bluuebunny/crossref_metadata_2025 huggingface dataset:
Principle and application of diagonal reducing method in the complex noise fields
Authors: Xia Hui-Jun  , Ma Yuan-Liang  , Liu Ya-Xiong  ,   School of Marine Science and Technology, Northwestern Polytechnical University, Xi'an 710072, China;,   Key Laboratory of Ocean Acoustics and Sensing(Northwestern Polytechnical University), Ministry of Industry and Information Technology, Xi'an 710072, China;,   Kingsignal Defence, Changsha 410000, China
Published: December 2020

Acoustic environment has low signal-to-noise ratio(SNR); hence, array signal processing is always used for reducing noise and enhancing signal. Because the delay-and-sum beam forming method is robust, so it is almost widely used, but the array gain is limited by the array aperture. The actual underwater ambient noise is complex, which includes uncorrelated noise and correlated noise. The noise powers of array elements are unequal to each other. The noise covariance matrix is not a scaled identity matrix. Consequently, the performance of array signal processing method decreases obviously. Aiming at these two problems, a diagonal reducing method of the covariance matrix in the complex noise field is proposed. Firstly, a reducing matrix, which is defined as a diagonal matrix with unequal diagonal elements, is subtracted from the covariance matrix so as to reduce the noise, and a new matrix is obtained. Secondly, the delay-and-sum beamforming is done by using the new matrix to obtain the beaming output. The analytic solution and approximate solution of reducing matrix are obtained under the constraint condition that the output SNR attains its maximum. Thirdly, the estimation of the reducing matrix is determined by minimizing the function that is defined as the error between the covariance matrix and the estimated covariance matrix. This minimization problem is accomplished in an iterative method. Fourthly, if the noise is uniform white noise or the nonuniform white noise, this proposed method performs well. While, under the complex noise field the performance of the proposed method may be deteriorated. So the effects of the correlation of the noise field and the input SNR on the estimated error are analyzed. In fact, the weaker the correlation is, or the larger the input SNR is, the smaller the estimated error is. Lastly, the simulation experiment and the lake trial are implemented. The simulation results show that the diagonal reducing method of the covariance matrix reduces some ambient noises, the noise output power decreases, the output SNR increases, and the proposed method improves the performance of array signal processing. The experimental results show that the output SNR of the target by using the proposed method is increased by about 14 dB. The diagonal reducing method of covariance matrix has definite value for engineering application, and is computationally attractive.
Variations of the electrical conductivity and the Fermi velocity of epitaxial graphene with temperature
Authors: Du Yi-Shuai  , Kang Wei  , Zheng Rui-Lun  ,   College of Electronic and Electrical Engineering, Chongqing University of Arts and Sciences, Yongchuan 402160, China;,   College of science, Chongqing University of Posts and Telecommunication, Chongqing 400065, China
Published: December 2020

The atomic anharmonic vibration and the electron-phonon interaction are considered, and then a physical model about the metal-based epitaxial graphene is built. Variations of the electrical conductivity and the Fermi velocity with temperature for the metal-based epitaxial graphene are given based on the solid state physics theory or method. The alkali-metal epitaxial graphene is selected as the substrate, and then the influences of substrate material, electron-phonon interaction and the anharmonic vibration on the electrical conductivity and the Fermi velocity of epitaxial graphene are discussed. Some results are shown as follows. Firstly, at zero temperature, the electrical conductivity and the Fermi velocity of the alkali-metal-base epitaxial graphene increase with the number of the atoms in substrate material increasing. Secondly, the electrical conductivity of epitaxial graphene decreases with temperature rising. Furthermore, the variation rate also decreases with temperature rising. Generally, the electrical conductivity originates mainly from electrons and phones. The electronic contribution to the electrical conductivity varies with temperature slowly, but the phone contribution to electrical conductivity varies with temperature evidently. Therefore, the contribution of phonons to electrical conductivity is much larger than that of electrons. Furthermore, the contribution increases with the number of atoms in basal elements. The phonon contribution to conductivity decreases with temperature rising, but it is unrelated to the basal elements. Thirdly, the Fermi velocity of the epitaxial graphene increases with temperature slowly. The variation of the Fermi velocity with temperature decreases with the increase of interaction between the graphene and the basal atoms. However, it increases with the number of atoms of the basal materials. The anharmonic effect causes important influences on the electrical conductivity and the Fermi velocity. Under the harmonic approximation the velocity is constant. However, the conductance increases rapidly with temperature. With considering the atomic anharmonic terms, the Fermi velocity increases with temperature. The variation of the electrical conductivity with temperature increasing becomes slower. If the temperature is higher, the anharmonic effects become more evident.
Receptivity of the steady cross-flow vortices in three-dimensional boundary layer
Authors: Shen Lu-Yu  , Lu Chang-Gen  ,   School of Marine Science, Nanjing University of Information Science and Technology, Nanjing 210044, China
Published: December 2020

