bluuebunny/crossref_metadata_2025 huggingface dataset:
Global analysis of stochastic bifurcation in permanent magnet synchronous generator for wind turbine system
Authors: Yang Li-Hui  , Ge Yang  , Ma Xi-Kui  ,   State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xian Jiaotong University, Xi'an 710049, China
Published: December 2020

The permanent magnet synchronous generator (PMSG) for wind turbine system operating under inevitable stochastic disturbance from wind power is a nonlinear stochastic dynamical system. With the random interaction and nonlinearity, the intense nonlinear stochastic oscillation is likely to happen in such a system, causing the system to be unstable or even collapse. However, the PMSG is usually considered as a deterministic system when analyzing its nonlinear dynamic behaviors in the past researches. Such a simplification can lead to wrong predictions for the system stability and reliability. This paper aims to discuss the effect of the stochastic disturbance on the nonlinear dynamic behavior of the PMSG. Based on the derived PMSG model considering the stochastic disturbance from the input mechanical torque, the evolution of the system global structure with the stochastic intensity is investigated using the generalized cell mapping digraph method. Meanwhile, the occurrence process and development process of the stochastic bifurcation are illustrated. Based on this global analysis, the intrinsic mechanism for the effect of the stochastic disturbance on the operating performances of the PMSG is revealed. Finally, the numerical simulations based on the Euler-Maruyama algorithm are carried out to validate the results of the theoretical analysis. The results present that as the intensity of the stochastic disturbance increases, two kinds of stochastic bifurcations can be observed in the PMSG system according to the definition of a sudden change in characteristic of the stochastic attractor. One is the stochastic interior crisis that occurs when a stochastic attractor collides with a stochastic saddle in its attraction basin interior, leading to the abrupt increase of the attractor and the disappearance of the saddle. This kind of bifurcation results in the intense stochastic oscillation and instability of the PMSG system. Another stochastic bifurcation is the stochastic boundary crisis which occurs when a stochastic attractor collides with the boundary of its attraction basin and results in the disappearance of the attractor. This sudden change of the number of stochastic attractors induces the stable solution set to vanish and thus the PMSG system to collapse. In a word, even the stochastic disturbance with small intensity may lead to the complete destruction of the stable structure of the PMSG, inducing the system to suffer a strong disordered oscillation or the operation to collapse. The results of this paper can provide significant theoretic reference for both practically operating and designing the PMSG for wind turbine systems.
Research and design of continuous-wave Nd:YVO4 self-Raman laser in-band pumped by a wavelength-locked laser diode
Authors: Zhang Xin  , Zhang Yun-Chuan  , Li Jian  , Li Ren-Jie  , Song Qing-Kun  , Zhang Jia-Le  , Fan Li  ,   College of Physics Science and Technology, Institute of Applied Photonic Technology, Yangzhou University, Yangzhou 225002, China;,   National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China}
Published: December 2020

In this paper, a continuous-wave all-solid-state Nd:YVO4 self-Raman laser in-band pumped by a wavelength-locked laser diode at 878.9 nm is theoretically investigated in detail. Considering the thermal lens effect in the laser crystal, cavity mode parameters are calculated for several output couplers with different radii of curvature, by employing the standard ABCD matrix approach and equivalent G parameter method. The influence of cavity structure on the output characteristic of the Raman laser is investigated by analyzing mode matching between the pump and the fundamental beams, as well as the fundamental intensities in the Raman crystal. This provides theoretical explanations for the experimental results, and based on the analysis above, laser cavity is optimized. Finally, a highest Raman output of 5.3 W is obtained at 1175 nm, corresponding to a diode-to-Stokes optical conversion efficiency of 20%.
Monte Carlo simulations of electron transport coefficients in low temperature streamer discharge plasmas
Authors: Sun An-Bang  , Li Han-Wei  , Xu Peng  , Zhang Guan-Jun  ,   State Key Laboratory of Electrical Insulation and Power Equipment, Xi'an Jiaotong University, Xi'an 710049, China
Published: December 2020

