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Ultrafast photodissociation dynamics of butanone in 3s Rydberg state
Luo Jin-Long, Ling Feng-Zi, Li Shuai, Wang Yan-Mei, Zhang Bing
Acta Physica Sinica, 2017, 66 (2): 023301
Dependences of valence electronic structure on magnetic moment and electrical resistivity of metals
Qi Wei-Hua, Ma Li, Li Zhuang-Zhi, Tang Gui-De, Wu Guang-Heng
Acta Physica Sinica, 2017, 66 (2): 027101
Progress of electrical control magnetization reversal and domain wall motion
Zhang Nan, Zhang Bao, Yang Mei-Yin, Cai Kai-Ming, Sheng Yu, Li Yu-Cai, Deng Yong-Cheng, Wang Kai-You
Acta Physica Sinica, 2017, 66 (2): 027501
Acta Physica Sinica  
  Acta Physica Sinica--2017, 66 (2)   Published: 20 January 2017
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INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

Study of three-dimensional nonlinear backward-wave interaction model and numerical simulation for helical traveling wave tube

Hu Yu-Lu, Hu Quan, Zhu Xiao-Fang, Li Bin, Qiu Hai-Jian, Gao Luan-Feng
Acta Physica Sinica. 2017, 66 (2): 028401 doi: 10.7498/aps.66.028401
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The wide band high power traveling wave tubes (TWTs) employed in radar, communication systems, etc. are always facing the backward wave oscillation (BWO) problem. However, it takes much time and computer resource to simulate BWO by the large electromagnetic software. Thus, several parametric models are developed to solve the problem faster. Most of those models do not discuss the saturated oscillation power. In this paper, a three-dimensional (3D) nonlinear backward-wave interaction model is presented, by which the BWO phenomenon can be accurately studied in TWTs and the oscillation power is also analyzed. This model is established with the equation of 3D excitation fields combined with 3D motion equations and 3D space charge force. The oscillation frequencies and the start-oscillation lengths are calculated by one-dimensional (1D) and 3D models, respectively, and they are carefully compared in the cases of with and without the space charge force, indicating that the space charge force in 1D model is much weaker than in 3D model. The reason for that is the model of current density for space charge model in 1D model is supposed to be proportional to particle radius, but the one in 3D model is almost uniform, which is indicated by 3D beam trace distribution analysis. The BWO saturated powers and the oscillation frequencies are studied by this nonlinear 3D backward-wave interaction model. The simulation results show that the BWO saturated power increases as the beam-wave interaction length extends before many trajectories intercept the helix. While the oscillation frequencies decrease, the large saturated power supplies more energy to the beam at the very beginning in beam-wave interaction starting region. Then the BWO suppression induced by the magnetic field effect of the beam ripple is also under consideration. As the magnetic force increases, not only some cross area of interaction beam is suppressed, but also the interaction impedance of -1 space harmonic decreases. So increasing magnetic field strength can obviously reduce BWO, while the effect on forward wave interaction should be balanced. Finally, a Ka-band tube is used to validate the 1D and 3D nonlinear backward-wave interaction models. The BWO frequencies at different voltages are compared among the experimental results and the calculations by 1D and 3D models. The results from the 3D model in the test voltage range are 4.8% lower than the experimental data, while the difference from the results of the 1D model is 6.7%. The 3D model seems to be more accurate than the 1D model.

Effect of degree correlations on controllability of undirected networks

Xu Ming, Xu Chuan-Yun, Cao Ke-Fei
Acta Physica Sinica. 2017, 66 (2): 028901 doi: 10.7498/aps.66.028901
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The controllability analysis of complex networks is of great importance for modern network science and engineering. Existing research shows that the controllability of a complex network is affected not only by the degree distribution of the network,but also by the degree correlation.Although the effect of degree correlations on the network controllability is well studied for directed networks,it is not yet very clear for the case of undirected networks.To explore the impact of degree correlations on the controllability of undirected networks and their corresponding generalized (bidirectional and directed) networks,in this paper,we use the simulated annealing algorithm to change the network degree correlation coefficients by link rewiring.First,the undirected Erdős-Rényi random network and the modified scale-free network are taken as example models to be investigated.Numerical simulations show that the controllability measure (density of driver nodes) of undirected networks decreases monotonically with the increase of the degree correlation coefficient under a constant degree distribution.Specifically,when the degree correlation coefficient changes from -1 to 0,the controllability measure decreases rapidly;while the decrease in the controllability measure is not obvious when the degree correlation coefficient changes from 0 to 1.Next,the bidirectional networks and some directed networks are considered;in these networks,the in-degree of each node is equal to its out-degree,thus link rewiring results in the simultaneous changes of various degree correlations (i.e.,in-in,in-out,out-in,and out-out degree correlations).Further investigations show that these bidirectional and directed networks also follow the above rule,which is verified by the two real networks.The increase of the degree correlation coefficient in undirected networks also implies the increases of various degree correlation coefficients in the corresponding directed networks.Although the effect of a single degree correlation on the controllability of directed networks is clear,the comprehensive effect of the simultaneous changes in various degree correlations on the network controllability cannot be additively and therefore directly estimated by the relevant results in the corresponding directed networks;namely,the effect of the degree correlation on the controllability in an undirected network has its special rule.Some explanations are given for this phenomenon.Moreover,for a large sparse network without self-loops,no matter how assortative or disassortative it is,its structural controllability and exact controllability are verified to be almost the same.These studies will deepen the understanding of the relationship between the network controllability and the network structure.
REVIEW

Progress of electrical control magnetization reversal and domain wall motion Hot!

Zhang Nan, Zhang Bao, Yang Mei-Yin, Cai Kai-Ming, Sheng Yu, Li Yu-Cai, Deng Yong-Cheng, Wang Kai-You
Acta Physica Sinica. 2017, 66 (2): 027501 doi: 10.7498/aps.66.027501
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Electrical control of spins in magnetic materials and devices is one of the most important research topics in spintronics. We briefly describe the recent progress of electrical manipulations of magnetization reversal and domain wall motion.This review consists of three parts:basic concepts,magnetization manipulation by electrical current and voltage methods,and the future prospects of the field.
The basic concepts,including the generation of the spin current,the interaction between the spin current and localized magnetization,and the magnetic dynamic Landau-Lifshitz-Gilbert-Slonczewski equation are introduced first.
In the second part,we reviewed the progress of the magnetization controlled by electrical current and voltage. Firstly we review the electrical current control of the magnetization and domain wall motion.Three widely used structures, single-layer magnets,ferromagnet/heavy metal and ferromagnet/nonmagnetic metal/ferromagnet,are reviewed when current is used to induce magnetization reversal or drive domain wall motion.In a single-layer magnetic material structure,domain wall can be effectively driven by electrical current through spin transfer torque.The factors influencing the domain wall trapping and motion are also discussed.The electrical current control of the skyrmions has big potential applications due to much lower current density.Using the Dresselhaus and Rashba spin orbital coupling,the electrical current can also directly reverse the magnetization of single magnetic or antiferromagnetic layer.Then,we review the electrical current switching the magnetization of the ferromagnetic layer in ferromagnetic/heavy metal structures,where both spin Hall effect and Rashba effect can contribute to the current switching magnetization in such device structures. To identify the relative contributions of these two mechanisms,several quantitative studies are carried,concluding that spin Hall effect plays a major role,which is summarized in this review.Finally,we review the current switching magnetization of free layers in spin valve and magnetic tunnel junctions (MTJs) by spin transfer torque.We also discuss the approaches to the decrease of the critical current density in MTJs,which is desired for future applications.
Alternatively,the electric field can also be used to manipulate the magnetization,where three methods are reviewed. Applying an electric field to the ferromagnetic/piezoelectric heterostructures,which changes the crystal structure of magnetic film through piezoelectric effects,realizes the change of the magnetic anisotropy of the ferromagnetic layer.In ferromagnetic/ferroelectric heterostructures,electric field changes the spin distribution and orbital hybridization at the surface of magnetic film through the magnet-electric coupling effects,and then controls the magnetization of the ferromagnetic layer.In ferromagnetic metal (semiconductor)/dielectric/metal structure,electric field controls the electron accumulation or depletion at the surface of the ferromagnetic metal or semiconductor,the change of the electron density in the magnetic layer in turn affects the magnetic exchange interaction and magnetic anisotropy.
Finally,we present the prospects for the development of electrical control magnetization reversal and domain wall motion for future applications.

Recent research progress of relaxation performances of defects in ZnO-Bi2O3 varistor ceamics

Zhao Xiao-Qiang, Zhao Xue-Tong, Xu Chao, Li Wei-Wei, Ren Lu-Lu, Liao Rui-Jin, Li Jian-Ying
Acta Physica Sinica. 2017, 66 (2): 027701 doi: 10.7498/aps.66.027701
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ZnO varistor ceramics have been widely applied to surge absorption and over-voltage protection in electronic circuit and power system because of their excellent non-ohmic characteristics.Therefore,the reaserch on ZnO varistor ceramic has long been a subject of interest for scholars and industrial circles.At present,the conductance theory of ZnO varistor ceramic has been widely studied and reviewed,and several models such as space charge limited current model,NordheimFowler tunneling current model,and Schottky barrier model have been proposed to describe the electronic transmission process and explain the non-ohmic behavior of ZnO ceramic varistor.However,the relationships of the defect structure and defect relaxation with the electrical property of ZnO varistor ceramic remain unclear,which becomes a challenge to developing new ZnO varistor ceramics.In this paper,comments on defect structures and defect types of ZnO ceramics are given,and the theortical calculation of the intrinsic point defects is discussed.Besides,the characterization technologies of the defect relaxations are introduced.The results show that the dielectric loss spectra are widely used to describe the relaxation of ZnO ceramic varistor,especially the spectra in the low frequency can provide more information about defect relaxation of ZnO ceramic varistor.It is also found that the frequency spectra of admittance in a wide temperature range and the temperature spectra of admittance in a wide frequency range play an equivalent role in characterizing the defect relaxation of ZnO ceramic varistor.The thermally stimulated current is considered to be an effective method to verify the relaxation polarization mechanism of the defects.The deep level transient spectroscopy can characterize the intrinsic and extrinsic defect relaxation processes.Moreover,several theories of relaxation mechanisms such as the Cole-Cole theory,Havriliak-Negami theory and Cole-Davidson theory are proposed to analyze the relaxation phenomena of ZnO ceramic varistors.It is suggested that the electric modulus spectrum combined with Cole-Davidson theory is more effective to characterize the defect relaxations in a wide temperature range.From the electrical degradation results, it is found that the extrinsic defect relaxation at grain boundary interface is closely related to the electrical property of ZnO ceramic varistor.A circuit model is also obtained to establish the correlation between defect relaxation and electrical performance of ZnO ceramic varistor.Therefore,the review on defect relaxations may offer some new ideas to optimize the electrical properties of ZnO ceramic varistors by modifying the defect structures.