The prediction and control of the laminar-turbulent transition are always one of the most concerned frontiers and hot topics.Receptivity is the initial stage of the laminar-turbulent transition process in the boundary layer,which decides the physical process of the turbulent formation.To date,the researches of receptivity in the three-dimensional boundary layer are much less than those in the two-dimensional boundary layer;while most of the real laminar-turbulent transition in practical engineering occurs in three-dimensional boundary layers.Therefore,receptivity under the threedimensional wall local roughness in a typical three-dimensional boundary layer,i.e.,a 45° back swept infinite flat plate, is numerically studied.And a numerical method for direct numerical simulation (DNS) is constructed in this paper by using fourth order modified Runge-Kutta scheme for temporal march and high-order compact finite difference schemes based on non-uniform mesh for spatial discretization:the convective term is discretized by fifth-order upwind compact finite difference schemes;the pressure term is discretized by sixth-order compact finite difference schemes;the viscous term is discretized by fifth-order compact finite difference schemes;and the pressure equation is solved by third-order finite difference schemes based on non-uniform mesh.As a result,the excited steady cross-flow vortices are observed in the three-dimensional boundary layer.In addition,the relations of three-dimensional boundary-layer receptivity with the length,the width,and the height of three-dimensional wall localized roughness respectively are also ascertained.Then, the influences of the different distributions,the geometrical shapes,and the location to the flat-plate leading-edge of the three-dimensional wall local roughness,and multiple three-dimensional wall local roughness distributed in streamwise and spanwise directions on three-dimensional boundary-layer receptivity are considered.Finally,the effect of the distance between the midpoint of the three-dimensional wall localized roughness and the back-swept angle on three-dimensional boundary-layer receptivity is studied.The intensive research of receptivity in the three-dimensional boundary-layer receptivity will provide the basic theory for awareness and understanding of the laminar-turbulent transition.
Effects of the doping of Al and O interstitial atoms on thermodynamic properties of -Al2O3:first-principles calculations
Authors: Huang Ao  , Lu Zhi-Peng  , Zhou Meng  , Zhou Xiao-Yun  , Tao Ying-Qi  , Sun Peng  , Zhang Jun-Tao  , Zhang Ting-Bo  ,   Department of Mathematics and Physics, Officers College of CAPF, Chengdu 610213, China;,   Institute of Chemical Materials, China Academy of Engineering Physics(CAEP), Mianyang 621900, China
Published: December 2020

Al particles are widely used as a metal reductant in the thermite, and a native Al2O3 film always forms on the particle surface as a passivating oxide shell. The diffusions of Al and O atom through the oxide shell will influence the structure and thermodynamic properties of Al2O3, and thus the ignition process of the thermite. In this work, the thermodynamics properties of -Al2O3, -Al2O3 doped by Al interstitial atom and -Al2O3 doped by O interstitial atom under high pressure and temperature are comparatively investigated by the first-principles calculations based on density-functional theory and quasi-harhmonic Debye model. The effects of the doping of Al and O interstitial atoms on the thermodynamic properties of -Al2O3 are discussed. The results indicate that the doping of the Al and O interstitial atoms will reduce the bulk modulus, and increase the volume thermal expansion coefficient and constant volume heat capacity of -Al2O3. Therefore, the diffusions of Al and O atom will make the oxide shell more ductile, and adverse to the spallation during the ignition of Al particles.
Latest studies on resistance switching of molecular thin films embedded with nanoparticles
Authors: Li Jian-Chang  , Shao Si-Jia  ,   Vacuum and Fluid Engineering Research Center, Northeastern University, Shenyang 110819, China
Published: December 2020

