bluuebunny/crossref_metadata_2025 huggingface dataset:
Order parameters of non-centrosymmetric superconductors
Authors: Zhang Jing-Lei  , Jiao Lin  , Pang Gui-Ming  , Yuan Hui-Qiu  ,   High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, China;,   Center for Correlated Matter, Zhejiang University, Hangzhou 310058, China;,   Department of Physics, Zhejiang University, Hangzhou 310027, China
Published: December 2020

The non-centrosymmetric (NCS) superconductors (SCs), a class of novel superconducting materials, have recently attracted considerable interests. As a result of antisymmetric spin-orbital coupling (ASOC) arising from the absence of inversion symmetry, the superconducting pairing state of these compounds allows the admixture of spin-singlet and spin-triplet components. This is in contrast to other previously studied superconductors, which usually possess an inversion symmetry in their crystal structure, and therefore their pairing state is of either spin-singlet/even parity or the spin-triplet/odd parity due to the restrictions of the Pauli principles and parity conservation.#br#Determination of the gap structure is crucial for unveiling the pairing state of NCS SCs. In this article, we first describe a method of measuring the precise temperature dependence of the changes in the London penetration depth using the tunnel-diode-oscillator (TDO), which provides an important evidence for the superconducting gap structures. Then the pairing states of NCS SCs are briefly reviewed, putting the emphasis on a few compounds with different ASOC strengths. It is proposed that the ASOC may tune the ratio of the spin-triplet to the spin-singlet component and, therefore, the spin-triplet state may become dominant while the ASOC effect is sufficiently strong in NCS SCs. However, our investigations demonstrate that the actual case is more complicated and there is no simple correspondence between the ASOC size and the pairing states. Instead, it is found that the band splitting due to the ASOC effect divided by the superconducting transition temperature Tc may better characterize of the superconducting pairing states in NCS SCs.
Recent experimental progress in low-dimensional superconductors
Authors: Zhang Xi  , Liu Chao-Fei  , Wang Jian  ,   International Center for Quantum Material, School of Physics, Peking University, Beijing 100871, China;,   Collaborative Innovation Center of Quantum Matter, Beijing 100871, China
Published: December 2020

Superconductivity is one of the most important research fields in condensed matter physics. The rapid development of material preparation technology in last few years has made the experimental study of low-dimensional physical superconducting properties feasible. This article gives a brief introduction on superconductivity and technology of low-dimensional material fabrication, and mainly focuses on the experimental progress in electrical transport studies on one-and two-dimensional superconductors, especially the results from our group. As for one-dimensional superconductivity, we review the superconductivities in single crystal Bi nanowires, crystalline Pb nano-belts, and amorphous W nanobelts, and the proximity effects in superconducting nanowires, metallic nanowires, and ferromagnetic nanowires. Surface superconductivity is revealed for crystalline Bi nanowire. The step-like voltage platforms in V-I curves are observed in Pb nano-belts and may be attributed to phase slip centers. Besides, vortex glass (VG) phase transition is discovered in amorphous W nano-belts. Inverse proximity effect is detected in crystalline Pb nanowires with normal electrodes, and proximity induced mini-gap is found in crystalline Au nanowire with superconducting electrodes. Furthermore, in crystalline ferromagnetic Co nanowire contacted by superconducting electrodes, unconventional long range proximity effect is observed. As for two-dimensional superconductivity, we review the superconductivities in Pb thin films on Si substrates, 2 atomic layer Ga films on GaN substrates, and one-unit-cell thick FeSe film on STO substrates grown by molecular beam epitaxy (MBE) method. By both in situ scanning tunneling microscopy/spectroscopy and ex situ transport and magnetization measurements, the two-atomic-layer Ga film with graphene-like structure on wide band-gap semiconductor GaN is found to be superconducting with Tc up to 5.4 K. By direct transport and magnetic measurements, the strong evidences for high temperature superconductivities in the 1-UC FeSe films on insulating STO substrates with the onset Tc and critical current density much higher than those for bulk FeSe are revealed. Finally, we give a summary and present a perspective on the future of low dimensional superconductors.
Phenomena and findings in pressurized alkaline iron selenide superconductors
Authors: Guo Jing  , Sun Li-Ling  ,   Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