Streamer is usually present at the initial stage of atmospheric pressure air discharge, which occurs in nature as a precursor to lightning, transient luminous events in upper atmosphere and has much potential applications in industry, such as the treatment of polluted gases/liquids, assisted combustion, plasma enhanced deposition etc. Streamer is a multi-scale problem both in time and in space, which brings much difficulty to the conventional diagnostic approaches. In past decades, fluid or particle-fluid hybrid models have been frequently used for understanding the mechanisms of streamer discharges because of their high efficiencies of calculations. Accuracies of the electron transport coefficients (including drift/diffusion coefficient, ionization/attachment coefficient, electron mean energy and extra) play a key role in ensuring the correctness of the fluid or hybrid simulations. As far as we know, BOLSIG+ and MAGBOLTZ are two typical tools for obtaining the electron transport coefficients and have been widely utilized in previous models. BOLSIG+ uses two-term approximation which is not sufficient for some molecular gases, MAGBOLTZ cannot calculate the bulk transport coefficients:these data are required for some models. METHES is an additional tool for computing electron transport coefficients, however, specific platform is required which is not very user-friendly. As sorts of drawbacks exist in currently available calculating tools, in the paper, a Monte Carlo model is developed for computing the electron transport coefficients in gases, the model is flexible to choose any type of gas mixture and its accuracy has been validated by comparing with BOLSIG+ and METHES. Furthermore, the influences of N2-O2 mixture and three-body attachment process in high gas pressures on the transport coefficient are investigated. It is worth mentioning that three-body attachment process can significantly change the electron transport properties at a relatively low reduced electric field. Thus, specific attention must be paid to the transport coefficients if simulation is performed at a high pressure. In addition, differences between the bulk and flux coefficients are analyzed which are not distinguished in some previous models. Finally, we further validate the present Monte Carlo model by performing simulation of streamer discharge in atmospheric N2, which shows that the improved electron transport coefficient from our Monte Carlo model can improve the simulated plasma properties, in particular at the interior of the streamer channel. The existence of divergence at the tip of the streamer channel might be due to our local field approximation; if a density gradient term is included in the impact ionization term and local electron energy approximation of the electron transport coefficients is used, the accuracy of the fluid can be improved further.
Quantitative calculation of atomic-scale frictional behavior of two-dimensional material based on sliding potential energy surface
Authors: Shi Ruo-Yu  , Wang Lin-Feng  , Gao Lei  , Song Ai-Sheng  , Liu Yan-Min  , Hu Yuan-Zhong  , Ma Tian-Bao  ,   State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China;,   Institute of Bio-inspired Structure and Surface Engineering, College of Astronautics, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China;,   Corrosion and Protection Center, Key Laboratory for Environmental Fracture (MOE, University of Science and Technology Beijing), Beijing 100083, China}
Published: December 2020

The excellent tribological characteristics of two-dimensional (2D) materials have received great attention, however, how to effectively predict their frictions is still lacking. Here, we propose to obtain the sliding potential energy surface by density functional theory calculations, instead of simplified potential energy function. Thus it is able to solve the frictional behaviors of 2D materials with irregular complex potential energy surfaces. Firstly, we reveal the mechanism of dual-scale stick-slip behavior between a tip and a graphene/Ru(0001) heterostructure. With a dual-wavelength potential energy surface, we observe a similar frictional behavior to those captured in atomic force microscopy experiments, in which a significant long-range stick-slip sawtooth modulation emerges with a period coinciding with the Moir superlattice structure. Secondly, we discuss the interlayer frictions of 2D materials, including graphene/graphene, fluorinated graphene/fluorinated graphene, MoS2/MoS2, graphene/MoS2 and fluorinated graphene/MoS2. With sliding potential energy surface obtained by density functional theory calculations, the interlayer friction is estimated according to the Prandtl-Tomlinson model calculation method. Compared with the friction between homostructures, the friction between heterostructures is lowered by orders of magnitude, which could be attributed to its ultralow sliding potential barrier. The stick-slip instability could be observed in homostructure, while heterostructure exihibits smooth friction loops. The 2D sliding path between the layers is recorded in the sliding process, showing its dependence on both the potential energy barrier and the spring constant. The sliding path shift increases with the increase of potential energy barrier and the decrease of spring constant in the y direction. This method is also applicable to tribological systems with dominated interfacial van der Waals interaction.
High-performance AlGaN/GaN MIS-HEMT device based on in situ plasma nitriding and low power chemical vapor deposition Si3N4 gate dielectrics
Authors: Li Shu-Ping  , Zhang Zhi-Li  , Fu Kai  , Yu Guo-Hao  , Cai Yong  , Zhang Bao-Shun  ,   Suzhou Industrial Park Institute of Services Outsourcing, Suzhou 215123, China;,   Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China}
Published: December 2020

Gallium nitride (GaN)-based high electron mobility transistor (HEMT) power devices have demonstrated great potential applications due to high current density, high switching speed, and low ON-resistance in comparison to the established silicon (Si)-based semiconductor devices. These superior characteristics make GaN HEMT a promising candidate for next-generation power converters. Many of the early GaN HEMTs are devices with Schottky gate, which suffer a high gate leakage and a small gate swing. By inserting an insulator under gate metal, the MIS-HEMT is highly preferred over the Schottky-gate HEMT for high-voltage power switche, owing to the suppressed gate leakage and enlarged gate swing. However, the insertion of the gate dielectric creates an additional dielectric/(Al) GaN interface that presents some great challenges to AlGaN/GaN MIS-HEMT, such as the threshold voltage (Vth) hysteresis, current collapse and the reliability of the devices. It has been reported that the poor-quality native oxide (GaOx) is detrimental to the dielectric/(Al) GaN interface quality that accounted for the Vth instability issue in the GaN based device. Meanwhile, it has been proved that in-situ plasma pretreatment is capable of removing the surface native oxide. On the other hand, low power chemical vapor deposition (LPCVD)-Si3N4 with free of plasma-induced damage, high film quality, and high thermal stability, shows great potential applications and advantages as a choice for the GaN MIS-HEMTs gate dielectric and the passivation layer. In this work, an in-situ pre-deposition plasma nitridation process is adopted to remove the native oxide and reduce surface dangling bonds prior to LPCVD-Si3N4 deposition. The LPCVD-Si3N4/GaN/AlGaN/GaN MIS-HEMT with a high-quality LPCVD-Si3N4/GaN interface is demonstrated. The fabricated MIS-HEMT exhibits a very-low Vth hysteresis of 186 mV at VG-sweep=(-30 V, +24 V), a high breakdown voltage of 881 V, with the substrate grounded. The hysteresis of our device at a higher positive end of gate sweep voltage (VG +20 V) is the best to our knowledge. Switched off after an off-state VDS stress of 400 V, the device has a dynamic on-resistance Ron only 36% larger than the static Ron.
Charged paricle activation analysis for characterizing parameters of laser-accelerated protons
Authors: He Shu-Kai  , Liu Dong-Xiao  , Jiao Jin-Long  , Deng Zhi-Gang  , Teng Jian  , Zhang Zhi-Meng  , Hong Wei  , Gu Yu-Qiu  ,   Key Laboratory of Plasma Physics, Laser Fusion Research Center, China Academy of Engineering Physics, Mianyang 621900, China;,   International Fusion Sciences and Applications(IFSA) Collaborative Innovation Center, Shanghai Jiao Tong University, Shanghai 200240, China;,   Center for Applied Physics and Technology, Peking University, Beijing 100871, China
Published: December 2020