Advances in applications of positron annihilation spectroscopy to investigating semiconductor microstructures

Cao Xing-Zhong, Song Li-Gang, Jin Shuo-Xue, Zhang Ren-Gang, Wang Bao-Yi, Wei Long
Acta Physica Sinica. 2017, 66 (2): 027801 doi: 10.7498/aps.66.027801
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Positron annihilation spectroscopy has unique advantage for detecting the micro-defects and microstructures in materials,especially for investigating the negatively charged defects such as cation vacancies in semiconductors.It is a powerful tool to characterize the important features for vacancy-type defects localized electron states within the forbidden energy gap and cation vacancy which provides the key information about the type and distribution of microdefects. Positron annihilation lifetime and Doppler broadening spectroscopy are the major methods of analyzing the vacancy formation,evolution and distribution mechanism.Importantly,the slow positron beam technique can provide the dependences of surface,defect and interface microstructure information on depth distribution in semiconductor thin film.Vacancy and impurity elements can change the ambient electron density in material.They also induce the middle band,which will have dramatic effects on optical and electrical performance.And the variation of electron density will exert furtherinfluences on the positron-electron annihilation mechanism and process.For the fundamental experiments in semiconductors,fabrication technology,thermal treatment,ion implantation/doping,irradiation etc, positron annihilation spectroscopy technology has been extensively applied to detecting the detailed electron density and momentum distribution,and gained the information about microstructure and defects.It can guide the fundamental researches in experiment and give optimal design of the technology and properties about semiconductors.In principle, defect concentrations can be derived and an indication can be obtained about the nature of the defect.Results are presented showing that cation vacancies can be easily detected.Also charge states and defect levels in the band gap are accessible.By combining the positron annihilation spectroscopy with optical spectroscopies or other experimental methods,it is possible to give detailed identifications of the defects and their chemical surroundings.The positron annihilation spectroscopy technology is a very special and effective nuclear spectroscopy analysis method in studying semiconductor microstructure.In this review,the research progress in applications of positron annihilation spectroscopy technology to semiconductors is reported,which focuses on the experimental results from the Positron Research Platform located in Institute of High Energy Physics,Chinese Academy of Sciences.Under different growth modes and ways of treating semiconductors,the experimental results about the internal micro-defect formation mechanism of material, evolution mechanism,and defect feature research progress are reviewed Future challenges including the analysis of electropositivity vacancy (i.e.oxygen vacancy) and of multi-ion implantation phenomena are also presented new technologies such as digitization and new theory will make the positron annihilation spectroscopy portable and reliable.
GENERAL

The design and realization of continuous-variable quantum key distribution system based on real-time shot noise variance monitoring

Cao Zheng-Wen, Zhang Shuang-Hao, Feng Xiao-Yi, Zhao Guang, Chai Geng, Li Dong-Wei
Acta Physica Sinica. 2017, 66 (2): 020301 doi: 10.7498/aps.66.020301
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In the safety assessment of the actual CVQKD (continuous-variable quantum key distribution) system,the preparation measurement model is generally equivalent to the entanglement-based model,whose major drawback is that the shot noise variance is treated as a constant.As the attacks on the LO (local oscillator) from the Eve,the shot noise variance will change with LO.And in the process of safety analysis based on the shot noise variance calibration technology,there are loopholes in which the shot noise variance for calculating secret key rate is obtained by the linear relationship between the shot noise variance and the LO before distributing the quantum key.However,the shot noise variance is not accurate nor real-time.In the security analysis of system,all the noise parameters of the system are normalized to the shot noise variance.The Eve can reduce the shot noise variance by controlling the strength of LO,thus actual excess noise of system will increase.But legal communicating parties are still normalized based on previous larger shot noise variance,so that the excess noise of system is substantially underestimated.As a consequence,the Eve can obtain secret key information without attracting the attention of legal communicating parties by adopting some attacks, such as intercept-resend attack.Thus it is an essential factor for ensuring the system security to evaluate real-time shot noise variance accurately.In order to effectively resist the above mentioned attacks on the LO from the Eve,a scheme of CVQKD system based on real-time shot noise variance monitoring is presented to improve the security of CVQKD system.The shot noise variance calibration technology is adopted in this system.By adding the real-time shot noise variance monitoring modules to the primary CVQKD system,the real-time shot noise variance is assessed by the linear relationship between the shot noise variance and the LO.In the hardware system,independent clocks are adopted. Sampling in peak algorithm is applied to software system,and this effectively solves the problem that CVQKD system with LO clock source is at risk of shot noise variance calibration attack.The scheme prevents the hazards that the Eve changes previously calibrated linear relationship by regulating the pulse delay of the LO,and thus judges whether the system is safe through calculating the accurate and real-time secret key rate.The system can analyze the real-time security of quantum key distribution and display safety status of system.The experimental results show that this system can defend effectively the LO attacks from the Eve and improve the security performance of the CVQKD system.

Bursting oscillations as well as the mechanism with codimension-1 non-smooth bifurcation

Zhang Zheng-Di, Liu Yang, Zhang Su-Zhen, Bi Qin-Sheng
Acta Physica Sinica. 2017, 66 (2): 020501 doi: 10.7498/aps.66.020501
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The coupling of different scales in nonlinear systems may lead to some special dynamical phenomena, which always behaves in the combination between large-amplitude oscillations and small-amplitude oscillations, namely bursting oscillations. Up to now, most of therelevant reports have focused on the smooth dynamical systems. However, the coupling of different scales in non-smooth systems may lead to more complicated forms of bursting oscillations because of the existences of different types of non-conventional bifurcations in non-smooth systems. The main purpose of the paper is to explore the coupling effects of multiple scales in non-smooth dynamical systems with non-conventional bifurcations which may occur at the non-smooth boundaries. According to the typical generalized Chua's electrical circuit which contains two non-smooth boundaries, we establish a four-dimensional piecewise-linear dynamical model with different scales in frequency domain. In the model, we introduce a periodically changed current source as well as a capacity for controlling. We select suitable parameter values such that an order gap exists between the exciting frequency and the natural frequency. The state space is divided into several regions in which different types of equilibrium points of the fast sub-system can be observed. By employing the generalized Clarke derivative, different forms of non-smooth bifurcations as well as the conditions are derived when the trajectory passes across the non-smooth boundaries. The case of codimension-1 non-conventional bifurcation is taken for example to investigate the effects of multiple scales on the dynamics of the system. Periodic bursting oscillations can be observed in which codimension-1 bifurcation causes the transitions between the quiescent states and the spiking states. The structure analysis of the attractor points out that the trajectory can be divided into three segments located in different regions. The theoretical period of the movement as well as the amplitudes of the spiking oscillations is derived accordingly, which agrees well with the numerical result. Based on the envelope analysis, the mechanism of the bursting oscillations is presented, which reveals the characteristics of the quiescent states and the repetitive spiking oscillations. Furthermore, unlike the fold bifurcations which may lead to jumping phenomena between two different equilibrium points of the system, the non-smooth fold bifurcation may cause the jumping phenomenon between two equilibrium points located in two regions divided by the non-smooth boundaries. When the trajectory of the system passes across the non-smooth boundaries, non-smooth fold bifurcations may cause the system to tend to different equilibrium points, corresponding to the transitions between quiescent states and spiking states, which may lead to the bursting oscillations.

Meminductive Wein-bridge chaotic oscillator

Xu Bi-Rong, Wang Guang-Yi
Acta Physica Sinica. 2017, 66 (2): 020502 doi: 10.7498/aps.66.020502
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A meminductor is a new type of memory device. It is of importance to study meminductor model and its application in nonlinear circuit prospectively. For this purpose, we present a novel mathematical model of meminductor, which considers the effects of internal state variable and therefore will be more consistent with future actual meminductor device. By using several operational amplifiers, multipliers, capacitors and resistors, the equivalent circuit of the model is designed for exploring its characteristics. This equivalent circuit can be employed to design meminductor-based application circuits as a meminductor emulator. By employing simulation experiment, we investigate the characteristics of this meminductor driven by sinusoidal excitation. The characteristic curves of current-flux (i-φ), voltage-flux (v-φ), v-ρ (internal variable of meminductor) and φ-ρ for the meminductor model are given by theoretical analyses and simulations. The curve of current-flux (i-φ) is a pinched hysteretic loop passing through the origin. The area bounding each sub-loop deforms as the frequency varies, and with the increase of frequency, the shape of the pinched hysteretic loop tends to be a straight line, indicating a dependence on frequency for the meminductor. Based on the meminductor model, a meminductive Wien-bridge chaotic oscillator is designed and analyzed. Some dynamical properties, including equilibrium points and the stability, bifurcation and Lyapunov exponent of the oscillator, are investigated in detail by theoretical analyses and simulations. By utilizing Lyapunov spectrum, bifurcation diagram and dynamical map, it is found that the system has periodic, quasi-periodic and chaotic states. Furthermore, there exist some complicated nonlinear phenomena for the system, such as constant Lyapunov exponent spectrum and nonlinear amplitude modulation of chaotic signals. Moreover, we also find the nonlinear phenomena of coexisting bifurcation and coexisting attractors, including coexistence of two different chaotic attractors and coexistence of two different periodic attractors. The phenomenon shows that the state of this oscilator is highly sensitive to its initial valuse, not only for chaotic state but also for periodic state, which is called coexistent oscillation in this paper. The basic mechanism and potential applications of the existing attractors are illustrated, which can be used to generate robust pseudo random sequence, or multiplexed pseudo random sequence. Finally, by using the equivalent circuit of the proposed meminducive model, we realize an analog electronic circuit of the meminductive Wien-bridge chaotic system. The results of circuit experiment are displayed by the oscilloscope, which can verify the chaotic characteristics of the oscillator. The oscillator, as a pseudo random signal source, can be used to generate chaotic signals for the applications in chaotic cryptography and secret communications.

An improved direct position determination method based on correlation accumulation of short-time signals with variable velocity receivers

Lu Zhi-Yu, Ren Yan-Qing, Ba Bin, Wang Da-Ming, Zhang Jie
Acta Physica Sinica. 2017, 66 (2): 020503 doi: 10.7498/aps.66.020503
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Wireless target location technology has been widely used in civil and military fields. In the two-step localization algorithms, the signal measurements, such as thetime of arrival, theangle of arrival, the frequency difference of arrival, etc., should be extracted first from the receivedsource signal. Then the target position is identified by calculating the location equation. Compared with the two-step localization algorithms, the direct position determination (DPD) method, which need not estimate the signal parameters and calculate the position step by step, but obtainsthe source position from the received signals directly based on the maximum likelihood criterion, has been shown to have a goodestimation accuracy and robustness, especially under low signal-to-noise ratio (SNR) conditions. So it has been widely studied in recent years and has made remarkable achievements in academic research. However, the DPD algorithm of wideband signals emitters is not performing well with moving receivers in the joint positioning based on time delay and Doppler shift under the low SNRs. To obtaina better positioning performance, in this paper we present a DPD algorithm with variable velocity receivers based on coherent summation of short-time signal segments, and derive the source position Cramer-Rao lower bound (CRLB). The algorithm designs a positioning model in which the multiple variable velocity receivers are usedtoobtain the source signal, then the signal received at the same receiver is patitioned into multiple non-overlapping short-time signal segments, based on which, an approximate maximum likelihood estimator for the new DPD algorithm is developed. The algorithm makes full use of the location information contained in the coherency among the signals segments, while extra target position information is acquired through the speed variability in the positioning model, and thus the problem of location ambiguity is solved. The simulation results show thatthe algorithm proposed in this paper further improves the positioning performance, and outperforms the traditional DPD algorithms with more accurate results. Especially in the low SNR, it is closer to the CRLB.

SM4 key scheme algorithm based on chaotic system

Wang Chuan-Fu, Ding Qun
Acta Physica Sinica. 2017, 66 (2): 020504 doi: 10.7498/aps.66.020504
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Block cipher is a widely used encryption method. In order to improve the security of information in the network data encryption systems, the initial key should be guaranteed to be large enough. In order to overcome the threat of quantum computer to short initial keys, a key scheme based on chaotic map is proposed. The chaotic map is introduced into the original SM4 key scheme, which effectively increases the initial key space and greatly improves the resistance to key scheme attacks.#br#Due to the limited logic resources in hardware implementation, a logistic map is chosen as a chaotic system in this paper. Although the logistic map has many excellent properties of chaotic system, such as initial value sensitivity, randomness, ergodic, etc, there are still a lot of problems that we need to pay attention to. The parameter μ is the system parameter in the logistic map. The value of μ controls chaotic characteristics in the logistic map. When μ is equal to 4, the dynamic characteristics of logistic map are best. The values of data transmitted in the network are all quantified as 0 and 1. In order to implement the logistic map in a digital circuit, the digital quantization is needed. The bit sequence design quantization is very simple and saves resource consumption. Compared with other quantization methods, bit sequence design quantization can be implemented in hardware parallelly. United States National Institute of Standards and Technology launched the test program package to test the random numbers. The test program package includes frequency detection, block frequency detection, run test, etc. Those tests are used to detect the randomness in binary sequence of arbitrary length. The test program package proves that the sequence generated by the logistic map has a great randomness characteristic. After the security analysis of logistic map, the hardware implementation of logistic map is carried out in this paper. Based on the theoretical analysis and hardware implementation in the logistic map, a new SM4 key scheme combined with the logistic map is proposed. The proposed key scheme has less hardware resource consumption, larger key space and higher security than other key schemes combined with chaotic systems. The output of key scheme in this paper is tested by the test program package. The results show that the random number produced by new key scheme is larger. In the end, a key scheme attack is introduced in this paper. It is proved that the new key scheme in this paper can effectively resist existing key scheme attacks.