Resistive switching of molecular film incorporated with nanoparticles(NPs) has become a hot topic in the information storage industry, which is systematically reviewed from the aspects of electrodes, film structure, NPs, switching mechanism and mechanical properties. There are three sorts of structures i.e., layered, core-shell and complexed films, in which the film thickness affects the device charge transport and switching performance to a large extent. Usually, higher on/off ratio and lower threshold voltage can be expected for device with less-conductive active layers than that with more conductive ones. As a key factor, the interfaces of electrode/organic and molecule/NPs may largely affect the switching performance. It is shown that the type, size and distribution of NPs and molecular structure govern the interfacial behaviors, which in turn influences the switching mechanisms including filament formation/ rupture, charge trapping/ detrapping or charge transfer. For the case of filament theory, it may be ascribed to metallic, oxygen vacant or carbon-rich model. The as-embedded NPs can be classified as metal, metal oxide and/or carbon-like materials such as Au, Ag, Al, ZnO, TiO2, or graphene etc. The Au NPs show distinguishing features of little diameter, high chemical stability and large work function. On the other hand, the metal oxide NPs may form deep interfacial barrier with the target molecules and thus improve the switching characteristics. Small molecular-weight organics are also studied as embedding materials complexed with polymers as to strengthen the switching properties, and charge transfer is believed to be responsible for such an enhancement. Except for concentration and diameter of the NPs, their distribution in the active layer critically influences the memory behavior. The NPs can be made onto the molecular layer in-situ by vacuum thermal evaporation of different metals or sputtering deposition of various metal oxides. In such cases, the thickness of the deposition layer is a key parameter to obtain good switching performance. Although great progress has been made for static devices in small-scale, it is crucial to develop roll-to-roll manufacturing, precise NPs' distribution and dynamic mechanical properties in order to fabricate large-scale, low-cost and flexible memory devices. It still needs hard work on understanding the switching mechanism and engineering the interfacial properties of molecule/electrode and molecule/NPs, especially under bending conditions. New techniques should be developed to fabricate organic memory films embedded with NPs so as to avoid the problems of pinhole, effects of solvent and dust normally existing in traditional spin-coating films.
Signal-to-noise ratio of spin noise spectroscopy in rubidium vapor
Authors: Shi Ping  , Ma Jian  , Qian Xuan  , Ji Yang  , Li Wei  ,   SKLSM, Institute of Semiconductors, Chinese Academy of Science, Beijing 100083, China;,   Faculty of Maritime Technology and Operations, Norwegian University of Science and Technology, Aalesund 6025, Norway
Published: December 2020

Spin noise spectroscopy is a non-demolition technique to detect the spin dynamics, and it is a good way to realize spin property under thermal equilibrium. Since spin noise arises from spin fluctuation at thermal equilibrium, it is a weak signal, therefore, various methods are used to enhance the signal-to-noise ratio(SNR) of the measurement system. To study the influence from different factors on the quality of spin noise spectroscopy, we report spin noise spectroscopy measurements in Rubidium vapor with three methods: a commercial frequency analyzer, a data acquisition card(DAC) with fast Fourier transform(FFT) done by a computer, and a DAC with real-time FFT based on FPGA(field-programmable gate array), respectively. According to the experimental results, we discuss several parameters and their influences on the SNR of the spectrum, including spectrum accumulation time, measurement efficiency and acquisition resolution. We find that the accumulation time is the most important factor for achieving high-quality spectrum. Measurement efficiency indicates how a good quality of the spin noise spectroscopy can be achieved in a finite time period, and we make a comparison of measurement efficiency among three methods. However, improvement of acquisition resolution does not make much more contribution to the quality of spin noise spectroscopy. Taken all into account, the DAC with real-time FFT performs best due to its bigger data utilization ratio, higher measurement efficiency and the multiplex advantage, thus it is more helpful for spin noise spectroscopy measurement in the study of spin dynamics.
Transmission characteristics of surface plasmon polaritons in -shaped resonator
Authors: Wang Wei  , Gao She-Sheng  , Meng Yang  ,   School of Automatics, Northwestern Polytechnical University, Xi'an 710072, China
Published: December 2020