The protons accelerated by ultra-high intensity laser have been extensively studied. The most commonly used detectors for measuring laser-driven proton are Tomspon parabola ion energy analyser (TP) and filtered nuclear track detectors, such as radiochromic films (RCF). The TP uses a parallel magneto-electric field to distinguish ions. This conventional technique can precisely identify the species and energy spectra of ions. However, the strong electromagnetic field produced by the laser-plasma interaction has an effect on TP, which results in no spatial resolution of TP. The RCF can give the spatial integration spectrum of proton, but it is easy to be saturated and cannot be reused anymore. In this paper, we present a method based on the traditional charged particle activation analysis and the gamma-gamma coincidence measurement to measure the spectrum of protons accelerated by ultra intense lasers. In this method, a copper plate stack is placed in the proton emission direction. Colliding with MeV proton converts 63Cu in the copper plates into radionuclide 63Zn whose decay can be easily observed and measured. Proton spectrum is then recovered from 63Zn decay counts from layers in the copper stack. The layout of diagnostics and the method to solve proton spectrum are discussed in detail and a self-consistent test is given. This spectrum analysis method is used in a laser-driven proton acceleration experiment carried out on XG-Ⅲ laser facility. The results show that protons up to 18 MeV are obtained, and the spatial integrated spectrum and a laser-proton conversion efficiency of 1.07% are achieved. In conclusion, our method has some advantages as a laser-driven ion diagnostic tool. It has no saturation problem and is not affected by strong electromagnetic fields. The basic principle of charged particle activation analysis is based on nuclear reaction, and can be extended to the measuring of other charged particle beams besides protons, such as deuterons, helium ions produced by ultra-high intensity laser.
Chemical functionalization of silicene
Authors: Yang Shuo  , Cheng Peng  , Chen Lan  , Wu Ke-Hui  ,   Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

Silicene exhibits extraordinary physical properties especially Dirac fermion characteristics. However, the zero-gap band structure of silicene hinders its applications in nanoelectronic and optoelectronic devices. It is thus desirable to open a finite band gap in silicene. Chemical functionalization is a commonly used method to tailor the structures and electronic properties of two-dimensional materials. In this paper we review the recent 3-year progress of silicene, including its hydrogenation, oxidization, halogenation, and other methods to modify silicene.
Novel p-n junctions based on ambipolar two-dimensional crystals
Authors: Zhang Zeng-Xing  , Li Dong  ,   Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, School of Physics Science and Engineering, Tongji University, Shanghai 200092, China
Published: December 2020

Two-dimensional (2D) materials have a unique crystal structure and excellent properties, which renders it possible to be used to construct novel artificial nanostructures and design novel nanodevices, thereby achieving a breakthrough in the semiconductor field. In this review paper, the basic behaviors of the ambipolar 2D crystals and the fabrication method of the van der Waals heterostructures are first introduced. We mainly summarize the applications of the ambipolar 2D crystals for novel electrical-field-tunable 2D p-n junctions and p-n heterojunctions (field-effect p-n heterojunction transistor) and non-volatile storable p-n junctions, and other aspects of the relevant structural design, electronic and optoelectronic properties. Then we further introduce their potential applications of logic rectifiers, field-effect optoelectronic transistors, multi-mode non-volatile memories, rectifier memories, optoelectronic memories, photovoltaics, etc. Finally, we provide an outlook of the future possible studies of this new type of p-n junctions in the relevant fields.
Temperature dependent characteristics of photo-induced birefringence in different types of azo materials
Authors: Lyu Zi-Yao  , Pan Yu-Jia  , Wang Chang-Shun  ,   State Key Laboratory of Advanced Optical Communication Systems and Networks, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
Published: December 2020