In the frontiers of condensed matter physics, pressure is widely adopted as an independent control parameter for tuning states of matters and plays an important role in finding new phenomena and corresponding physics, as well as in testing the relevant theories. Remarkably, a great deal of success has been achieved in searching for new superconductors and uncovering the microphysics for known superconductors. In this brief review, we attempt to describe the progress in high pressure studies of alkaline selenide superconductors AxFe2-ySe2 (A=K, Rb, Tl/Rb).#br#The high-pressure studies of Tl0.6Rb0.4Fe1.67Se2, K0.8Fe1.7Se2 and K0.8Fe1.78Se2 superconductors show that after the ambient-pressure superconducting phase is completely suppressed under about 9 GPa, the reemergence of a pressure-induced superconductivity with a maximum Tc of 48.7 K is observed at ～11 GPa, which is the highest Tc in this kind of superconductor. The systematic investigations on transport and structural properties for K0.8FeySe2 (y=1.7 and 1.78) reveal that a pressure-induced quantum phase transition occurs at pressure between 9.2 GPa and 10.3 GPa, where the antiferromagnetic state with Fermi liquid behavior converts into the paramagnetic state with non-Fermi liquid behavior. Therefore, it is proposed that the observed reemergence of superconductivity at high pressure is probably driven by the quantum critical transition.#br#In addition, some intriguing puzzles on these superconductors and corresponding possible answers are also reviewed from the perspective of high-pressure studies, including the roles of the insulating magnetic phase in developing/stabilizing ambient-pressure and high-pressure superconducting phases and the significance of the pressure-induced antiferromagnetic fluctuation state for the emergency of superconductivity in the high-pressure superconducting phase.
Magnetic and electrical properties of K2NiF4-type Sr2CrO4
Authors: Cao Li-Peng  , Wang Xian-Cheng  , Liu Qing-Qing  , Pan Li-Qing  , Gu Chang-Zhi  , Jin Chang-Qing  ,   Department of Physics, University of Science and Technology Beijing, Beijing 100083, China;,   Key Laboratory of Extreme Conditions Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   College of Science and Research Institute for New Energy, China Three Gorges University, Yichang 443002, China
Published: December 2020

Sr2CrO4 with a K2NiF4 structure can be synthesized by different methods under ambient pressure and high pressure, while the properties reported are quite different. In this paper, pure phase Sr2CrO4 with K2NiF4 structure is obtained by one-step solid state reaction under high pressure and high temperature. Powders of SrO and CrO2 are used as the starting materials. The Sr2CrO4 structure at room temperature is determined by powder XRD measurement and GSAS Rietveld refinement. Sr2CrO4 crystal is of tetragonal symmetry with space group I4/mmm and its lattice parameters a = 3.8191 Å and c=12.5046 Å. There are two oxygen sites, apical O1 and equatorial O2. The CrO6 octahedron is slightly elongated along the c-axis, forming a longer bond Cr–O1=1.9180 Å and a shorter bond Cr–O2=1.9096 Å. Temperature dependence of the magnetic susceptibility is measured in the temperature range of 2-300 K under the magnetic field 1 T. A weak antiferromagnetic transition can be seen at TN=95 K. Above TN, the susceptibility obeys Curie-Weiss law. The Curie-Weiss fitting gives the Weiss constant θ =-364 K and the effective magnetic moment μeff=2.9 μB, in good agreement with the theoretical value of localized Cr4+, indicating the localized electronic state. Field dependence of susceptibility has been measured at different temperatures. The magnetic properties here are different from those in the previous reports, and this discrepancy is attributed to the quite different sample synthesis methods. Temperature dependence of electrical resistivity of Sr2CrO4 shows insulating behavior. Activation energy Δ is estimated by the relation ρ ∝ exp(Δ/kBT) at temperature range 150-300 K. In the temperature range 150-200 K and 200-300 K the activation energies are ΔL=0.134 eV and ΔH=0.168 eV, respectively. The insulating behavior is consistent with the previous experiment reports and the theoretical calculation, which is possibly attributed to the suppression of orbital degree of freedom, resulting from the elongation of CrO6 octahedron and the narrow band width induced by the two-dimensional crystal structure.
Effects of Co-doping on multiferroic properties of Bi6Fe2-xCoxTi3O18 ceramics
Authors: Mao Xiang-Yu  , Zou Bao-Wen  , Sun Hui  , Chen Chun-Yan  , Chen Xiao-Bing  ,   College of Physics Science and Technology, Yangzhou University, Yangzhou 225002, China
Published: December 2020