To improve the efficiency of transmission, in this paper, we propose a structure of the surface plasmon polariton embedded in a sliver circular resonator with a sliver nanoellispod(-shaped resonator), and also investigate its optical properties by the finite element method. Firstly, we study the optical properties of -shaped resonator at a=120 nm and =0 with different values of b. The results show that the -shaped resonator structure has the narrow transmission peaks, and the transmittance spectrum can be tuned by modifying the structure parameters. So this nanostructure would find applications in the designing of the novel filter. Secondly, compared with the former Fano resonance which results from the localized plasmon resonance coupling, the Fano resonance which results from the resonance of the surface plasmon polaritons coupling is represented by this structure. When the symmetry of -shaped resonator is broken, the Fano resonance will be observed clearly. Like the Fano resonance which results from the localized plasmon resonance coupling between the bright mode of metallic nanostructure and the dark mode of metallic nanostructure, the results show that the dipolar, quadrupolar, and octupolar Fano resonances of -shaped resonator structure occur, which are caused by the destructive interference between the bright dipolar mode and the dark dipolar mode, quadrupolar mode, and octupolar mode. When we take the rotation angle as 0 and 90, 15 and 75, 30 and 90 respectively, the Fano asymmetric transmittance spectra of -shaped resonator are similar, which result from the same degree of asymmetry. The larger the degree of asymmetry of the -shaped resonator structure, the more obvious the Fano resonance is. Thirdly, the size of this structure has significant effects on the transmission peak positions, line width, and intensity of the Fano resonance, in particular, in the case that =0 corresponds to the generation of FR(FR on) and in the case corresponding to the vanishing of FR(FR off). therefore, this phenomenon of -shaped resonator will provide a new strategy for the surface plasmon polariton Fano switch. We hope that this nanostructure has potential applications in designing filter, biological sensors, and Fano switch.
Ferromagnetism of Al-doped 6H-SiC and theoretical calculation
Authors: Huang Yi-Hua  , Jiang Dong-Liang  , Zhang Hui  , Chen Zhong-Ming  , Huang Zheng-Ren  ,   Structural Ceramics Engineering Research Center, The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
Published: December 2020

SiC with d0 ferromagnetism is thought to be one of the most important materials in the spintronics field, and it has received widespread attention. In this paper, Al: SiC magnetic powder is fabricated by high temperature calcination method with the protection of Ar gas. X-ray diffraction results show that the obtained powder is of 6H-SiC phase, and Al is proposed to enter into the 6H-SiC crystalline. Raman results show that Ar gas plays a crucial role in impeding the SiC from decomposing at high temperature. With the protection of Ar gas, it maintains round shape after calcination about 2200℃, no any other peakis detected in the Raman spectrum. Without the protection of Ar gas, SiC particle would decompose into graphite, and the instinct peak of graphite is detected in the Raman spectrum. Energy dispersive spectrometer results show that there is 0.96 at% Al in the powder. The obtained powder shows magnificent magnetic hysteresis loop and large coercive force. Its saturation magnetic moment reaches 0.07 emu/g after calcination at 1800℃. Its coercive force reaches a maximum after calcination at 2000℃, while the saturation magnetic moment is 0.012 emu/g. With the rise of calcination temperature, the magnetism of the powder changes from diamagnetism to ferromagnetism. But when the calcination temperature rises to 2200℃ or more, it would change back to diamagnetism. The phenomenon of ferromagnetism disappearing is similar to that in ZnO as reported. The total quantity of magnetic impurities(Fe, Co, Ni) is evaluated to be less than 5 ppm. Saturation magnetic moments arising from these impurities can be calculated to be less than 10-5 emu/g according to the reported results, which is impossible to affect the accuracy in the experiment. Thus it is proposed that the ferromagnetism originates from the doping of Al in SiC powder. To understand the origin of the observed magnetism, we carry out first principles calculations based on spin polarized density functional theory. All the calculations are performed by using the generalized gradient approximation in the form of the Perdew-Burke-Ernzerhof function, which is implemented in the Viemma ab initio simulation package. A supercell consisting of 331 unit cells of 6H-SiC containing one AlSi-VSi, corresponding to a defect concentration of 0.93 at%, is built for calculations. The origin of its ferromagnetism is studied, and its spin situation in the space is mapped. The results show that the combination of Al and vacancy leads to a local magnetic moment of 1.0 B, and magnetic coupling is steady in the c axis direction. It is found that the p electron of carbon is the origin of the net spin.
An in-situ real time study of the perovskite film micro-structural evolution in a humid environment by using synchrotron based characterization technique
Authors: Yang Ying-Guo  , Yin Guang-Zhi  , Feng Shang-Lei  , Li Meng  , Ji Geng-Wu  , Song Fei  , Wen Wen  , Gao Xing-Yu  ,   Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China;,   Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano and Soft Materials(FUNSOM), Soochow University, Suzhou 215123, China
Published: December 2020