At different temperatures, a semiconductor laser with a wavelength of 650 nm is used as probe light, and an Nd:YAG continuous laser with a wavelength of 532 nm is selected as pump light. The azo samples are placed between a pair of orthogonal polarizers with the vertical direction clockwise and counterclockwise 45 degrees, respectively. The polarization direction of the pump light is set to be the vertical direction. In order to reduce the effect of the stray light, a chopper is placed in the optical path of the probe light. The signal of photo-induced birefringence is recorded by a phase-locked amplifier (NF-LI5640). The photo-induced birefringences of the doped azo material, the azo polymer and the azo liquid crystal polymer are measured respectively, and the dynamic processes of photo-induced birefringence are fitted by a double e-index model. The experimental results show that with the influence of the pump light, photo-induced birefringences of the three types of azo materials rise rapidly at first and then gradually tend to reach their own saturation state because of the photo-induced cis and trans isomerism and the photo-induced molecular orientation properties of azo molecules. The photo-induced birefringence shows a tendency to increase at first and then decrease with the temperature increasing, which can be understood as a competitive mechanism. The photo-induced birefringence depends on the photo-induced orientation and irregular thermal motions of azo groups. In the range below the glass transition temperature of the samples, the increase of the temperature of samples contributes to the rearrangement of the azo molecules due to the influence of the pump light. When the temperature of the samples is higher than the glass transition temperature, molecular chains begin to move. The irregular thermal motions of azo components and polymer molecules are aggravated. This destroys the orientations of the polymer molecules and results in the drop of the photo-induced birefringence. Comparing the doped azo material with the azo polymer sample, the azo liquid crystal polymer sample exhibits not only a larger photo-birefringence, but also the photo-induced birefringence that does not change obviously after the pump light has been turned off, which means that the azo liquid crystal polymer sample has long optical storage properties. This shows that the azo liquid crystal polymer material is an ideal polarization-sensitive optical recording medium, which is expected to be used in the fields of optical storage, polarization holography and optical information processing.
Performances of time-delay signature and bandwidth of the chaos generated by a vertical-cavity surface-emitting laser under chaotic optical injection
Authors: Su Bin-Bin  , Chen Jian-Jun  , Wu Zheng-Mao  , Xia Guang-Qiong  ,   School of Physical Science and Technology, Southwest University, Chongqing 400715 China;,   School of Medical Engineering Technology, Xinjiang Medical University, Urumqi 830011, China
Published: December 2020

Time-delay signature (TDS) and effective bandwidth (EBW) are two key performance indexes to evaluate a chaos signal generated by a laser system including delay-time feedback. In this paper, we propose and simulate a technical scheme to optimize the TDS and EBW of chaotic signal generated by a slave vertical-cavity surface-emitting laser (S-VCSEL) under chaotic optical injection from a master vertical-cavity surface-emitting laser (M-VCSEL), which is subjected to double external-cavity feedback. First, based on the spin-flip model of a VCSEL subjected to two double external-cavity feedback, the time series of two orthogonal polarization components (referred to as X-component (X-PC) and Y-component (Y-PC), respectively) in the M-VCSEL can be obtained. Furthermore, with the help of self-correlation function (SF) analysis method, the TDSs of X-PC and Y-PC can be evaluated. The results show that through selecting suitable system operation parameters, X-PC and Y-PC in the M-VCSEL can simultaneously output chaotic signals with equivalently average intensity and weak TDS. Under optimized operation parameters, the peak values of the SF (σ) of the chaotic signal are 0.20 for X-PC and 0.16 for Y-PC, respectively, and the EBWs of the chaotic signal are 10.72 GHz for X-PC and 10.10 GHz for Y-PC, respectively. The chaotic signals output from the M-VCSEL under optimized operation parameters are injected into the S-VCSEL for further weakening TDS and enhancing EBW. Through examining the evolution rules of TDS and EBW of polarization-resolved chaotic signals in the parameter space composed of injection strength and frequency detuning, the ranges of optimizing injection parameters are determined for achieving two-channel chaotic signals with well suppressed TDS (σ 15 GHz).
Characteristics of chaotic output from a Gaussian apodized fiber Bragg grating external-cavity semiconductor laser
Authors: Qi Jun-Feng  , Zhong Zhu-Qiang  , Wang Guang-Na  , Xia Guang-Qiong  , Wu Zheng-Mao  ,   School of Physical Science and Technology, Southwest University, Chongqing 400715 China
Published: December 2020