Invesitgation and experiments of wavelet thresholding in ensemble-based background error variance

Liu Bai-Nian, Huang Qun-Bo, Zhang Wei-Min, Ren Kai-Jun, Cao Xiao-Qun, Zhao Jun
Acta Physica Sinica. 2017, 66 (2): 020505 doi: 10.7498/aps.66.020505
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A large amount of sampling noise which exists in the ensemble-based background error variance need be reduced effectively before being applied to operational data assimilation system.Unlike the typical Gaussian white noise,the sampling noise is scaled and space-dependent,thus making its energy level on some scales much larger than the average. Although previous denoising methods such as spectral filtering or wavelet thresholding have been successfully used for denoising Gaussian white noise,they are no longer applicable for dealing with this kind of sampling noise.One can use a different threshold for each scale,but it will bring a big error especially on larger scales.Another modified method is to use a global multiplicative factor,α, to adjust the filtering strength based on the optimization of trade-off between removal of the noise and averaging of the useful signal.However,tuning α is not so easy,especially in real operational numerical weather prediction context.It motivates us to develop a new nearly cost-free filter whose threshold can be automatically calculated.#br#According to the characteristics of sampling noise in background error variance,a heterogeneous filtering method similar to wavelet threshold technology is employed.The threshold,TA,determined by iterative algorithm is used to estimate the truncated remainder whose norm is smaller than TA.The standard deviation of truncated remainder term is regard as first guess of sampling noise.Non-Guassian term of sampling noise,whose coefficient modulus is above TA,is regarded as a small probability event.In order to incorporate such a coefficient into the domain of[-T,T],a semi-empirical formula is used to calculate and approach the ideal threshold.#br#According to the characteristics of sampling noise in background error variance,a heterogeneous filtering method similar to wavelet threshold technology is employed.The threshold,TA,determined by iterative algorithm is used to estimate the truncated remainder whose norm is smaller than TA.The standard deviation of truncated remainder term is regard as first guess of sampling noise.Non-Guassian term of sampling noise,whose coefficient modulus is above TA,is regarded as a small probability event.In order to incorporate such a coefficient into the domain of[-T,T],a semi-empirical formula is used to calculate and approach the ideal threshold.#br#A new nearly cost-free filter is proposed to reduce the scale and space-dependent sampling noise in ensemble-based background error variance.It is able to remove most of the sampling noises,while extracting the signal of interest. Compared with those of primal wavelet filter and spectral filter,the performance and efficiency of proposed method are improved in 1D framework and real data assimilation system experiments.Further work should focus on the sphere wavelets,which is appropriate for analysing and reconstructing the signals on the sphere in global spectral models.
NUCLEAR PHYSICS

Event mixing constraints for Bose-Einstein correlations in reactions with three particles in the final state

He Qing-Hua
Acta Physica Sinica. 2017, 66 (2): 022501 doi: 10.7498/aps.66.022501
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Bose-Einstein correlations (BEC) are widely used to gain an insight into the spatiotemporal characteristics of boson emitters. It was used for the first time in the 1950s by R. Hanbury-Brown and R. Q. Twiss[Hanbury-Brown R, Twiss R Q 1954 Phil. Mag. 45 663] in astronomy to measure the dimension of distant astronomical objects emitting photons, and hence is also known as Hanbury-Brown-Twiss effect (HBT). In nuclear and particle physics field, BEC also has important applications in the investigation of the space-time properties of subatomic reaction region, especially in elementary-particle collisions and relativistic heavy-ion collisions with large multiplicity at high energies. Its potential application in exclusive reactions with low multiplicity in the non-perturbative QCD energy region may offer complementary information like duration and size of nucleon resonances, which are generally excited by hadronic or electromagnetic probes and usually decay into the ground states accompanied by emission of identical mesons. However, the event mixing technique, which is highly adopted for BEC observations in inclusive reactions at high energies with large multiplicity cannot be directly applied to the BEC measurement in exclusive reactions with very limited multiplicity at low energies. The event mixing method produces un-correlated samples from original sample through making mixed events by randomly selecting the momenta of two bosons from different original events. It works well for the high multiplicity case because the degree of freedom of final state particles is large compared with that of the low multiplicity case. In exclusive reactions with a very limited number of identical bosons in the final state, this method is however strongly interfered by non-BEC factors such as global conservation laws and decays of resonances. Appropriate constraints are required to control the event mixing process in order to eliminate the influence of those non-BEC factors. In this study, we are trying to develop an event mixing method for BEC measurement in reactions having only three final state particles and only two identical bosons among them. For this end, five constraint modes for the event mixing are proposed and investigated via Monte Carlo simulation. Each mode employs one or a combination of the following cut conditions:1) missing mass cut (MM) that requires the missing mass of the mixed event to be equal to that of the original event; 2) polar angle consistency cut (PAC) that requires that the swapping particles should come from the same polar angle bin; 3) azimuthal angle consistency cut (AAC); 4) momentum consistency cut (MC); 5) energy upper limit cut (EU) that requires that any boson energy should not exceed a given upper limit. The double neutral pion photoproduction on the proton around 1 GeV is taken for example to demonstrate the effects of these constraints on the event mixing. In the simulation, one event sample free of BEC effects and four samples in the presence of BEC effects are generated for testing the ability for these constraints to extract BEC parameters. It is found the constraint mode using the MM and PAC cuts, and the mode employing the MM and AAC cuts, and the mode adopting the MM and the EU cuts can be used to observe BEC effects and extract BEC parameters. Among them, optimum results can be achieved by the combination of the MM and EU cuts.
ATOMIC AND MOLECULAR PHYSICS

Spectroscopic properties of low-lying excited electronic states for CF- anion based on ab initio calculation

Zhou Rui, Li Chuan-Liang, He Xiao-Hu, Qiu Xuan-Bing, Meng Hui-Yan, Li Ya-Chao, Lai Yun-Zhong, Wei Ji-Lin, Deng Lun-Hua
Acta Physica Sinica. 2017, 66 (2): 023101 doi: 10.7498/aps.66.023101
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CF- anion is very important for collisional ionization reactions, electron transfer from Rydberg atoms and electron attachment. Potential energy curves (PECs) of five low-lying excited electronic states, X3Σ-, a1Δ, b1Σ+, A3Π and c31Π of CF-, are calculated by using the internally contracted multireference configuration interaction (icMRCI) approach. Ro-vibrational levels of these electronic states are derived through solving the radial Schrödinger ro-vibrational equation, and then the molecular parameters are obtained by fitting. Our results for X3Σ- agree well with those in the references. We compute the electronic dipole moments (EDMs) of these states with different bound lengths, and analyze the relationship between the electronic configurations and EDMs. The electronic transition dipole moment matrix elements, Franck-Condon factors and oscillator strengths f00 of A3Π-X3Σ- are evaluated, and radiative lifetimes of five lowest vibrational levels of A3Π state are derived. Finally the predissociation mechanism of A3Π state is discussed in detail, and the dissociation lifetimes of high vibrational levels are obtained.

Ultrafast photodissociation dynamics of butanone in 3s Rydberg state Hot!

Luo Jin-Long, Ling Feng-Zi, Li Shuai, Wang Yan-Mei, Zhang Bing
Acta Physica Sinica. 2017, 66 (2): 023301 doi: 10.7498/aps.66.023301
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The initiation and subsequent control or exploration study of chemical transformation in real time by using ultrashort laser pulses aim at femtochemistry. The real-time investigations of ultrafast dynamics of excited molecules in gas and condensed phases have attracted a great deal of attention over the last two decades. As a kind of important organic compound, aliphatic ketone is an area of much interest for many research fields, especially for atmospheric photochemistry. Via photodissociation reaction, it can release carbonyl radical whose chemical character is active and can react with hydroxyl easily. As a typical aliphatic ketone, butanone has been a research focus over the past decades. The ultrafast dissociation dynamics of butanone after excitation to the second electronically excited state (S2) with a 195.8 nm pump pulse is studied by the femtosecond pump-probe technique combined with the time-of-flight mass spectrometry (TOF-MS). Time-resolved mass spectrometry (TRMS) has proven to be a powerful technique to study the ultrafast dynamics of excited states in molecules. In this technique, the MCP detector is capable of recording time-resolved ion yield measurements of different cations by monitoring the current output directly from the anode by using an oscilloscope. This enables a time-of-flight mass spectrum to be recorded at each delay time, which is controlled by a delay stage, and the measured total signal is then integrated, yielding a time-resolved ion yield transient, which is conducted by LABVIEW software. The pump wavelength in this work is set to be 195.8 nm and the probe laser wavelength is centered at 800 nm. The complex ultrafast dynamics in butanone with 3s Rydberg state excitation and its possible decay paths and following dissociation mechanism are given. Experimental results show that the Norrish I type dissociation kinetics of butanone exhibit rich features, for it has a methyl group and an ethyl group at α position. The decay time constant of the parent transient is approximately 2.23 ps±0.02 ps. There is only one time constant of 2.15 ps±0.02 ps for the fitting of the propionyl transient. The best fit of acetyltransient is obtained with four time constants:τ1=(2.40±0.15) ps, τ2=(1.10±0.25) ps, τ3=(0.08±0.02) ps, and τ4=(17.72±0.80) ps, corresponding to S2→S1 internal conversion, the primary dissociation of the S1 state generating CH3CO(Ã), Ã internal conversion and secondary dissociation of CH3CO() respectively. Two competitive α-CC bond dissociation processes are observed and discussed. They are dissociation channels through intramolecular vibrational energy redistribution (IVR) and/or by getting over the dissociation barrier in α-cleavage of butanone. But hereunder the condition of this experiment, the dissociation is the result of IVR.

Influence of collision energy on the stereodynamics of the H+CH+→C++H2 reaction

Tang Xiao-Ping, Zhou Can-Hua, He Xiao-Hu, Yu Dong-Qi, Yang Yang
Acta Physica Sinica. 2017, 66 (2): 023401 doi: 10.7498/aps.66.023401
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The reactive cross section and stereodynamics at selected collision energies for the H(2S)+CH+(X1Σ+)→C+(2P)+H2(X1Σg+) reaction on a globally smooth ab initio potential surface of the 2A' state are calculated in detail by the quasi-classical trajectory(QCT) method. The calculated cross section decreases with the increase of the collision energy, which is found to be in overall good agreement with the previous time-dependent quantum results in the high collision energy regime (Ec>20 meV). The discrepancy between the QCT and previous quantum cross section below 20 meV can be attributed to the limitations of the classical trajectory method, because the QCT method cannot handle the effect of zero point energy. In general, QCT results show qualitative agreement with the quantum results, which confirmsthe validity of the QCT method. The research shows that the product rotational angular momentum vector is aligned and oriented. The alignment of the product rotational angular momentum vector j' depends very sensitively on the collision energy. With the increase of the collision energy, the alignment effect recedesin the low collision energy region (1500 meV), while it is enhanced in the high collision energy region (500-1000 meV). Moreover, the k-k'-j' distributions tend to be asymmetric with respect to the k-k' scattering plane (or about φr=180°), with two peaks appearing at φr=90° and φr=270°, respectively. This indicates that the product rotational angular momentum is not only in the Y-axis direction but also along the positive Y-axis direction. The peak intensity decreases with the collision energy increasing from 1 meV to 100 meV, while it increases with collision energy increasing from 100 meV to 1000 meV. Therefore the Y-axis orientation effect turns weak with the enhancement of the collision energy in the low energy region, while it becomes strong in the high energy region. In addition, the polarization dependent differential cross sections (PDDCSs) (2π/σ)(dσ00/dωt) and (2π/σ)(dσ20/dωt) are calculated. PDDCS (2π/σ)(dσ00/dωt) results indicate that the products have almost symmetrically scattered forward and backward, and the intensity of the scattering increases with the increase of the collision energy. The PDDCS (2π/σ)(dσ20/dωt) shows that the alignment effect of the rotational angular momentum of the products is stronger at the terminal of the scattering angle than at the other directions.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

Effect of grating groove density error on the output pulses of the tiled grating compressor and corresponding compensation scheme

Zhao Dan, Wang Xiao, Mu Jie, Zuo Yan-Lei, Zhou Song, Zhou Kai-Nan, Zeng Xiao-Ming, Li Zhi-Lin, Su Jing-Qin, Zhu Qi-Hua
Acta Physica Sinica. 2017, 66 (2): 024201 doi: 10.7498/aps.66.024201
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The grating groove density error (GGDE) will degrade the performance of the tiled-grating compressor, and the compensation for GGDE is of significance for improving the characteristics of the output pulses. With the ray-tracing method, analytical expressions considering GGDE are derived to predict the output beam drift and the output pulse broadening. According to the numerical results, we propose a compensation method to reduce the degradation of the tiled-grating compressor by applying angular tilt error and longitudinal piston error simultaneously. The tilt angle and the translation distance of the grating, as well as the allowable tolerance range of GGDE are obtained with this compensation method. By using the equiphase lines in the spatial-spectral interference patterns, the experimental results demonstrate that this compensation method can correct the angular drift of the output beams effectively, and compensate for the second-order and the third-order dispersion error well. Our investigation provides an efficient way to guide the adjustment of the tiled grating with GGDE.