Multiferroic materials have drawn increasing interest due to the coexistence of ferromagnetism (FM) and ferroelectricity (FE), which provides significant potentials for applications in spintronics, information storage, and sensors, etc. In this paper, the multiferroic Bi6Fe2-xCoxTi3O18 (BFCT-x,x=0-2.0) ceramics are prepared by the solid-state reaction. The BFCT-x samples belong to Aurivillius structure containing five perovskite layers clapped between two Bi-O layers. The lattice constants a, b, and c of BFCT-x samples increase simultaneously with increasing cobalt content up to 0.6 and then decrease with further addition of cobalt. The magnetic and ferroelectric properties, and their corresponding Curie temperatures are measured. At room temperature (RT), the magnetism of the BFCT-0, BFCT-1.8 and BFCT-2.0 samples can be understood by the presence of the antiferromagnetic (AFM) interaction with the dominant paramagnetism (PM) state, which is consistent with the linear behavior of the M-H plot. The Fe3+-O-Fe3+ and Co3+-O-Co3+ interactions present in the BFCT-x samples lead to AFM. The BFCT-0.21.0 samples show saturated magnetic loops, while the BFCT-1.2 sample is far from saturation even under an applied magnetic field of 10 kOe. The M-H curve of BFCT-1.6 sample shows a weak ferromagnetism. The Co content (x=0.2-1.6) dependences of 2Ms and 2Mr have been recorded. Both the 2Ms and 2Mr experience first-increase-then-decrease variation tendency with their maximal values of ～ 4.49 emu/g and ～ 0.89 emu/g located at x =0.6 and x =1.0, respectively. As the cobalt content varies from x=0.2 to x=1.2, the paramagnetic-ferromagnetic phase transition temperature (TMC) decreases from 752 to 372 K. At RT, the BFCT-x samples are ferroelectric, and the maximum and minimum values of remnant polarization (2Pr) are about 8.0 up C/cm2 (x=0.6) and 1.1 up C/cm2(x=1.2), respectively. The 2Pr of the BFCT-0.6 is about three times larger than that of Bi5Fe2Ti3O18 (x=0) sample. Furthermore, the dependence of 2Pr on Co content first increases with Co doping when x qslant 0.6, and decreases from x=0.8 to x=1.2, and then increases again. The ferroelectric Curie temperature Tc of the BFCT-x samples increases with increasing x up to 0.8 and then decreases with further increasing cobalt content. It is noteworthy that the Tc of BFCT-1.0 is 2 K lower than that of BFCT-0.6, while the 2Pr decreases by 63%. It is seen that the 2Pr and 2Mr increase simultaneously with increasing Co content (below 0.6). When 0.8 x qslant 1.0, the 2Mr increases while 2Pr decreases with increasing Co content. After x1.2, the 2Mr decreases while 2Pr increases with increasing Co content. The repelling between the FE and FM as discussed above may result from the magnetic-crystalline and ferroelectric-crystalline anisotropy. The mechanism of this phenomenon is not quite clear and needs further investigation.
Effect of interfacial hydrogen bonds on the structure and dynamics of confined water
Authors: Wang Ming  , Duan Fang-Li  ,   State Key Laboratory of Mechanical Transmissions, Chongqing University, Chongqing 400030, China
Published: December 2020