Optical chaos based on semiconductor laser (SL) has some vital applications such as optical chaos secure communication, high-speed physical random number generation, chaos lidar, etc. Among various schemes to drive an SL into chaos, the introduction of external cavity feedback is one of the most popular techniques, which can generate chaos signals with high dimension and complexity. For the chaos output from an external cavity feedback SL, a time-delay signature (TDS) and bandwidth are two key indexes to assess the chaos signal quality. In this work, according to the rate-equation model of an optical feedback SL, we theoretically investigate the characteristics of TDS and effective bandwidth (EWB) of chaotic output from a Gaussian apodized fiber Bragg grating (GAFBG) feedback SL (GAFBGF-SL). The results show that with the increase of feedback strength, the GAFBGF-SL experiences a quasi-periodic route to chaos. Through selecting the suitable feedback strength and the frequency detuning between the Bragg frequency of the GAFBG and the peak frequency of the free-running SL, the TDS of chaotic output from the GAFBGF-SL can be efficiently suppressed to a level below 0.02. Furthermore, by mapping the TDS and EWB in the parameter space of the feedback strength and the frequency detuning between the Bragg frequency of the GAFBG and the peak frequency of the free-running SL, the optimized parameter region, which is suitable for achieving chaotic signal with weak TDS and wide bandwidth, can be determined. We believe that this work will be helpful in acquiring the high quality chaotic signals and relevant applications.
Spectroscopic exploration of upconversion luminescence behavior of rare earth-doped single-particle micro/nanocrystals
Authors: Zhang Xiang-Yu  , Ma Ying-Xiang  , Xu Chun-Long  , Ding Jian  , Quan Hong-Juan  , Hou Zhao-Yang  , Shi Gang  , Qin Ning  , Gao Dang-Li  ,   College of Science, Chang'an University, Xi'an 710064, China;,   College of Science, Xi'an University of Architecture and Technology, Xi'an 710055, China
Published: December 2020

In recent years, rare earth-doped upconversion (UC) micro/nanocrystals are useful for many applications, especially in biology because of their unique luminescent properties and specific geometry. The luminescence efficiency of lanthanide-doped UC nanoparticles is of particular importance for their applications. However, the unsatisfactory UC efficiency is still one of the main hurdles. In the present article, a series of Yb3+/Er3+ doped NaYF4 micro/nanoparticles with different ratios of length to diameter are successfully synthesized by a facile hydrothermal route. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDX) analyses, photoluminescence spectra, and the dynamic process of the luminescence are used to characterize the samples. The intrinsic structural feature of fluoride, the solution pH value, and organic additive Cit3- account for the ultimate shape evolution of the final products. The ratio of length to diameter of NaYF4 microrod can be tuned only by varying the value of pH or the amount of an organic additive (Cit3-). The UC characteristics of a single NaYF4:Yb3+/Er3+ microrod obtained by tuning the value of pH or the amount of Cit3- are investigated by laser confocal microscopy with a 980 nm laser. The two series of codoped fluoride crystals both exhibit the characteristic UC luminescence from Er3+ ions and display the rich luminescence patterns in space. The UC luminescence from a single NaYF4:Yb3+/Er3+ microrod obtained by tuning the value of pH is brighter than that from a single NaYF4:Yb3+/Er3+ microrod with the same size obtained by tuning the amount of Cit3-. The EDX analysis indicates that the number of Na+ defects depends on the specific synthesis conditions of the sample. The Na+ defects of samples obtained by tuning the values of pH are lower than those of samples with the same size obtained by tuning the amount of Cit3-. It conduces to reducing Na+ defects at lower pH value. The parameters of the luminescence kinetics are found to be unambiguously dependent on the size of sample, which relates to higher energy phonon of surface and Na+ defects. The mechanism of luminescence enhancement by pH controlling is explored, and a mechanism based on the reduced intrinsic defects of Na+ is proposed. The investigation not only enriches the controllable synthesis approach of fluoride micro/nanomaterials, but also indicates the potential applications of rare earth materials with a rich luminescence pattern in the photonic devices and anti-counterfeiting devices.
Entanglement characteristics of output optical fields in double-cavity optomechanics
Authors: Zhang Xiu-Long  , Bao Qian-Qian  , Yang Ming-Zhu  , Tian Xue-Song  ,   College of Electronic Science, Northeast Petroleum University, Daqing 163318, China;,   College of Physics, Liaoning University, Shenyang 110036, China;,   College of Science, Heilongjiang University of Science and Technology, Harbin 150001, China
Published: December 2020

Radiation pressure in an optomechanical system can be used to generate various quantum entanglements between the subsystems. Recently, one paid more attention to the study of quantum entanglement in an optomechanical system. Here in this work, we study the properties of output entanglement between two filtered output optical fields by the logarithmic negativity method in a double-cavity optomechanical system. Our calculations show that the decay rate of the mechanical resonator, the bandwidth of filter function, and non-equal-coupling will evidently affect the value of the output entanglement. In particular, under the parameters of equal-coupling and zero filter bandwidth, the output entanglement in the vicinity of resonant frequency (=0 in the rotating frame) will decease with mechanical decay rate increasing. But under the parameters of equal-coupling and non-zero filter bandwidth, the output entanglement will be suppressed if the center frequency of output field is in the vicinity of the resonant frequency. However, the output entanglement can be enhanced if we adopt a non-equal-coupling to counteract the suppression effect of the filter bandwidth. Furthermore, we find that there are three peaks in the whole center frequency domain of the output field if we adopt strong non-equal-coupling. This is because the normal mode of Hamiltonian Hint will split into three normal modes in this case. Our results can also be used in other parametrically coupled three-mode bosonic systems and may be applied to realizing the state transfer process and quantum teleportation in an optomechanical system.
First principle study of electron and band structure of BixBa1-xTiO3
Authors: Fang Yu-Zhen  , Kong Xiang-Jin  , Wang Dong-Ting  , Cui Shou-Xin  , Liu Jun-Hai  ,   School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, China;,   School of Physics Science and Information Technology, Liaocheng University, Liaocheng 252059, China
Published: December 2020