The research of δ13CO2 by use of wavelet de-noising at 2.008 μm based on tunable diode laser absorption spectroscopy

Niu Ming-Sheng, Wang Gui-Shi
Acta Physica Sinica. 2017, 66 (2): 024202 doi: 10.7498/aps.66.024202
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Development of optical isotope techniques has provided scientists with a set of powerful tools for investigating the sources and sink of atmospheric CO2. Here we describe a continuous, high precision, compact and portable carbon dioxide isotope ratio laser multi-pass cell spectrometer with a tunable distribute feedback laser at 2.008 μm based on tunable diode laser absorption spectroscopy and, the spectrometer has good temperature and pressure stability. In order to deduce the noise, drift effect and background changes associated with low level signals, a superior signal processing technique of wavelet denoising, which possesses multi-level analytical resolutions both in time and frequency-domains, is introduced. After evaluating the method, evaluation ability and applicabilities of several common wavelet functions are analyzed and tested, the wavelet function of Haar is selected as an optimal wavelet basis function. Based on the analysis of the optimal decomposition level of Haa wavelet function, the VISU function is selected as an optimal wavelet threshold function. The denoising effect and measurement precision are evaluated by use of the VISU threshold function in the measurement process of carbon dioxide stable isotope ratio. The measurement results of carbon dioxide stable isotope ratio before and after suppressing the noises are compared in the same experiment conditions and, the inconsistent reasons of the measured results are theoretically analyzed. This technique allows the measurement of the δ-value for carbon dioxide isotopic ratios with a precision of -12.5‰ and and the measuremnt results show that the wavelet denoising measuring results have higher measurement accuracy, and the measurement precise of carbon dioxide isotope ratio is 7.3 times the original measurement results. The application of the wavelet denoising to the carbon dioxide isotope ratio measurement for the first time proves that the capability of the new near-infrared direct absorption technique to measure isotope ratio can permit high-frequency, near-continuous isotope measurement and obtain the high precision and accurate real-time stable isotope data directly in the field. This technique provides an important tool for studying the resource and sink of green house gases in the future.

Sheared-beam imaging target reconstruction based on all-phase spectrum analysis

Chen Ming-Lai, Luo Xiu-Juan, Zhang Yu, Lan Fu-Yang, Liu Hui, Cao Bei, Xia Ai-Li
Acta Physica Sinica. 2017, 66 (2): 024203 doi: 10.7498/aps.66.024203
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Sheared-beam imaging technique is considered to be a non-conventional speckle technique for remote imaging through turbulent medium. In this high resolution imaging technique, three beams are splitted from one laser source and illuminate a remote target simultaneously in shearing distribution. Each beam is modulated by a tiny frequency shift so that these beams can interfere and beat together. The returning speckle signals are received by an array of detectors. The primary algorithm for the signal processing and image reconstruction has been developed previously. However, the reconstructed image is deteriorated by the frequency drifting error and spectrum leakage. These frequency errors are always from the transmitter and scattered signals that are caused by spectrum-shift errors from acoustic-optic modulators, atmospheric turbulence, Doppler effects of moving targets, etc. To solve the problems mentioned above, in this paper we propose a new image reconstruction algorithm based on the all-phase spectrum analysis theory. The all-phase fast Fourier transform (FFT) spectrum analysis theory, which can effectively inhibit spectral leakage and correct speckle spectrum, is used to process the scattered signals. By searching for the accurate positions of the beat frequency components in the transformed frequency domain data, the speckle amplitude and phase difference frames can be extracted accurately. Based on the speckle phase-difference frames, the phase distribution of the wavefront is derived by least-square algorithm. The phase distribution in grid is highly coherent, in which each point is related to the phases of its four nearest neighbors. If an initial phase map is given or preset, the phase map of the wavefront can be estimated accurately by Gauss-Seidel method. Meanwhile, the amplitude of wavefront is obtained by the algebraic operation of speckle amplitude frames. The reconstructed wavefront is inverse Fourier transformed to yield a two dimensional image. A series of speckled images of the same object are averaged to reduce the speckle noise. The proposed method improves the ability of system imaging in the actual imaging environment. Simulation experiments validate the effectiveness of the proposed algorithm, and simulation results show that the proposed image reconstruction algorithm can inhibit the frequency errors from influencing imaging quality when there exist frequency errors in scattered signals. Thus, the imaging quality of the algorithm based on the all-phase FFT method is much better than that of the algorithm based on the traditional FFT method. The substantial usage of this technique is widely spread after the reconstruction algorithm has been optimized.

Influence of gradient phased interfaces on the laws of light propagation

Xiao Xiao, Xie Shi-Wei, Zhang Zhi-You, Du Jing-Lei
Acta Physica Sinica. 2017, 66 (2): 024204 doi: 10.7498/aps.66.024204
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The gradient phased interface is characterized by a non-zero phase variation along the interface between two optical media,which could generate a phase shit between the emitted and incident light beams.Unlike common ones,gradient phased interfaces have a great influence on the laws of light propagation,including light reflection and refraction,and some novel phenomena are observed.For a comprehensive understanding the optical characteristics of those gradient surfaces,the universal laws of light propagation at gradient phased interfaces are derived and discussed in detail in this paper.According to Fermat's principle,we use the stationary phase method to successively acquire the two-dimensional (2D) and three-dimensional (3D) generalized laws of reflection and refraction.In the 2D generalized laws,the interfacial phase gradient lies in the plane of incidence,which is coplanar with the incident,refracted and reflected light beams. But in the 3D case,the phase gradient does not lie in the plane of incidence,and the non-planar reflection and refraction phenomena are observed.These generalized reflection and refraction laws indicate that the interface between two media could be an important factor when light traverses it,and gradient phased interfaces provide new degrees of freedom for manipulating the wavefront of light beams.Based on the generalized reflection and refraction laws,we analyze the influence of phase gradient on light propagation,then obtain critical angles of incidence for reflection and refraction (i.e.the critical angles for total internal reflection and total transmission) in 2D and 3D cases,and explain the reasons for some novel phenomena,such as reflection angle unequal to incidence angle,anomalous reflection and refraction, out-of-plane reflection and refraction,etc.These analysis results show that generalized laws of reflection and refraction have important value in optical design.In addition,we propose an optical design idea based on generalized laws of reflection and refraction,in which gradient phased interfaces are used as core components of optical elements to perform optical transform.And then a flat lens and flat axicon are taken for example to illustrate this idea,the design process of the two flat optical elements are shown in detail.Moreover,we experimentally simulate the gradient surfaces of the two elements by spatial light modulator,and experimental results agree well with theoretical values.It proves that this design idea is practicable.Our research is useful to understand comprehensively the generalized reflection and refraction laws,and extend the applications of generalized laws to flat optics,freeform optics and the accurate control of complex wavefront.

Low-noise optical field phase-shifting manipulated using a coherently-prepared three-level atomic medium

Guan Jia, Gu Yi-Sheng, Zhu Cheng-Jie, Yang Ya-Ping
Acta Physica Sinica. 2017, 66 (2): 024205 doi: 10.7498/aps.66.024205
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We propose a multifunction phase-shifting manipulator with low noise at a single-photon level,by using a threelevel atomic scheme.This three-level system interacts with a strong pumping field and a weak probe field with a large detuning.Due to this large detuning,two lower states can be coherently prepared prior to the injection of the pump and probe fields.In our configuration,the duration of the pumping field is much longer than that of the probe field. By solving the Heisenberg-Langevin equations of our system under the steady state approximation,we calculate the linear susceptibility of the system and examine the quantum noise properties of the probe field in detail.We show that this scheme,which rests on the process of two-wave mixing with initial atomic coherence,exhibits many interesting properties that neither typical electromagnetically induced transparency (EIT) schemes nor active Raman gain (ARG) schemes possess.Although both EIT-and ARG-based schemes have been widely investigated in atomic medium,the direct generalizations of these schemes to the single/few photon limit prove to be more problematic.The low fidelity due to the significant probe-field attenuation in EIT medium and the large quantum noise due to the amplification of the probe field in an active Raman gain medium are the main obstacles that prohibit a high-fidelity,low-noise phase shifter from being realized in the single/few photon limit.Physically,this scheme can be viewed as a hybrid scheme in which two processes of different physical principles are allowed to interfere with each other to achieve many desired functionalities. For instance,it can be used as a lossless two-photon-broadband phase-shifter with suitable system parameters.It can also be used as an attenuator/amplifier and a total transparency with a zero phase shift.In particular,we show that by locking the pump field intensity and the two-photon detuning simultaneously a flat constant π-phase shift can be realized with unit probe fidelity in a broad probe field frequency range.Applying the quantum regression theorem,we calculate the noise spectrum of the outgoing probe field as a large phase shift is achieved,and show that this two-photon-insensitive π-phase shift may significantly reduce the quantum noise fluctuations associated with a Raman gain process,and have a lot of potential applications for quantum information processing and optical telecommunication.The realization of this broadband π-phase-shift with significantly reduced quantum noise fluctuations makes this scheme attractive for the realization of low-noise phase-gate/polarization-gate at single-photon level.

Experimental study on increasing signal-to-noise ratio of a beat note by cascading an Yb-doped fiber in an Er-fiber comb

Liu Huan, Cao Shi-Ying, Yu Yang, Lin Bai-Ke, Fang Zhan-Jun
Acta Physica Sinica. 2017, 66 (2): 024206 doi: 10.7498/aps.66.024206
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The harmonic optical frequency chain is the only tool for measuring optical frequency till the advent of a femtosecond optical frequency comb (FOFC). However, its disadvantages are obvious, such as high cost, difficult construction, complex usage, and complicated maintenance. The emergence of femtosecond optical frequency combs (FOFCs) makes it possible to measure the absolute frequency of a laser, which greatly simplifies the quantity traceability of the absolute frequency value and comparison, and allows the length unit “m” to be directly traced back to the time unit “s”. The beat note (fb) between an FOFC and a test laser is one of the most important data in measuring absolute frequency of the test laser. In order to ensure the accuracy and reliability of the measurement, the signal-to-noise ratio (SNR) of fb should be above 30 dB at 300 kHz resolution bandwidth. Among the wavelength standards recommended to replicate “meter” (SI), iodine-stabilized 633 nm lasers and iodine-stabilized 532 nm lasers have been widely used. Compared with iodine-stabilized 633 nm lasers, iodine-stabilized 532 nm lasers have the advantages of high stability, high output power, no modulation and fiber coupled output. Therefore, it is of great importance to measure and monitor the absolute frequency of an iodine-stabilized 532 nm laser. Aiming at the specific requirements for absolute frequency measurement of an iodine-stabilized 532 nm laser, the absolute frequency measurement of its fundamental 1064 nm laser has been studied. In this paper, a high-repetition-rate Er-doped femtosecond fiber laser is adopted as an optical source in the system. The repetition rate of the fiber laser is 303 MHz, the output power in the continuous-wave state is 130 mW and the average output power in the mode-locking state is 80 mW. The highest SNR of fb between the comb light and a 1064 nm laser generated by an iodine-stabilized 532 nm laser is only 30 dB due to the low intensity at 1 μm wavelength in the supercontinuum, which just reaches the SNR threshold meeting the counter's working condition. In order to improve the accuracy and reliability of absolute frequency measurement, the technique of cascading an Yb-doped fiber amplifier after spectral broadening is adopted to enhance the spectral intensity at 1 μm wavelength. The experimental results indicate that the SNR of fb between a 1 μm laser after spectral enhancement and a 1064 nm laser is increased by 5 dB and kept at 35 dB for several days, meeting requirements for long-term continuous monitoring. This method can effectively reduce the intensity requirements at 1 μm wavelength when the spectrum is directly broadened in the Er-FOFC.

