifetime spectrum as an input. In principle, the method has the potential to unfold an unknown number of lifetimes and their intensities from a measured spectrum. So far, only a proof-of-principle type preliminary investigation was made by unfolding three or four discrete lifetimes. The present study aims to design the network. Besides, the performance of this method requires both the accurate design of the BPN structure and a long training time. In addition, the performance of the method in practical applications is dependent on the quality of the simulation model. However, the chances of satisfying the above criteria appear to be high. When appropriately developed, a trained network could be a very efficient alternative to the existing methods, with a very short identification time. We have used the artificial neural network codes to analyze data such as the positron lifetime spectra for single crystal materials and monocrystalline silicon. Some meaningful results are obtained.

Self-consistent field method and non-self-consistent field method for calculating the positron lifetime

this paper, we flrst introduce the two basic methods and then, we take Si as an example and give our calculation results, these results coincide with our latest experimental results, flnally, we discuss the advantages and disadvantages of the two methods.

Evolution of native point defects in ZnO bulk probed by positron annihilation spectroscopy

This paper studies the evolution of native point defects with temperature in ZnO single crystals by positron lifetime and coincidence Doppler broadening (CDB) spectroscopy, combined with the calculated results of positron lifetime and electron momentum distribution. The calculated and experimental results of the positron lifetime in ZnO bulk ensure the presence of zinc monovacancy, and zinc monovacancy concentration begins to decrease above 600 °C annealing treatment. CDB is an effective method to distinguish the elemental species, here we combine this technique with calculated electron momentum distribution to determine the oxygen vacancies, which do not trap positrons due to their positive charge. The CDB spectra show that oxygen vacancies do not appear until 600 °C annealing treatment, and increase with the increase of annealing temperature. This study supports the idea that green luminescence has a close relation with oxygen vacancies.

Assessment of Several Calculation Methods for Positron Lifetime

We test seven different local density approximation (LDA) or generalized gradient approximation (GGA) forms of the enhancement factor and correlation potential for positron-lifetime calculations by using a useful database of experimental values based on the all-electrons approach: full-potential linearized augmented plane wave (FLAPW). To make a numerical assessment of these calculation methods, we use the mean-deviation and the reduced chi-squared as model selection criterions. We find that the two recent LDA forms of the enhancement factor make distinct improvements upon the calculations for positron-lifetime compared with the older LDA form proposed by Arponen and Pajanne. However, all the LDA forms are still disfavored by the experimental data compared with the GGA forms. In addition, the two recent GGA forms do not yield any improvement when compared to experimental data over the original from given by Barbiellini et al., which is found to give the best agreement.