Reactive molecular dynamics (MD) is used to simulate the equilibrium process of water confined between two fully hydroxylated α-quartz (001) surfaces with separation distances from 7 to 20 Å. Effect of different patterns of interfacial hydrogen bonds on the structure and dynamics of confined water is investigated. Density profiles, radial distribution functions, number of interfacial hydrogen bonds, and mean square displacements are calculated. The α-quartz (001) surface is cut from an α-quartz crystal at a certain depth to construct a surface with geminal silanols after being fully hydroxylated. The silanol groups on the surface are treated in two different ways in the MD simulations. One of the silanol groups are treated as to be fixed, and the other one is treated as no constraint for the movement of surface silanols. Our results show that different patterns of hydrogen bonds are formed at the interface between SiO2 surface and water. For the fixed silanol surface there is one type of strong hydrogen bonds interacting between the oxygen atoms of water and the hydrogen atoms of surface silanols, leading to the dipole moment of water molecules pointing out from the surface. For the movable silanol surface there are two types of strong hydrogen bonds formed at the interface. One is between the oxygen atoms of water and the hydrogen atoms of surface silanols, and the other is between the oxygen atoms of surface silanols and the hydrogen atoms of water. The number of hydrogen bonds of the first type is much less than those of the second type, leading to the dipole moment of water molecules pointing to the surface. Moreover, the total number of interfacial hydrogen bonds formed on the fixed silanol surfaces is larger than that on the movable silanol surfaces. The density profiles of the confined water indicate the formation of a strong layering of water in the vicinity of the fixed silanol surface, and the water layer is also more ordered with an ice-like structure, as compared with a dense water layer with a liquid-like structure in the case of movable silanol surfaces. Thus the mean square displacements of confined water show that, as compared with interfacial hydrogen bonds formed on the fixed silanol surfaces, the weaker and the lesser interfacial hydrogen bonds formed on the movable silanol surfaces may be responsible for more intense movement of confined water between the movable silanol surfaces. Our simulation suggests that the different pattern of interfacial hydrogen bonds could signifiantly affect the structure and dynamic behaviors of the confined water between two fully hydroxylated silica surfaces.
Robust cubature Kalman filter target tracking algorithm based on genernalized M-estiamtion
Authors: Wu Hao  , Chen Shu-Xin  , Yang Bin-Feng  , Chen Kun  ,   Information and Navigation College, Air Force Engineering University, Xi'an 710077, China
Published: December 2020

Target tracking has been introduced as a key point in the physical applications, such as passive sonar and chaotic communication etc. It is typically a nonlinear filtering problem to estimate the position and the velocity of a target from noise-corrupted measurements. Some approaches have been proposed for the problem, such as the extended Kalman filter, the unscented Kalman filter, and the cubature Kalman filter (CKF). However, they are effective only in the Gaussian and white assumption for the measurements. Actually, the measurements are easily polluted by the measurement outliers in practice. The measurement outliers may lead to inaccurate performance due to non-symmetrical or non-Gaussian property. In order to cope with the measurement outliers in nonlinear target tracking system, a robust filtering algorithm called the M-estimation based robust cubature Kalman filter (MR-CKF) is proposed for the target tracking problem. Firstly, the nonlinear measurement equation is transformed into an equivalently linear form according to the orthogonal vector, and then the Gaussian extremal function of the target tracking can be obtained by the constrained total least square (CTLS) criterion. By employing the Huber's robust score function, the Gaussian extremal function is further rendered into a robust extremal function, thus the generalized M-estimation can be introduced to the CKF without linearization approximation. The only difference between the Gaussian extremal function and the robust extremal function is the weight matrix, implying that the CKF solution framework does not change and the virtues of both the CKF and M-estimation can be fully utilized such as derivative-free, high accuracy and robust performance. Furthermore, an improved Huber equivalent weight function is designed for the MR-CKF based on the Mahalanobis distance. The outliers' judge threshold is determined according to the confidence level of Chi-square distribution and improper empirical value of the Huber's method can be avoided. In addition, the improved Huber weight function reduces weights of small outliers and removes large outliers, and this is more robust and reasonable than the Huber's method. Moreover, the statistical information of outliers is also not required. Theoretical analysis and numerical results show that the proposed filtering algorithm can improve the accuracy and robustness than the conventional robust algorithms.
Community detection based on joint matrix factorization in networks with node attributes
Authors: Chang Zhen-Chao  , Chen Hong-Chang  , Liu Yang  , Yu Hong-Tao  , Huang Rui-Yang  ,   National Digital Switching System Engineering & Technological Research Center, Zhengzhou 450002, China
Published: December 2020