Characterization of Brillouin scattering in a few-mode fiber

Zhang Yan-Jun, Gao Hao-Lei, Fu Xing-Hu, Tian Yong-Sheng
Acta Physica Sinica. 2017, 66 (2): 024207 doi: 10.7498/aps.66.024207
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The few-mode fiber can be used to transmit limited orthogonal modes, which has the advantages of small modular interference and easily controlled modes. The Brillouin scattering sensor based on the few-mode fiber can effectively reduce the cross sensitivity of multi parameter measurement, and realize the measurement of multi physical quantity. In this paper, based on the wave optics theory, the Brillouin scattering spectrum parameters of the step-index few-mode fiber are analyzed, such as frequency shift, line width and peak gain and so on. Firstly, the transmission modes of the few-mode fiber are analyzed. The finite element analysis result shows that there are 5 kinds of transmission modes:LP01, LP11, LP21, LP02 and LP31, and their effective refractive indexes are 1.4664, 1.4652, 1.4637, 1.4630 and 1.4616, respectively. Secondly, the mathematical models of the Brillouin frequency shift, line width and peak gain of different modes in the few-mode fiber are analyzed. Finally, the parameters of Brillouin scattering spectrum with different modes' superposition are also discussed. In the few-mode fiber, due to the different effective refractive index, the light of each mode is propagated along its respective path and interacts with the particles in the fiber, thus producing different Brillouin scattering spectrum. The simulation results show that the frequency shift of the Brillouin scattering spectrum of each mode is in a range of 10.19-10.23 GHz, and the frequency shift increases with the decrease of the mode order. The Brillouin line width of each mode is in a range of 32-34 MHz, and the line width also increaseswith the decrease of the mode order. Moreover, the relative amplitude of the Brillouin scattering gain spectrum increases with the decrease of the mode order. The mathematical models of this paper are used respectively to analyze the Brillouin scattering spectra of other types of step-index few-mode fibers. It is shown that the Brillouin frequency shift, Brillouin line width and peak gain of other types of step-index few-mode fibers also increase with the decrease of the mode order. In a step-index few-mode fiber, intramodal Brillouin scattering spectrum and the intermodal Brillouin scattering spectrum are both in line with the distribution of Lorenz curve. However, the intermodal Brillouin scattering spectrum of modes' superposition leads to the line width broadening of the Brillouin scattering spectrum, and the relative amplitude of the intermodal Brillouin scattering spectrum of modes' superposition being generally smaller than that of intramodal.

First-principles study on the electronic structures and the absorption spectra of In: Mn: LiNbO3 crystals

Zhang Yun, Wang Xue-Wei, Bai Hong-Mei
Acta Physica Sinica. 2017, 66 (2): 024208 doi: 10.7498/aps.66.024208
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The electronic structures and the absorption spectra of the indium and manganese codoped LiNbO3 crystals and their comparative groups are investigated by first-principles based on the density functional theory. The supercell crystal structures are established with 60 atoms, including four models:the near-stoichiometric pure LiNbO3 crystal (LN), the manganese doped LiNbO3 crystal (Mn:LN, charge compensation model as MnLi+-VLi+), the indium and manganese codoped LiNbO3 crystal (In:Mn:LN, charge compensation model as InLi2+-MnLi+-3VLi+), and the other indium and manganese codoped LiNbO3 crystal (In(E):Mn:LN, charge compensation model as InLi2+-InNb2--MnLi+-VLi+). The results show that the extrinsic defect levels within the forbidden band of Mn:LN crystal are mainly contributed by Mn 3d orbital electrons, which also affect the top of the valence band. The band gap of Mn:LN about 3.18 eV is narrower than that of LN; the band gaps of In:Mn:LN and In(E):Mn:LN sample are 2.82 and 2.93 eV respectively. The electron density of state (DOS) of manganese codoped LiNbO3 crystal shows that the orbits of Mn 3d, Nb 4d and O 2p superpose each other, i.e., forming covalent bonds, which result in conduction and valence bands shifting toward low energy. The indium ion does not contribute the extrinsic energy level within forbidden band, it affects the band gap through changing O2- electron cloud shape. The band gap narrows down if the indium ions occupy lithium ion positions, and becomes broad if the indium ions occupy niobium ion positions. It is found that the Mn:LN, In:Mn:LN and In(E):Mn:LN samples display the absorption peaks at 3.25, 3.11, 2.97, 2.85, 2.13 and 1.66 eV. The last absorption peak is contributed by the electron transferring from the Mn2+ energy level to conduction band, and the doping of indium ions leads to attenuation of this peak. The peak at 2.13 eV relates to the Mn3+, it is enhanced by the doped indium ions. The indium ions in crystal would influence the absorption, which relates to manganese ions, by transforming manganese ion valence via the formula as m Mn2++In3+→Mn3++In2+, that is, with the doping of the indium ions, the photorefractive center Mn2+ concentration decreases, which is responsible for the absorption peak at 1.66 eV. It must be mentioned that the Mn2+ possesses not only the shallow levels as mentioned previously, but also the deep ones which are responsible for the absorptions at 2.85 eV and other high energies. For the indium and manganese codoped LiNbO3 crystals, if the recording light is chosen at near 1.66 eV (748 nm), the relatively low concentration of indium ions is proposed to be chosen to achieve the high recording sensitivity.

Dual-core terahertz polarization splitter based on porous fibers with near-tie units

Wang Jing-Li, Liu Yang, Zhong Kai
Acta Physica Sinica. 2017, 66 (2): 024209 doi: 10.7498/aps.66.024209
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Terahertz (THz) radiation, which is defined as the electromagnetic wave with a frequency ranging from 0.1 THz to 10 THz, has attracted widespread attention in recent years because of its unique possibilities in many fields. High-performance THz polarization splitter, a key device in THz manipulation, is of great significance for studying the THz devices. In the present paper, a novel dual-core THz polarization splitter is proposed, which is based on porous fiber with near-tie units. The introduction of near-tie units into the fiber core can enhance asymmetry to realize high mode birefringence. And the results show that the porous THz fiber exhibits high birefringence at a level of 10-2 over a wide frequency range. An index converse matching coupling (ICMC) method, which exhibits several advantages (such as short splitting length, high extinction ratio, low loss, and broad operation bandwidth), is used to allow for the coupling of one polarization mode within a broad operation band, while the coupling of the other polarization component is effectively inhibited. The splitting length is equal to one coupling length of x- or y-polarization component for which inter-core coupling occurs, and short splitting length means low transmission loss. Unlike the reported filling method, an adjusting structure method is proposed in the paper to satisfy the condition of index converse matching coupling. The full vector finite element method (FEM), which is based on the variational principle and the subdivision interpolation, is used to analyze the guiding properties of the proposed THz polarization splitter. The FEM is a widely used numerical method in physical modeling and simulation. Simulation results show that the THz polarization splitter operates within a wide frequency range of 0.5-2.5 THz. The splitting length does not exceed 2.5 cm in the whole frequency range and the minimum is only 0.428 cm. At 2.3 THz, the material absorption losses of x- and y-polarization are both less than 0.35 dB, and the extinction ratios for x- and y-polarization are 2.9 and 19.2 dB, respectively. Moreover, by comparing with a THz polarization splitter with filling method, the proposed THz polarization with adjusting structure method is easier to realize, the operating frequency range is wider, the splitting length is shorter, and the material absorption loss is lower. Finally, we note that the fabrication of such THz porous fiber designs could be realized by several methods, such as a capillary stacking technique, a polymer casting technique, a hole drilling technique, etc.

Dual-mode large-mode-area multi-core fiber with circularly arranged airhole cores

Jin Wen-Xing, Ren Guo-Bin, Pei Li, Jiang You-Chao, Wu Yue, Shen Ya, Yang Yu-Guang, Ren Wen-Hua, Jian Shui-Sheng
Acta Physica Sinica. 2017, 66 (2): 024210 doi: 10.7498/aps.66.024210
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Multi-core fiber has aroused considerable interest as one of potential candidates for space division multiplexing that provides an additional freedom degree to increase optical fiber capacity to overcome the transmission bottleneck of current single-mode fiber optical networks. Few-mode fiber is also under intense study as a means to achieve space division multiplexing. We propose a novel dual-mode large-mode-area multi-core fiber (DMLMAMCF), which uses multi-core structure to realize few-mode condition when pursuing large mode-area. The proposed fiber consists of 5 conventional silica-based cores in the center region and 14 air hole cores surrounding the center cores. The outer circle with 12 air hole cores, which function similarly to the fluorine doping region in the bend-insensitive fiber, can mitigate the bending loss when keeping large mode area. The symmetrically distributed two cores on both sides of the center core in central region can reduce the half second-order LP11 mode consisting of two degenerate HE11 modes, TE01 mode, two degenerate HE21 modes and TM01 mode, thus leading to the remaining four vector modes, i.e. two degenerate HE11 modes and two degenerate HE21 modes. That is the reason why we call it strict dual-mode. We focus on large-mode-area properties and bending characteristics of the dual-mode. The influence of structural parameters that include corepitch Λ, refractive index difference between core and cladding Δn, and fiber core radius a, on mode characteristics and mode area of HE11 mode and HE21 mode is investigated in detail. The results reveal that it is helpful to increase the effective area of fundamental mode when we increase the value of corepitch, reduce the refractive index and fiber core radius. The effective mode area of HE11 is about 285.10 μm2 under the strict dual-mode condition. In addition, the relationship between bending loss and bending radius, and the relationship between effective mode area and bending radius of two modes are both investigated. For the HE11 mode, the least bending loss is about 5×10-5 dB/m while the least effective mode area with bending radius larger than 0.6 m is about 285.10 μm2. The HE21 mode is more sensitive to bend effect. The least bending loss is about 0.028 dB/m and the effective mode area is larger than 280.00 μm2 except for resonant coupling points. Large effective areas of both modes with low bending loss can be realized. Larger effective mode area with larger corepitch, appropriate refractive index difference and fiber core radius can be achieved. This fiber may find its usage in high power fiber lasers and amplifiers.