Some perovskite structured catalysts have narrower forbidden band widths than pure TiO2, and they have been widely used in a number of photo-catalytic reactions. The ions in the perovskite may be replaced by other ions while maintaining the structure unchanged for its tailorable character. BiTiO can form into the typical perovskite composite oxide BiTiO3 under specific preparation conditions. The regulation of the energy gap of the perovskite BaTiO3 can be realized by substituting Bi for Ba to form the BixBa1-xTiO3 perovskite structure to improve its photo-catalytic activity. But the improvement mechanism and the electron and band structures of BixBa1-xTiO3 are still not very clear.
In this study, we exhibit a detailed theoretical investigation to predict the electronic structure, band gap and optical absorption properties of BixBa1-xTiO3 structures based on the first-principles plane-wave ultrasoft pseudopotential method. The exchange and correlation interactions are modeled using the generalized gradient approximation and the Perdew-Burke-Ernzerhof exchange-correlation functional. The cutoff kinetic energy of the electron wave function is 340 eV, and the k-point sampling sets 333 division of the reciprocal unit cell based on the Monkhorst-Pack scheme. In the geometrical optimization, all forces on atoms are converged into less than 110-5 eV/atom, the maximum ionic displacement is within 0.001 and the total stress tensor decreases to the order of 0.05 GPa.
The DFT calculation results reveal that the symmetry and binding energy decline in the BixBa1-xTiO3 structure, and the bond lengths of BaO and TiO decrease a little after Ba has been substituted by Bi atom, except for the structure of Bi0.5Ba0.5TiO3. The photo-catalysts of BixBa1-xTiO3 are direct band gap semiconductors, and the substitution Bi can regulate the band gaps of BixBa1-xTiO3. The band gaps become wider from x=0.125 to x=0.750 with the carrier concentration decreasing, and then decreases with the higher carrier concentration increasing when x=0.875. It is predicted that the band width of Bi-based perovskite will be much lower than that of Ba-based perovskite. In the case of the density of states we reveal that the top of the valence band is hybrided by O-2p and Bi-6s and the bottom of the conduction band state is mainly constituted by the Ti-3d state. The electron transport properties and carrier types are mainly determined by Ti-3d, O-2p state and Ba-5p electronic states in BaTiO3 and Ti-3d, O-2p, Bi-6s and Bi-6p electronic states in BixBa1-xTiO3 respectively. The absorption spectra indicate that the ultraviolet absorption performance can be improved in BixBa1-xTiO3 system, which may effectively improve the photo-catalytic activity of BaTiO3.
Mechanism of a nonlinear bistable piezoelectric cantilever beam under narrow-band random excitations and its energy harvesting
Authors: Wu Juan-Juan  , Leng Yong-Gang  , Qiao Hai  , Liu Jin-Jun  , Zhang Yu-Yang  ,   School of Mechanical Engineering, Tianjin University, Tianjin 300350, China;,   Caterpillar technology R & D Co., Ltd., Wuxi 214028, China
Published: December 2020

While wireless sensors, data transmission devices and medical implant devices tend to miniaturization and low consumption, energy supply modes such as batteries, solar energy and wind energy are limited due to their defects. Instead, vibration energy harvesting can open up new possibilities for self-supplying the low-consumption devices. The narrow-band random vibration with center frequency is a typical vibration in the environment, and its characteristics are closely related to the environment.This paper takes the energy harvesting system with bi-stable piezoelectric cantilever beam as a research object, and the characteristics of system's equivalent linear natural frequency, linear and nonlinear stiffness under different intervals between magnets are analyzed. By using the narrow-band random excitation with a certain bandwidth output of the bandpass filter to simulate environment vibration and using Runge-Kutta method to solve the system equation numerically, the response of system and the characteristics of energy harvesting are studied.It is observed that the variation of the magnet spacings at peak output voltage, which possesses a central frequency, is related to the variation of the equivalent linear natural frequency of the system with the interval between magnets. When the variation of magnet spacing is triggered by the narrow-band random excitation with a certain bandwidth, there is always a constant interval between magnets, making the system produce a peak output, which is like a bi-stable system that produces the peak output at optimal spacing under broad-band excitation. On the other hand, there are also more than one or two different magnet spacings making the system produce peak outputs while excitation's center frequency changes in a certain range, and the peak outputs are formed by bi-stable or single-stable “resonance” of the system, induced at the equivalent linear natural frequency. And the demarcation point spacing of the single-stable and bi-stable vibration of the system are the magnet spacing when linear stiffness is zero.Therefore, for the narrow-band random excitation in the actual environment, the magnet spacing of the energy harvesting system can be reasonably arranged according to the specific working conditions to achieve better electromechanical energy conversion. The findings in this paper can provide some theoretical and technical support for the study of harvesting the vibration energy with characteristics of narrow-band random excitation.
Magnetoelastic phenomena and mechanisms of magnetic skyrmion crystal
Authors: Hu Yang-Fan  , Wan Xue-Jin  , Wang Biao  ,   Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-sen University, Zhuhai 519082, China
Published: December 2020