Humid environment plays a vital role in affecting the performance stability of the organic metal halide perovskite solar cells. Therefore, in situ monitoring the micro-structural evolution of perovskite film in real time will help to reveal the micro-mechanism for the device performance decay induced by humidity. A device providing a controllable humid environment has been set up at X-ray diffraction beamline of Shanghai Synchrotron Radiation Facility, which is used to monitor in situ the perovskite film micro-structural evolution in real time in a humid environment by using grazing incidence X-ray diffraction(GIXRD). After a perovskite film is exposed to the environment with a relative humidity of 60%±2%, a new component emerges near the perovskite(110) diffraction peak in the early stage of the exposure, which is observed for the first time. This new component is attributed to the perovskite intermediate phase structure transformed from the gradual degradation of the perovskite crystalline. Meanwhile, UV-Vis absorption measurements show that humidity causes the absorption of the film to decrease slightly with the blue shift of the absorption edge at ~770 nm, which indicates a reduced amount of perovskite crystalline or a decrease of perovskite crystallinity. Scan electron microscope further demonstrates that the film after the humid exposure presents a worse morphology with a lower coverage, bigger pores, and larger voids between crystalline than the pristine film. The current-voltage(J-V) measurements of the solar cells fabricated on the perovskite films before and after the humid exposure show that both the filling factor and the power conversion efficiencydecrease by over 30% due to the humidity. The present work demonstrates that the close relationship between the device performance and the perovskite film microstructure as well as their morphologies can be studied very well by in-situ synchrotron based characterization technique. The present study could lay a good foundation for the understanding of the degradation mechanism for the organic metal halide perovskites.
heoretical model of influence of frequency on thermal breakdown in semiconductor device
Authors: Zhang Cun-Bo  , Yan Tao  , Yang Zhi-Qiang  , Ren Wei-Tao  , Zhu Zhan-Ping  ,   Science and Technology on High Power Microwave Laboratory, Northwest Institute of Nuclear Technology, Xi'an 710024, China
Published: December 2020

In order to analyze the influence of frequency on thermal breakdown in semiconductor device, the influences of frequency on heat generation and heat conduction in the hot zone are introduced into the theoretical model. The heat transfer equation is solved by the Green's function method, and the error function is approximated. Then, the expressions of temperature in the hot zone and failure power of semiconductor device including frequency and pulse width are derived. The change rules of failure power with the increasing of pulse width under different frequencies and with the increasing of frequency under different pulse widths are obtained. The result shows that the expression for center temperature in hot zone caused by the failure power is divided into four time regions, i.e., regions I-IV, by three thermal diffusion times ta, tb, and tc. The three diffusion times ta, tb, and tc are related to the side lengths a, b and c(c≤b≤a) of the hot zone represented by a rectangular parallelepiped, respectively. In region I(0≤t≤tc), the relation between failure power Pf and failure time t is Pf∝t-1. In this region, the failure time is short and little heat is lost from the surface of hot zone so that the adiabatic term(t-1) dominates. In region Ⅱ(tcttb), the relation between failure power Pf and failure time t is Pf t-1/2. In this region, it is indicative of heat loss from the hot zone to its surrounding medium. In region Ⅲ(tb≤t≤ta), the relation between failure power Pf and failure time t is Pf∝1/ln t. In region IV(t >ta), the failure power Pf is constant. In this region, the failure time is very large and thermal equilibrium can be established so that the steady state term dominates. The relation between failure power and frequency is divided into two parts. In part one, the failure power increases with the increasing of frequency; in part two, the failure power is nearly constant with the increasing of frequency. Meanwhile, the physical interpretation of the influence of frequency on failure power is given. From region I to region IV, each heat transfer rate increases with pulse width. The lower the frequency, the more the injection energy during region I or region Ⅱ is, when the total injection energy is constant. The heat transfer rate is slower in region I or region Ⅱ, so the energy converted into heat will be more and the temperature in the hot zone will be higher, thus the device is burned out easily.
The design and realization of continuous-variable quantum key distribution system based on real-time shot noise variance monitoring
Authors: Cao Zheng-Wen  , Zhang Shuang-Hao  , Feng Xiao-Yi  , Zhao Guang  , Chai Geng  , Li Dong-Wei  ,   School of Information Science and Technology, Northwest University, Xi'an 710127, China;,   School of Electronics and Information, Northwestern Polytechnical University, Xi'an 710072, China
Published: December 2020