Inverse Doppler effect of acoustic metamaterial with negative mass density

Liu Song, Luo Chun-Rong, Zhai Shi-Long, Chen Huai-Jun, Zhao Xiao-Peng
Acta Physica Sinica. 2017, 66 (2): 024301 doi: 10.7498/aps.66.024301
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It is always an issue for researchers to control the propagation of sound wave at will. A kind of acoustic metamaterial built with artificial microunits attracts the attention of researchers, because it possesses many unique properties that cannot be realized by natural materials, such as negative refractive index, slab focusing, and cloak. The Doppler effect leads to the frequency change of a wave because of the relative motion between the observer and the source. In 1968, Veselago[Veselago V G 1968 Soviet Physics Uspekhi 10 509] theoretically proposed that a metamaterial with a negative refraction can result in an inverse Doppler effect. The investigation of inverse Doppler effect has been developed with the improvement of metamaterials. However, the design methods of these metamaterials generally need ideal material parameters, which are difficult to obtain experimentally. Besides, although the inverse Doppler effects are realized by some electromagnetic metamaterials in optical and microwave frequencies, the relevant researches in acoustic metamaterials make slow progress. In this work, a 2D acoustic metamaterial with negative mass density is fabricated. Our previous work has demonstrated that the air in the internal cavity of the unit cell will vibrate back and forth to generate the vibration velocity when the air is driven by a sound source. As the source frequency reaches the resonant frequency, large amounts of energy will be stored in the internal cavity. This accumulation of energy will cause the acceleration of the air in opposite direction to the sound pressure, thus this metamaterial will exhibit negative mass density. In this case, the direction of the phase velocity is exactly opposite to that of the group velocity of the sound wave. Therefore, the inverse Doppler effect of sound wave can be realized by this metamaterial. Since the unit cells with different lengths have different resonant frequencies and there is only weak interaction among the adjacent unit cells, the frequency band of the metamaterial with negative mass density can be broaden by combining several different unit cells. Our previous experiments have demonstrated that the mass density and refractive index of this metamaterial are negative over a broad frequency range from 1560 Hz to 5580 Hz and 1500 Hz to 5480 Hz, respectively. A testing equipment is constructed to measure the Doppler effect of this metamaterial from 1200 Hz to 6500 Hz. The experimental results show that when the sound source witha frequency of 2000 Hz approaches to the detector, the detected frequency is 1999.27 Hz, which is 0.73 Hz smaller than the source frequency; when the sound source recedes from the detector, the detected frequency is 2000.68 Hz, which is 0.68 Hz larger than the source frequency. Therefore, the inverse Doppler effect appears at 2000 Hz. The experimental results within the whole frequency range of negative refractive index show broadband inverse Doppler phenomena.

Effects of poling state and direction on domain switching and phase transformation of Pb(Zr0.95Ti0.05)O3 ferroelectric ceramics under uniaxial compression

Jiang Zhao-Xiu, Wang Yong-Gang, Nie Heng-Chang, Liu Yu-Sheng
Acta Physica Sinica. 2017, 66 (2): 024601 doi: 10.7498/aps.66.024601
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The digital image correlation technique is used for full field measurements of axial strain and transverse strain of PZT95/5 ferroelectric ceramics under uniaxial compression. Based on the variations of the axial strain and transverse strain with axial stress, the effects of poling state and poling direction of PZT95/5 ferroelectric ceramics on the domain switching and phase transformation behaviors are explored. Domain switching occurs in unpoled and Z-axis poled PZT95/5 ferroelectric ceramics separately, while domain switching in the Y-axis poled PZT95/5 ferroelectric ceramic is not observed. Domain switching strain in the Z-axis poled PZT95/5 ferroelectric ceramic has obvious influences on the developments of axial strain and transverse strain, but the influence of domain switching strain in the unpoled PZT95/5 ferroelectric ceramic is very weak, which can be attributed to the different random distribution characteristics of domain orientation. By the strain decomposition analysis, it is proved that the domain switching and the phase transition process can be decoupled, and domain switching strain and phase transformation strain can be distinguished successfully. Compared with the Z-axis poled PZT95/5 ferroelectric ceramic, the unpoled PZT95/5 ferroelectric ceramic has a small critical stress of phase transformation, while the critical stress of the Y-axis poled PZT95/5 ferroelectric ceramics is big, which may be concluded that the domain switching behavior favors the phase transformation process. The polarization released behavior of PZT95/5 ferroelectric ceramic also depends on the poling direction. The depolarization mechanism of Z-axis poled PZT95/5 ferroelectric ceramic is caused by both domain switching and phase transformation, and the Y-axis poled PZT95/5 ferroelectric ceramic is caused by only phase transformation.

Experimental study on the density characteristics of a supersonic turbulent boundary layer

He Lin, Yi Shi-He, Lu Xiao-Ge
Acta Physica Sinica. 2017, 66 (2): 024701 doi: 10.7498/aps.66.024701
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An experimental study on the density characteristics of a zero-pressure-gradient flat plate turbulent boundary layer at Ma=3.0 is performed by the density field measurement method based on Nano-tracer planar laser scattering (NPLS) technology. The mean and the fluctuating characteristics of the density field of the boundary layer are analyzed. And the spectrum analyses of density fluctuations are performed by utilizing Taylor's hypothesis to convert spatial measurements into pseudo-temporal measurements. The mean density profile increases away from the wall, which accords well with the density profile deduced from the mean velocity distribution by using the adiabatic Crocco-Busemann relation. The root mean square (RMS) of the density fluctuations increases in the logarithmic region with a peak value of 0.2ρ, and its probability density distribution follows a normal distribution. However, the RMS of density fluctuations decreases in the outer region of the boundary layer. According to the spectrum analysis, the density fluctuations are characterized in a wide range of frequencies throughout the boundary layer, with the maximum frequency on the order of 1 MHz. The low frequency fluctuations are predominant near the wall and in the outer region of the turbulent boundary layer. However, the proportion of high-frequency fluctuations is nearly equal to that of low-frequency fluctuations in the logarithmic region. The combined NPLS and PIV technique provide a simultaneous density and velocity measurements of the present turbulent boundary layer. The high frequency fluctuations in the supersonic turbulent boundary layer may be induced by the density fluctuations, which are caused by the convection of the turbulent structures with nonuniform density distributions. And the contribution of the velocity fluctuations only to the low frequency fluctuations is observed. There are good similarities between the density fluctuations and the mass flux fluctuations for both the probability density distribution and the spectrum characteristics. On the contrary, a large difference between the fluctuations of velocity and density is identified. Therefore, the strong density fluctuations inside supersonic turbulent boundary layers, as well as its difference between the velocity fluctuations, should be one of the most important differences between compressible and incompressible turbulent boundary layers.

Simulation investigation of two droplets vertically impacting on solid surface simultaneously

Gao Ya-Jun, Jiang Han-Qiao, Li Jun-Jian, Zhao Yu-Yun, Hu Jin-Chuan, Chang Yuan-Hao
Acta Physica Sinica. 2017, 66 (2): 024702 doi: 10.7498/aps.66.024702
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The flow characteristic of the droplets impacting on solid surface is extremely significant for practical engineering applications. The problem is also very complicated since there are many parameters that may influence the process of droplets impacting on a solid surface. Therefore the numerical study of behaviors of droplets impacting on a solid surface is performed in this work. With a given impact velocity, two two-dimensional axisymmetric droplets subsequently interact on the solid surface. To conduct numerical simulations, a mass conserved level set method is adopted, and the gravity and surface tension are taken into consideration in the process of droplet development on the solid surface. The effects of Weber number, surface contact angle, the horizontal distance between the two droplets, and droplet arrangement on the dynamic behaviors of droplet impact are systematically investigated. It is found that two droplets vertically impacting on solid surface simultaneously can produce a columnar liquid jet column, and the horizontally spreading liquid on the solid surface will break up in several segments as time goes by. With the increase of Weber number, the secondary droplets are generated from liquid jet, and the columnar liquid jet rebounds away from the surface subsequently. If the Reynolds number, surface contact angle and the horizontal distance are set to be, respectively, 2000, 90°and 2, in particular, the non-dimensional length of liquid spread is unrelated to Weber number when the non-dimensional time T<2, and it increases as the Weber number increases when T>2. Meanwhile, the dynamic change characteristics of the non-dimensional liquid jet height are about the same during the jet rising, but the jet falling time becomes shorter as the Weber number decreases. Obviously, the bigger the Weber number, the bigger the biggest non-dimensional height of liquid jet and length of liquid spread are. On the other hand, with the increase of surface contact angle, the columnar liquid jet rebounds away from the surface and the spreading liquid breaks up much earlier on the surface. Also, the non-dimensional height of liquid jet and length of liquid spread grow with the increase of surface contact angle. In addition, in the case that the Weber number, Reynolds number and surface contact angle are set to be 32, 2000 and 90° respectively, we also find that the correlation between the biggest non-dimensional jet height and horizontal distance is not monotonic. Under the circumstances, the biggest non-dimensional height of liquid jet is achieved when the distance is set to be 2, and the phenomenon of liquid jet rebound occurs subsequently, whether the rebound phenomenon of the jet liquid column is related to the horizontal distance of the droplet or not. And finally, as the horizontal distance between the two droplets increases from 1.5 to 3, the non-dimensional length of liquid spread gradually increases.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

Three-dimensional direct numerical simulation of helicon discharge

Yang Xiong, Cheng Mou-Sen, Wang Mo-Ge, Li Xiao-Kang
Acta Physica Sinica. 2017, 66 (2): 025201 doi: 10.7498/aps.66.025201
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With the detailed consideration of electrochemical reactions and collision relations, a direct numerical simulation model of helicon plasma discharge with three-dimensional fluid-dynamic equations is proposed in the present work. It can improve the precision of results and widen the model applicability by discarding the small perturbation theory in previous helicon models which are partially analytical in essence. Under the assumption of weak ionization, the Maxwell equations coupled with the plasma parameters are directly solved in the whole computational domain. Thus the energy deposited from electromagnetic wave to plasma can be then easily calculated. The values of plasma parameters which include electron density, mean electron energy and heavy species density are obtained by solving a set of drift-diffusion equations. Meanwhile, seven kinds of chemical reactions in the plasma and two kinds of surface reactions on the wall are taken into account. All of the partial differential equations are solved by the finite element solver of COMSOL MultiphysicsTM with the full coupled method.#br#The results of numerical cases employing argon as the working medium show that there exists a sharp density jump from a low to high value as the radiofrequency power is raised. The density jump phenomenon is in accordance with the experimental results of Toki (Toki K, Shinohara S, Tanikawa T, Shamrai K P 2006 Thin Solid Films 506-507 597) and Chen (Chen F F 2007 Plasma Sources Sci. Technol. 16 593). The electron temperature decreases with an increase of the gas pressure, which is similar to Toki's (Toki K, Shinohara S, Tanikawa T, Shamrai K P 2006 Thin Solid Films 506-507 597) measurement by a RF compensation probe. In comparison with the classical sheath theory, the simulation result demonstrates that the distribution of parameters such as particle number density, the Deby length, electric potential and electron temperature can be solved exactly. In addition, the phenomenon of low-field density peak in helicon discharge was studied in the work. Previous research by Chen (Chen F F 2003 Phys. Plasmas 10 2586) suggests that this peak is caused by constructive interference from the reflected wave. The effect of length of the discharge chamber on the relation of electron density and background magnetic field is investigated numerically. The results validate the mechanism of wave interference reflected by endplates of the discharge chamber. Furthermore, the time-averaged magnetic energy density has more than one peak on the axial direction. Comparing the distribution of the magnetic energy density to that of the dimensionless amplitude of the helicon wave and the TG wave in the one-dimensional undamped condition, it found that the length of peak to peak of the helicon wave is just as twice as that of the magnetic energy density, which indicates that the substance of wave interference is involved in the standing wave generated by the helicon wave and its reflected wave from endplates.

Equivalence of energy deposition profile in target between electron beam of multi-energy composite spectrum and X-ray

Liu Xue, Ran Xian-Wen, Xu Zhi-Hong, Tang Wen-Hui
Acta Physica Sinica. 2017, 66 (2): 025202 doi: 10.7498/aps.66.025202
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It has great significance to study the thermal-mechanical effects of X-ray in assessing the viability of space-crafts, the penetration ability of missiles and testing the effectiveness of the anti-nuclear reinforcement measures. However, it is rather difficult to construct a suitable X-ray source in laboratory. During recent decades, pulsed electron beam with multi-energy composite spectrum has become a most important simulation source of X-ray to study its thermal-mechanical effects. And energy deposition profile in target material is the basis for studying the thermo-mechanical effects. However, under the same incident conditions, the energy deposition profile of pulsed electron beam with multi-energy composite spectrum in target material is extremely different from X-ray's, and the equivalence between the two beams is quite low. Thus, it is very important to adjust the energy spectrum and the incident mode of pulsed electron beam so as to improve their equivalence. In this paper, we use the energy deposition profiles of electron beam and X-ray in different kinds of material. MCNP is used to calculate their energy deposition profiles in target materials. Two kinds of blackbody X-rays with the equivalent temperatures of 3 and 5 keV and energy density of 200 J/cm2 are chosen for an optimization target. Aluminum, copper and titaniumare chosen as the target materials. Based on the change law of electron beam's energy deposition profile when the electron beam hits the target material at different incident angles, a theoretical model is established. Then, taking advantage of simulated annealing algorithm, we use the MATLAB to carry out numerical calculation and finally the numerical optimization results about the incident angle spectrum and energy density of electron beam are obtained. After optimization, the energy deposition of pulsed electron beam with multi-energy composite spectrum is well adjusted. The peak energy deposition and change of gradient of electron beam are of wonderful consistency with X-ray's. The equivalence of pulsed electron beam with multi-energy composite spectrum in simulating X-ray is also effectively improved. However, the energy density of adjusted pulsed electron beam should be much higher than 200 J/cm2. Electron beam designed by this paper can be used to better simulate the thermal-mechanical effects of X-ray in different kinds of materials.