Recently, a novel two-dimensional spin structure with non-trivial topological properties, called magnetic skyrmion, has been found in many chiral magnets. In most cases, magnetic skyrmions assemble spontaneously and form a lattice structure, called magnetic skyrmion crystal (SkX). SkX, as a novel macroscopic magnetic phase, may interact with different types of external fields through the intrinsic multi-field coupling of the material, resulting in many peculiar physical phenomena. It is found that due to the intrinsic magnetoelastic coupling of chiral magnets, SkX not only influences the mechanical properties of the materials, but also has emergent elastic properties when subjected to external forces. In this review, we first introduce and categorize various types of SkX-related magnetoelastic phenomena, and then introduce a unified theoretical framework to analyze these magnetoelastic phenomena. Specifically, we establish the Landau-Ginzburg free energy functional with a comprehensive description of the magnetoelastic effect for B20 chiral magnets obtained through symmetry analysis, and prove that SkX should be described by a Fourier series due to its wave nature. We show quantitative agreement between theoretical results and experimental results for three types of phenomena:1) the temperature-magnetic field phase diagrams of MnSi suffering uniaxial compression, it is found that uniaxial compression in the direction[0, 0, 1]T constricts the stable region of the skyrmion phase in the phase diagram, while uniaxial compression in the direction[1, 1, 0]T extends the stable region of the skyrmion phase in the phase diagram; 2) the emergent elastic behavior of SkX, it is found that this property derives from the magnetoelastic effect of the underlying material, and the linear constitutive equation (with coefficient matrix ) which determines the emergent deformation of SkX, is briefly introduced; 3) the variations of elastic coefficients C11, C33, C44, and C66 with the external magnetic field for MnSi, and the predictions of the variation of C12 and C13 are provided by the theory. Based on the theoretical framework, the analytical solutions of the eigenstrain problems for chiral magnets hosting SkX and the surface configuration of SkX in a half-space magnet are introduced. In this process, we show how to use the theoretical framework to deal with different problems. Finally, we make a summary and suggest several directions for the future development of this field.
Stability switching behavior of thermoacoustic oscillation in Rijke tube
Authors: Dang Nan-Nan  , Zhang Zheng-Yuan  , Zhang Jia-Zhong  ,   School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China
Published: December 2020

Large-amplitude self-excited thermoacoustic oscillations arising due to the interaction between unsteady heat release and acoustic pressure fluctuations have been encountered in many thermal devices. These oscillations may lead to unwanted structural vibrations and efficiency reduction while emitting loud noises, and thus the predicting of such oscillations is very important. Physically, oscillation is a kind of instability, so stability analysis can be applied to understanding such a phenomenon. The present work focuses on the role of time delay between unsteady heat release and flow perturbation in the stability of thermoacoustic system. To this end, one-dimensional Rijke tube model with both open ends is numerically investigated. In the Rijke tube model, an electric heater is located at the first quarter of the Rijke tube and its unsteady heat release rate is modeled by an empirical model proposed by Heckl. Non-dimensional momentum equation and energy equation of the acoustic perturbation are derived and solved in time domain by using the Galerkin technique. The time evolution of the thermoacoustic oscillations with continuous increase in the time delay is calculated in two different acoustic damping cases, namely the heavily damped case and the weakly damped case, while other parameters are fixed. It is found that in both the heavily damped case and the weakly damped case, the system stability switches between stability and instability as the time delay increases, which is called stability switching and is a typical nonlinear phenomenon in a delay-dependent system. However, compared with in the heavily damped case, in the weakly damped case, the stability region is enlarged and the amplitude of the limit cycle oscillation is increased. Besides, in the weakly damped system, the dominating mode of system shifts in the first three modes instead of keeping in the first mode during increasing the time delay, which suggests that for the weakly damped system, the higher modes cannot be neglected and the system cannot be analyzed with a single-mode model either. Further, the bifurcation plots for the variation of the time delay for these two cases show that the system stability changes with time delay for a period of two, which is equal to the period of the first acoustic mode. As a conclusion, the results of present work indicate that the time delay between unsteady heat release and flow perturbations plays a critical role in generating thermoacoustic oscillations, and the findings of stability switching can help to understand the nonlinear phenomena in thermoacoustic systems.
Effect of Pb doping on metallic state of cubic pyrochlore Cd2Ru2O7
Authors: Jiao Yuan-Yuan  , Sun Jian-Ping  , Prashant Shahi  , Liu Zhe-Hong  , Wang Bo-Sen  , Long You-Wen  , Cheng Jin-Guang  ,   Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Many exotic phenomena in strongly correlated electron systems, such as unconventional superconductivity, metal-insulator transition, and quantum criticality, take place in the intermediate regime between localized and itinerant electronic state. To understand the electronic behaviors near the localized-to-itinerant crossover remains a challenging problem in condensed matter physics. The Ru5+ cubic pyrochlores A2Ru2O7 (A=Cd, Cd, Hg) constitute such a system that the Ru-4d electrons acquire characters of both itinerancy and localization. In addition, the magnetic Ru5+ ions that are situated on the vertices of corner-shared tetrahedral lattice are expected to experience strong geometrical frustration given an antiferromagnetic (AF) arrangement. In this work, we investigate the cubic pyrochlore Cd2Ru2O7, which develops a peculiar metallic state below the AF transition. We synthesize a series of Pb-doped Cd2-xPbxRu2O7 (0 x 2) polycrystalline samples under high-pressure condition, and study the effects of Pb doping on their crystal structure and physical properties. Although the Pb2Ru2O7 pyrochlore is a Pauli paramagnetic metal, we find that the substitution of 10% Pb2+ for Cd2+ in Cd1.8Pb0.2Ru2O7 converts the metallic state of Cd2Ru2O7 into an insulating ground state, in a manner similar to the consequence of exerting hydrostatic pressure or substituting 10% Ca2+ for Cd2+ ions as we found recently. We propose that the electronic state of Cd2Ru2O7 be located at the itinerancy to localization crossover, and the introduction of chemical disorder via Pb2+ substitution may enhance the localized character and induce the metal-to-insulator transition. Our results further demonstrate that the cubic Ru5+ pyrochlore oxides offer an important paradigm for studying the exotic physics of correlated electrons on the border of (de)localization in the presence of strong geometrical frustration.
Dual micro-holes-based in-fiber Fabry-Perot interferometer sensor
Authors: Zhang Wei  , Liu Ying-Gang  , Zhang Ting  , Liu Xin  , Fu Hai-Wei  , Jia Zhen-An  ,   School of Environmental and Municipal Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China;,   Key Laboratory of Photoelectric Oil and Gas Logging and Testing, Ministry of Education, Xi'an Shiyou University, Xi'an 710065, China
Published: December 2020