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
Authors: Zhang Zheng-Di  , Liu Yang  , Zhang Su-Zhen  , Bi Qin-Sheng  ,   Faculty of Science, Jiangsu University, Zhenjiang 212013, China
Published: December 2020

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.
The research of δ13CO2 by use of wavelet de-noising at 2.008 μm based on tunable diode laser absorption spectroscopy
Authors: Niu Ming-Sheng  , Wang Gui-Shi  ,   Shandong Provincial Key Laboratory of Laser Polarization and Information Technology, Laser Institute, Qufu Normal University, Qufu 273165, China;,   Laboratory of Atmospheric Physico-Chemistry, Anhui Institute of Optics & Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China
Published: December 2020

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.
Dual-core terahertz polarization splitter based on porous fibers with near-tie units
Authors: Wang Jing-Li  , Liu Yang  , Zhong Kai  ,   Department of Opto-Electronic Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China;,   Key Laboratory of Optoelectronic Information Science and Technology(Ministry of Education), Tianjin University, Tianjin 300072, China
Published: December 2020

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.
Dependences of valence electronic structure on magnetic moment and electrical resistivity of metals
Authors: Qi Wei-Hua  , Ma Li  , Li Zhuang-Zhi  , Tang Gui-De  , Wu Guang-Heng  ,   Hebei Advanced Thin Film Laboratory, College of Physics and Information Engineering, Hebei Normal University, Shijiazhuang 050024, China;,   Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

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 B, 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 -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.
High-pressure synthesis and special physical properties of several ordered perovskite structures
Authors: Yin Yun-Yu  , Wang Xiao  , Deng Hong-Shan  , Zhou Long  , Dai Jian-Hong  , Long You-Wen  ,   Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   Collaborative Innovation Center of Quantum Matter, Beijing 100190, China
Published: December 2020

Strongly correlated electronic systems with ABO3 perovskite and/or perovskite-like structures have received much attention. High pressure is an effective method to prepare perovskites, in particular A-site and/or B-site ordered perovskites. In these ordered perovskites, both A and B sites can accommodate transition-metal ions, giving rising to multiple magnetic and electrical interactions between A-A, B-B, and A-B sites. The presence of these new interactions can induce a wide variety of interesting physical properties. In this review paper, we will introduce an A-site ordered perovskite with chemical formula AA3'B4O12 and two A- and B-site ordered perovskites with chemical formula AA3'B2B2'O12. All of these compounds can be synthesized only under high pressure. In the A-site ordered LaMn3Cr4O12 with cubic perovskite structure, magnetoelectric multiferroicity with new multiferroic mechanism is found to occur. This is the first observation of multiferroicity appearing in cubic perovskite, thereby opening the way to exploring new multiferroic materials and mechanisms. In the A- and B-site ordered perovskite CaCu3Fe2Os2O12, a high ferrimagnetic Curie temperature is observed to be around 580 K. Moreover, this compound exhibits semiconducting conductivity with an energy band gap of about 1 eV. The CaCu3Fe2Os2O12 thus provides a rare single-phase ferrimagnetic semiconductor with high spin ordering temperature well above room temperature as well as considerable energy band gap. Moreover, theoretical calculations point out that the introducing of A'-site Cu2+ magnetic ions can generate strong Cu-Fe and Cu-Os spin interactions. As a result, this A- and B-site ordered perovskite has a much higher Curie temperature than that of the B-site only ordered perovskite Ca2FeOsO6 (~320 K). In addition, we also for the first time prepare another A- and B-site ordered perovskite LaMn3Ni2Mn2O12. In the reported ordered perovskites with Mn3+ at the A' site, the A'-B intersite spin interaction is usually negligible. In our LaMn3Ni2Mn2O12, however, there exists the considerable A'-B interaction, which is responsible for the rare formation of B-site orthogonal spin structure with net ferromagnetic moment.
High-pressure structural and optical properties of organic-inorganic hybrid perovskite CH3NH3PbI3
Authors: Guo Hong-Wei  , Liu Ran  , Wang Ling-Rui  , Cui Jin-Xing  , Song Bo  , Wang Kai  , Liu Bing-Bing  , Zou Bo  ,   State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China;,   College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
Published: December 2020