Simulations of the cathode falling characteristics and its influence factors in atmospheric pressure dielectric barrier glow discharge pulse

Yao Cong-Wei, Ma Heng-Chi, Chang Zheng-Shi, Li Ping, Mu Hai-Bao, Zhang Guan-Jun
Acta Physica Sinica. 2017, 66 (2): 025203 doi: 10.7498/aps.66.025203
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A fluid model is built in this paper to describe and study the atmospheric pressure dielectric barrier glow discharge pulse in helium. The collision excitation and ionization reactions between electron and helium atom, heavy particles reactions, and Penning reaction between N2 and metastable He are taken into account in the fluid model. It is found that there are cathode falling, negative glow, Faraday dark, positive column and anode glow areas in atmospheric pressure glow discharge pulse, and the ranges of different areas are changing during the current falling edge. The ranges of cathode falling area are defined according to electron production balance position (definition 1, set as dc1) and the electrical field distribution around cathode (definition 2, set as dc2), respectively. Both dc1 and dc2 decreaseas the current grows to its peak in one discharge pulse, which reflects the transition from Townsend discharge to glow discharge. Compared with negative glow peak position, the boundary of cathode falling area by definition 1 is closer to cathode. However, the dc1 cannot reflect the cathode potential falling value and lose its definition after current peak moment. The dc2 can reflect the cathode potential falling value but it causes the overlapping between cathode falling and negative glow areas. At the current peak moment, the glow peak is located at the boundary of cathode falling area according to definition 2 while the glow peak is always located in the cathode falling area during the current falling edge. The cathode falling area characteristics can be influenced by different factors, e. g. applied voltage, secondary electron emission coefficient γ and N2 content. By changing applied voltage, it is found that the electrical potential dropping in cathode falling area increases as the average current density decreases, which indicates that the atmospheric pressure dielectric barrier glow discharge pulse is a subnormal glow discharge, and it is close to the normal glow discharge region. When γ <0.02, both dc1 and dc2 increase sharply with γ decreasing. When γ >0.02, dc1 and dc2 increase slowly with γ increasing. When N2 content is greater than 4 ppm, dc1 and dc2 first decrease and then increase slowly. The electrical potential falling of cathode is changeless with N2 content changing. However, the voltage across the gas gap decreases with N2 content changing because the Penning effect lowers the breakdown voltage of the gas gap. The spatial average current density has a highest value when N2 content is about 35 ppm, which also means that the spatial average charged particle density has the highest value in the same situation. Moreover, when the secondary electron emission coefficient is a constant, both dc1 and dc2 have negative linear relationship with the average current density.
CONDENSED MATTER:STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

Irradiation-induced modifications in the mechanical properties of borosilicate glass

Wang Tie-Shan, Zhang Duo-Fei, Chen Liang, Lü Peng, Du Xin, Yuan Wei, Yang Di
Acta Physica Sinica. 2017, 66 (2): 026101 doi: 10.7498/aps.66.026101
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Understanding the evolutions of the mechanical properties of borosilicate glasses under irradiation is crucial for evaluating their performances after long-term interaction with the irradiation environment in the disposal of high level nuclear waste.The variations of the mechanical properties of borosilicate glasses,induced by irradiation have been extensively studied.However,the mechanisms of variations in mechanical properties,induced by irradiation have not been clarified yet,especially when considering the effects of electronic and nuclear processes,respectively.To clarify this issue,a commercial borosilicate glass is investigated through an external irradiation of 5 MeV Xe ions and 1.2 MeV electrons in this paper.The nano-indentation test is used to study the changes of the hardness and modulus.The microstructure evolutions of Xe ion irradiated borosilicate glasses are characterized by Fourier transform infrared (FTIR) spectroscopy to discuss the mechanisms in the evolutions of mechanical properties.The nano-indentation results indicate that the hardness is reduced by 24%,and the modulus is lessened by 7.4% after the glass has been irradiated by Xe ions.Both the hardness and modulus variations reach their stable states when the total deposited energy is around 6.6×1021 keV/cm3.Although hardness and modulus are also observed to decrease by about 4.7% and 2.9%,resepectively, when the total deposited energy reaches approximately 1.4×1022 keV/cm3 after the glass has experienced the electron irradiation,the results still emphasize that the nuclear energy deposition is the major factor for the evolutions of the hardness and modulus of the borosilicate glass under ion irradiation.The decreases of hardness and modulus after the glass has experienced ion irradiation can be attributed to the deformation of glass network and volume expansion, which are induced by reducing the average ring size and transforming from[BO4] to[BO3] units.By considering the recovery resistance,it is found that the toughness of the borosilicate glass is significantly strengthened,and therefore the mechanical properties of the borosilicate glass are enhanced after the glass has been irradiated by Xe ions.Compared with the results after ion irradiation,the mechanical properties have negligible changes after electron irradiation.The present work is important for understanding both the irradiation effects on the hardness/modulus and the variations in the mechanical properties during the high level waste disposal.

A cluster-formula composition design approach based on the local short-range order in solid solution structure

Jiang Bei-Bei, Wang Qing, Dong Chuang
Acta Physica Sinica. 2017, 66 (2): 026102 doi: 10.7498/aps.66.026102
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The composition design is of importance for developing high-performance complex alloys and is also the primary step to realize a new mode for material development via theoretical prediction and experimental verification, in comparison with the traditional experience-oriented experiments. Traditional alloy design approaches, including Hume-Rothery rule, electron theories, equivalent method, computer simulation, etc., are first reviewed from the viewpoints of their theoretical basis and applicability to limitations. Almost all the traditional alloys are based on solid solution structures, in which the typical characteristic is the chemical short-range order (CSRO) of the solute distribution. We propose a cluster-plus-glue-atom model for stable solid solutions in light of CSRO. A cluster-formula composition design approach is presented for developing the multi-component high-performance alloys. The cluster-plus-glue-atom model classifies the solid solution structure into two parts, i.e., the cluster part and the glue atom part, where the clusters are centered by solute atoms, showing the strong interactions of clusters with the solvent base and the weak interactions of clusters with solute atoms. The clusters are the nearest-neighbor polyhedrons, being cuboctahedron with a coordination number of 12 (CN12) in FCC structure and rhombic dodecahedron with a CN14 in BCC structure, respectively. Then a uniform cluster-formula of[CN12/14 cluster](glue atom)x is achieved from the cluster model. Its wide applications in different multi-component alloy systems confirm its universality as a simple and accurate tool for multiple-component complex alloy composition design. Such alloy systems include corrosion-resistant Cu alloys, high-performance Ni-base superalloys, high-strength maraging stainless steels, Ti/Zr alloys with low Young's modulus, high-entropy alloys, amorphous metallic glasses, quasicrystals, etc.. The specific alloy design steps are incarnated in the βup-Ti alloys with low Young's modulus. Firstly, the necessary alloying elements are chosen according to the service requirements (BCC stability and low Young's modulus). Secondly, the local cluster unit to present CSRO and the corresponding cluster formula of[(Mo, Sn)-(Ti, Zr)14](Nb, Ta)x are built, in which the occupations of the alloying elements in the cluster formula are determined by the enthalpy of mixing ΔH between them with the base Ti. Thirdly, these designed alloys are verified experimentally, and the lowest Young's modulus appears at the βup-[(Mo0.5Sn0.5)-(Ti13Zr1)]Nb1. Finally, a new Mo equivalent formula under the guidance of phase diagram features is proposed to characterize the structural stability of Ti alloy. Thus all the Ti alloy compositions with different structural types can be expressed with a uniform cluster formula, in which the structural types of alloys are determined by the Mo equivalent.

Damage effects of proton beam irradiation on single layer graphene

Zhang Ning, Zhang Xin, Yang Ai-Xiang, Ba De-Dong, Feng Zhan-Zu, Chen Yi-Feng, Shao Jian-Xiong, Chen Xi-Meng
Acta Physica Sinica. 2017, 66 (2): 026103 doi: 10.7498/aps.66.026103
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Graphene was first discovered in 2004 (Novoselov K S, et al. 2004 Science 306 666), it is a single atomic layer of sp2-bonded carbon atoms arranged in a honeycomb-like lattice. According to its extraordinary electronic, mechanical, thermal and optical properties, one can expect it to have a variety of applications in nanoscale electronics, composite materials, energy storage, and biomedicine fields. Although many experimental and theoretical studies on graphene have been carried, there still exist many obstacles to its applications. A representative example is nanoscale electronics (e.g., field-effect transistors and optoelectronic devices) that requires non-zero band-gap. Therefore, introducing defects into graphene and leading to band-gap opening are key steps for its technique applications.
Recently, ion beam irradiation as a defects introducing technique was performed by Lee et al. (2015 Appl. Surf. Sci. 344 52) and Zeng et al. (2016 Carbon 100 16) through 5, 10, and 15 MeV protons and highly charged ions (HCIs) irradiating the graphene separately. Considering the advantages of simplity for preparing samples and feasibility in atmospheric condition of Raman spectroscopy compared with common characterization techniques (high resolution transmission electron microscopy, scanning electron microscopy, atomic force microscopy) for nano-materials, in both studies, Raman spectroscopy is used to obtain the evolution of ID/IG (ID is the peak intensity excited by defects, IG is the peak intensity origining from lateral vibration of carbon atoms) with different energies and fluences, respectively. In this work, considered are the following points:1) the absence of quantitive characterization for defects in the above two studies; 2) the low displacement energy of 25 eV required for a carbon atom to be knocked out (Zhao S J, et al. 2012 Nanotechnology 23 285703); 3) the complex interaction between HCIs and material. The irradiation effects of single layer graphene on silicon substrate are investigated by 750 keV and 1 MeV proton bombarding. This introduces the defects into graphene and thus leads to band-gap opening. By comparing Raman spectra of the samples before and after irradiation, a quantitive characterization about defects in graphene is achieved. Detailed analysis shows that 1) the value of ID/IG increases with the energy loss of incident proton, which is consistent with the result of SRIM simulation; 2) the average distance of defects LD increases with the incident proton energy; 3) the defect density nD decreases with the incident proton energy. These indicate that the damage effect for MeV protons in single layer graphene with substrate is similar to those in three-dimensional materials. The method presented here may facilitate the understanding of the physical mechanism of MeV proton interaction with two-dimensional materials, and provide a potential way of controlling the electronic structure and band-gap.