A kind of dual micro-holes-based in-fiber Fabry-Perot interferometer sensor is proposed in this paper. The theoretical model of the reflection spectrum of proposed sensor is established based on the interference among four light beams, where both the relationships of the spectrum intensity with the length of micro-hole, refractive index (RI) of medium in cavity, transmission loss and reflection loss, and the characteristic parameters of fiber are demonstrated, and the temperature and RI responses of reflection spectra are also simulated. Through machining two micro-holes in single-mode fiber with 193 nm excimer laser, we fabricate the proposed fiber sensor which can be used for measuring the multi-physical quantities, and the corresponding experiments are demonstrated simultaneously. The results show that the sensor has better linear responses to temperature and RI change, and the corresponding linearity is superior to 99%. Due to having two sets of different temperature and RI sensitivities (i.e.-0.172 nm/℃ and 1050.700 nm/RIU; 0.004 nm/℃ and 48.775 nm/RIU) and better linearity, this kind of sensor can be used for measuring the temperature, the ambient RI and even the simultaneous discrimination of temperature and ambient RI. The RI and temperature resolutions are 1.010-5 RIU and 0.2℃, respectively. Furthermore, the sensor can also be used for sensing the gas pressure, and its measurement accuracy can reach to. 3 kPa. Owing to its high sensitivity, stability, small volume and easy fabrication, the sensor will be widely used in sensing technology.
A new charging model for exposed dielectric of spacecraft
Authors: Yuan Qing-Yun  , Wang Song  ,   National Key Laboratory of Electromagnetic Environment Effect, Army Engineering University of PLA, Shijiazhuang 050003, China;,   Unit 63618, Kuerle 841000, China
Published: December 2020

In order to consider comprehensively the effects of high-energy electron radiation and space plasma on the exposed dielectrics outside a spacecraft, in this paper, a model named surface and internal coupling charging model for the exposed dielectric of spacecraft is proposed, and its numerical solution is obtained. It is based on the deep dielectric charging model, with considering the interaction between the exposed dielectric surface and the ambient plasma by adding an incident charging current into the boundary in the proposed model, and the potential of infinite plasma is regarded as the referential potential (zero potential). The determinate solution of the model is analyzed and a numerical solution in one-dimensional case is provided by using an iterative algorithm to overcome the coupling between electric field and conductivity. The solution includes the potential of spacecraft body, the distribution of dielectric potential, and the electric field. Moreover, the new model is compared with surface charging model and internal charging model. The results show that the new model has an advatage of depicting the electric field exactly with respect to the surface charging model; if the internal deposition current is equal to zero, the new model degenerates into the one depicting the surface charging. It considers the effect of surface potential on charging results compared with the internal charging model. The three kinds of currents, namely the surface incident current, the internal deposition current and the leakage current, are considered comprehensively in the new model. Among them, the leakage current is the most complicated, which is determined by the potential and the dielectric conductivity affected by the electric field, radiation dose rate, and temperature. Using this new model, the surface and internal coupling charging simulation of the exposed dielectric can be performed. Therefore, the new model can provide a more comprehensive assessment for the charging of exposed dielectric of spacecraft.