Recent advance in highly efficient solar cells based on organic-inorganic hybrid perovskites has triggered intense research efforts to ascertain the fundamental properties of these materials. In this work, we utilize diamond anvil cell to investigate the pressure-induced structural and optical transformations in methylammonium lead iodide (CH3NH3PbI3) at pressures ranging from atmospheric pressure to 7 GPa at room temperature. The synchrotron X-ray diffraction experiment shows that the sample transforms from tetragonal (space group I4cm) to orthorhombic (space group Imm2) phase at 0.3 GPa and amorphizes above 4 GPa. Pressure dependence of the unit cell volume of CH3NH3PbI3 shows that the unit cell volume undergoes a sudden reduction at 0.3 GPa, which can prove the observed phase transition. We provide the high-pressure optical micrographs obtained from a diamond anvil cell. Upon compression, we can visually observe that the opaque black sample gradually transforms into a transparent red one above 4 GPa. We analyze the pressure dependence of the band gap energy based on the optical absorption and photoluminescence (PL) results. As pressure increases up to 0.25 GPa, the absorption edge and PL peak move to the longer wavelength region of 9 nm. However, abrupt blueshifts of the absorption edge and PL peak occur at 0.3 GPa, followed by a gradual blueshift up to 1 GPa, these phenomena correspond to the previously observed phase transitions. Phase transition increases the band gap energy of CH3NH3PbI3 as a result of reductions in symmetry and tilting of the[PbI6]4- octahedral. Upon further compression, the sample exhibits pressure-induced amorphization at about 4 GPa, which significantly affects its optical properties. Further high pressure Raman and infrared spectroscopy experiments illustrate the high pressure behavior of organic CH3NH3+ cations. Owing to the presence of hydrogen bonding between organic cations and the inorganic framework, all of the bending and rocking modes of CH3 and NH3 groups are gradually red-shifted with increasing pressure. The transition of NH stretching mode from blueshift to redshift as a result of the attractive interactions between hydrogen atoms and iodine atoms is gradually strengthened. Moreover, all the observed changes are fully reversible when the pressure is completely released. In situ high pressure studies provide essential information about the intrinsic properties and stabilities of organic-inorganic hybrid perovskites, which significantly affect the performances of perovskite solar cells.
Structures and novel superconductivity of hydrogen-rich compounds under high pressures
Authors: Duan De-Fang  , Ma Yan-Bin  , Shao Zi-Ji  , Xie Hui  , Huang Xiao-Li  , Liu Bing-Bing  , Cui Tian  ,   State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China
Published: December 2020

Metallic hydrogen can be realized theoretically at high pressure, which suggests that it will be a room-temperature superconductor due to the high vibrational frequencies of hydrogen atoms. However, the metallic state of hydrogen is not observed in experiment at up to 388 GPa. Scientists have been exploring various new ways to achieve hydrogen metallization. Hydrogen-rich compounds can be metallized at much lower pressures because of chemical pre-compression. Moreover, because such materials are dominated by hydrogen atoms, some novel properties can be found after metallization, such as high Tc superconductivity. Therefore, hydrogen-rich compounds are potential high-temperature superconductors, and this method is also believed to be an effective way to metalize hydrogen, which has aroused significant interest in lots of fields, such as physics, material science, etc. In a word, hydrogen-rich compounds are expected to become a new member of superconductor family:hydrogen-based superconductor. Very recently, the theoretical prediction and the successful experimental discovery of high-temperature superconductivity at 200 K in a sulfur hydride compound at high pressure have set a record, which inspired further efforts to study the superconductivity of hydrogen-rich compounds. The present review focuses on crystal structures, stabilities, interaction between atoms, metallization, and superconductivity of several typical hydrogen-rich compounds at high pressures. Furthermore, higher Tc superconductors can be expected to be found in hydrogen-rich compounds in the future.