Structure of NO dimer multilayer on Rh(111)

Wang Chen-Chao, Wu Tai-Quan, Wang Xin-Yan, Jiang Ying
Acta Physica Sinica. 2017, 66 (2): 026301 doi: 10.7498/aps.66.026301
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Molecular self-assembly is the spontaneous organization of molecules under thermodynamic equilibrium conditions into well-defined arrangements via cooperative effects between chemical bonds and weak noncovalent interactions. Molecules undergo self-association without external instruction to form hierarchical structures. Molecular self-assembly is ubiquitous in nature and has recently emerged as a new strategy in chemical biosynthesis, polymer science and engineering. NO monomer is apt to be absorbed on the surfaces of some metals such as Ir(111), Ni(111), Pd(111), Pt(111), Rh(111) and Au(111), and the interactions of NO monomer with the metal surfaces have been extensively studied. When NO monomer is weakly adsorbed on the noble-metal surface, it cannot be reduced completely but forms a stable structure, which is named NO dimer. The first-principle technique is employed to determine the structures of NO dimer ((NO)2) molecular chains and monolayers on virtual Rh(111), as well as (NO)2 monolayer and multilayer on Rh(111). First, (NO)2 monomers are assembled into two stable molecular chains on the virtual Rh(111) surface, whose bind energies are 0.309 and 0.266 eV, respectively. The molecular chains are self-assembly systems, in which (NO)2 monomers are parallel and ordered, and the O atoms and N atoms are shown to be of (100) and (111) structures, respectively. Then, the two molecular chains are assembled into two stable monolayers (denoted as M1 and M2) on the virtual Rh(111)-(1×√3), and the coverage is 1.00 ML. In the M1 monolayer, the angle between the N–N bond of (NO)2 monomer and the substrate is in a range of 70°-90°, and in the M2 monolayer, the N–N bond is parallel to the substrate.
In the adsorption system of M2/Rh(111), (NO)2 molecules can be adsorbed on the top as well as the hcp and fcc hollow sites. When (NO)2 molecules are adsorbed on the top site, the adsorption system is best described by the electron structure Rh+0.14–N0=O-0.14, and when (NO)2 molecules are absorbed on the two hollow sites, the adsorption system is described by the electron structure Rh+0.34–N-0.18=O-0.16. Therefore, (NO)2 molecules are more apt to be adsorbed on the two hollow sites than on the top site. In the adsorption systems of M1+M2/Rh(111) and M1+(M1+M2)/Rh(111), (NO)2 molecules are adsorbed vertically on the two hollow sites, the N–N bond is parallel to the substrate in the first monolayer, and the angle between the N–N bond and the substrate is in a range of 70°-90° in the second and third monolayers. The interaction between the neighbor monolayers is about 0.01 eV, and the thickness of the vacuum layer is 0.31 nm±0.02 nm.

CONDENSED MATTER:ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

Dependences of valence electronic structure on magnetic moment and electrical resistivity of metals Hot!

Qi Wei-Hua, Ma Li, Li Zhuang-Zhi, Tang Gui-De, Wu Guang-Heng
Acta Physica Sinica. 2017, 66 (2): 027101 doi: 10.7498/aps.66.027101
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Conventionally, the energy band theory is used to explain the magnetic and electrical transport properties of metals. However, so far, there has been no quantitative explanation of the relations between the average magnetic moment per atom and the resistivity for Fe, nor Ni, nor Co metals. In this paper, a new itinerant electron model for magnetic metal is proposed on the basis of electron distribution theory at the energy level. 1) In the process of free atoms forming the metal solid, most of the 4s electrons of Fe, Ni and Co enter into the 3d orbits subjected to the Pauli repulsive force, and the remaining 4s electrons form free electrons. 2) Since the average number of 3d electrons is not an integer, a part of atoms have one 3d electron more than the other atoms. These excess 3d electrons have a certain probability to itinerate between the 3d orbits of the adjacent atoms as itinerant electrons; and the other 3d electrons are local electrons. 3) The transition probability of itinerant electrons is very low, thus the contribution to metal resistivity from itinerant electrons is far lower than that from free electrons. Resistivity of metal decreases with increasing the number of free electrons. Therefore, using the observed values of average atomic magnetic moments, 2.22, 0.62 and 1.72 μB, the average numbers of free electrons in Fe, Ni and Co can be calculated to be 0.22, 0.62 and 0.72, respectively. This is the reason why the electrical resistivities of Fe, Ni and Co (8.6, 6.14 and 5.57 μΩ-cm) decease successively. In addition, according to this model, the average number of 3d electrons per atom in Ni metal is 9.38. This indicates that 38% of atoms in Ni metal have ten 3d electrons, forming a full 3d sub-shell, as in Cu or Zn atoms. The 3d electrons in these atoms are difficult to itinerate or exchange. This may be the reason why the Curie temperature of Ni metal (631 K) is far lower than those of Fe and Co metals (1043 and 1404 K). On the basis of the energy band theory, the numbers of 3d electrons in Fe, Ni and Co metals are 7.4, 9.4 and 8.3, which are close to our results (7.78, 9.38 and 8.28), respectively. This indicates that our model is consistent with the energy band theory. Compared with the complex energy band theory, a simple and effective method on investigating valence electron structures through the experimental average magnetic moments per atom in a metal is presented based on our model. Therefore, the new itinerant electron model may be a new clue to understanding the electronic structure of metals and alloys.

Microstructure, resistivity, and hardness of aged Ag-7wt.%Cu alloy

Li Rui, Zuo Xiao-Wei, Wang En-Gang
Acta Physica Sinica. 2017, 66 (2): 027401 doi: 10.7498/aps.66.027401
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Ag-Cu alloys are used as both decorative materials because of beautiful appearance, and conductors due to excellent combinations of strength and electrical conductivity. The strength and electrical conductivity of Ag-Cu alloy are closely related to precipitation behavior of Cu-rich phase in Ag matrix. The morphology, size and volume fraction of Cu-rich phase have been highly concerned. In this work, a series of aging temperatures is used in both supersaturated solid-solution and cold-rolled Ag-7wt.%Cu samples to investigate the relationship between the precipitation behavior of Cu-rich phase and property by using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, and properties measurements (hardness and resistivity). The DSC results of as-solid-solution Ag-7wt.%Cu alloy show a distinct exothermic precipitation reaction of Cu out of Ag matrix ranging from 300 °C to 350 °C, and the activation energy is estimated to be (111±1.6) kJ/mol according to Kissinger equation. Because of the existence of deformation energy, the DSC results of cold-rolled Ag-7wt.%Cu sample show a distinct exothermic precipitation reaction of Cu from Ag matrix between 290 °C and 330 °C, and the activation energy is (128±12) kJ/mol. XRD analysis indicates that the dissolved Cu in Ag is dependent on ageing temperature, and the change of solubility of Cu in Ag is calculated by XRD curve. Microstructural analysis demonstrates that spherical Cu-rich phases are precipitated from Ag-matrix at 450 °C in both solid-solution and cold-rolled Ag-7wt.%Cu alloys. Moreover, the banded structure of Cu-rich phase is found in the solid-solution sample after being aged at 450 °C. The deformation twinning Ag is found in the cold-rolled sample. The precipitation and dissolution of Cu-rich phase in Ag matrix play important roles in the resistivity and microhardness. With ageing temperature increasing (ageing temperatures range from 200 to 450 °C), the electrical resistivity of as-solid-solution aged sample decreases and the microhardness increases, however, both electrical resistivity and microhardness of as-cold-rolled aged sample decrease. With ageing temperature increasing further (over 450 °C), the electrical resistivity increases and the microhardness decreases in both aged samples. Because of the formations of dislocation and deformation twinning Ag, the microhardness of cold-rolled sample reaches to 217 HV, which is higher than that of solid-solution sample. Strengthening and electrical resistivity models are built based on the microstructural characterization and concentration contributions. These theoretical predictions are in good agreement with experimental values. Our model demonstrates that the precipitation and dissloution of Cu in Ag significantly affect the electrical conductivity, and dislocation and deformation twinning play important roles in microhardess in Ag-Cu alloy. This work clarifies the influencing mechanism of different microstructures on the microhardness and resistivity of Ag-Cu alloy.

Structural and photoelectrical properties of AZO thin films improved by Ag buffer layers

Cheng Jing-Yun, Kang Chao-Yang, Zong Hai-Tao, Cao Guo-Hua, Li Ming
Acta Physica Sinica. 2017, 66 (2): 027702 doi: 10.7498/aps.66.027702
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In order to obtain more excellent photoelectric properties of transparent conductive film, a series of high-quality AZO thin films and AZO/Ag/AZO thin films with various thickness values of Ag buffer layers are prepared on glass substrates by the radio frequency magnetron sputtering method at room temperature. The phase and surface morphologies of films are characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM) respectively. The technology of Hall effect measurement and ultraviolet, visible spectrophotometer are employed to investigate the photoelectric properties of films. The electrical properties (including sheet resistance, sheet concentration and mobility) of films are also determined by using non isothermal technique to explore their thermal stability performances. The results indicate that the thickness values of Ag buffer layers have a large influence on the crystalline structures and photoelectric properties of AZO thin films. The XRD results show that with the increase of the thickness of Ag, the diffraction peak of Ag (111) is gradually enhanced, the ZnO (002) diffraction peak is gradually weakened, and the preferred orientation of ZnO (002) crystal plane is weakened. AFM test indicates that the change of Ag layer thickness has a great influence on the surface growth mode of the upper layer AZO thin film. When the Ag layer thickness is less than 5 nm, AZO thin film surface is rough and the grain size is smaller. When the Ag layer thickness is larger than 10 nm, the continuous surfaces of multilayer films begin to be shaped, directly affecte the photoelectric properties of the films. Hall effect measurement and transmittance test show that with the increase of Ag layer thickness, the transmission of AZO/Ag/AZO multilayer film gradually decreases, and also the resistance gradually decreases. When the thickness of Ag layer is 10 nm, AZO(30 nm)/Ag(10 nm)/AZO(30 nm) thin film gains a best figure of merit of 1.59×10-1 Ω-1 an average transmittance of 84.2% and a sheet resistance of 0.75 Ω/sq. Hall effect measurement versus temperature indicates that AZO film without an Ag layer proves to be subjecte to the regular change of semiconductor resistance with temperature. When adding an Ag layer, the trend of the relationship of resistance with temperature presentes the characteristic of that metal resistance relating to temperature. Moreover, the sheet concentration of AZO with Ag layer is higher than that of AZO. The highest sheet concentration and the excellent thermal stability are obtained on AZO/Ag (10 nm)/AZO. The changes of the mobility of AZO under different temperatures turn out to be poorly stable. However, when adding an Ag layer, the better stability of AZO/Ag/AZO can be obtained. In conclusion, the photoelectric properties of films own excellent thermal stabilities with optimum thickness of Ag layer.

Mode identification via temperature variation in resonant ultrasonic spectroscopy technique for piezoelectric material

Tang Li-Guo
Acta Physica Sinica. 2017, 66 (2): 027703 doi: 10.7498/aps.66.027703
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The full matrix material constants of piezoelectric materials should be characterized first before they have been used to make actuators or sensors. Up to now, they are usually determined by the ultrasonic pulse-echo and electric impedance resonance techniques through using multiple samples with drastically different sizes. However, the constants determined by the aforementioned techniques are probably inconsistent because the sample-to-sample variation cannot be eliminated. The technique of resonant ultrasonic spectroscopy (RUS) only needs one sample to determine the full matrix constants of piezoelectric material. Therefore, the consistency of the constants is guaranteed. During the implementation of the RUS technique, the elastic stiffness cijE and piezoelectric constants cij can be determined from the resonance modes identified from the resonant ultrasonic spectrum. The free and clamped dielectric constants cannot be determined by the RUS technique because they have very weak influence on resonance frequency. However, they can be directly measured from the same sample by using an impedance analyzer. To ensure the reliable inversion of material constants, enough resonance modes should be identified from the measured resonant ultrasonic spectrum. However, there are many missing and overlapped modes in the spectrum, which makes mode identification become a biggest obstacle to the implementation of the RUS technique. The adjacent modes may overlap if the resonance frequencies corresponding to them have a very small difference. In addition, the lower the mechanical quality factor QM, the more likely to overlap the adjacent modes are. During the RUS measurement, the rectangular parallelepiped sample is placed between the transmitting and receiving transducers with contacts only at the opposite corners of the sample. Resonance modes would not be detected if the receiving point, i.e., one corner of the sample, is the node of these modes. Therefore, there are missing modes in the resonant ultrasonic spectrum. To overcome the difficulty in identifying the modes, caused by modes missing and overlapping, the mode identifying method via temperature variation is presented in this study. Note that a change of temperature may change the material properties of a piezoelectric sample. The material properties have a great influence on the resonance frequency of the sample. Moreover, the influences corresponding to resonance modes are different. Therefore, the variation of temperature may make the overlapped modes separated from each other and the missing modes appear, namely, the missing and overlapped modes may be identified by comparing the resonant ultrasonic spectra measured at different temperatures. The experimental results of piezoelectric ceramics (PZT-8) show that this method can effectively improve the accuracy of mode identification and guarantee the reliability of inversion in the RUS technique.

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