An important problem in the area of social networking is the community detection. In the problem of community detection, the goal is to partition the network into dense regions of the graph. Such dense regions typically correspond to entities which are closely related with each other, and can hence be said to belong to a community. Detecting communities is of great importance in computing biology and sociology networks. There have been lots of methods to detect community. When detecting communities in social media networks, there are two possible sources of information one can use: the network link structure, and the features and attributes of nodes. Nodes in social media networks have plenty of attributes information, which presents unprecedented opportunities and flexibility for the community detection process. Some community detection algorithms only use the links between the nodes in order to determine the dense regions in the graph. Such methods are typically based purely on the linkage structure of the underlying social media network. Some other community detection algorithms may utilize the nodes' attributes to cluster the nodes, i.e. which nodes with the same attributes would be put into the same cluster. While traditional methods only use one of the two sources or simple linearly combine the results of community detection based on different sources, they cannot detect community with node attributes effectively. In recent years, matrix factorization (MF) has received considerable interest from the data mining and information retrieval fields. MF has been successfully applied in document clustering, image representation, and other domains. In this paper, we use nodes attributes as a better supervision to the community detection process, and propose an algorithm based on joint matrix factorization (CDJMF). Our method is based on the assumption that the two different information sources of linkage and node attributes can get an identical nodes' affiliation matrix. This assumption is reasonable and can interpret the inner relationship between the two different information sources, based on which the performance of community detection can be greatly improved. We also conduct some experiments on three different real social networks; theoretical analysis and numerical simulation results show that our approach can get a superior performance than some classical algorithms, so our method is an effective way to explore community structure of social networks.
Multiple harmonic X-ray pulsar signal phase estimation method
Authors: Song Jia-Ning  , Xu Guo-Dong  , Li Peng-Fei  ,   Research Center of Satellite Technology, Harbin Institute of Technology, Harbin 150001, China;,   School of Electrical and Electronic Engineering, Harbin Institute of Science and Technology, Harbin 150001, China
Published: December 2020

Pulsars, a small portion of celestial sources that emit radiation at varying intensity, provide new possible navigation algorithms which are different from steady point sources. Time-delay estimation is one of the key aspects of pulsar-based navigation technology. Previous work for pulse phase estimation uses a maximum likelihood estimator (MLE) for the phase-in time domain, which is seen as one of the most useful phase estimators. However, the analytic solution for phase cannot be found using MLE. As a result, a brute-force method involving nested, iterative grid-searches is needed to solve this MLE issue, which leads to lots of computations. In order to solve this problem, a multiple harmonic X-ray pulsar signal phase estimation (MHSPE) method is proposed. This method uses the times of arrivals (TOAs) measured pulsar signal to estimate the time-delay in the frequency domain. In this paper, firstly we use the arrival time to derive the maximum-likehood (ML) estimation of phase-delay by fundamental frequency, then an analytic expression for the fundamental frequency phase is obtained. The MHSPE method based on the fundamental frequency phase equation, calculates different harmonic phases by generalizing the analytic expression of fundamental frequency phase, and the normalized amplitude of each harmonic in the spectrum is used as the weight of each harmonic phase. Finally, the weighted average of harmonic phases, which is given by the final analytic expression, is used as the estimation of the moment. To evaluate the MHSPE method, the error and variance equations are calculated and the MHSPE method is demonstrated to be unbiased and consistent. Moreover, by comparing with the ML estimation of the first harmonic, if the amplitudes of harmonic in the spectrum are almost the same, the signal-to-noise ratio (SNR) of MHSPE improves m1/2 times when the number of harmonic waves is m. Compared with the Cramer-Rao bound of pulsar time-delay estimation, the derivative of pulsar signal in the time domain reflects the number of harmonic waves in the frequency domain. Hence, the MHSPE can greatly approximate to the Cramer-Rao bound for the estimation of pulsar signal timedelay when the harmonic number increases. Finally, we utilize the TOAs of Crab pulsar, observed by Rossi X-ray timing explorer (RXTE) spacecraft, to verify the performance of MHSPE. The results show that for low SNR of pulsar signal, MHSPE can obtain high precision phase estimations. When the amplitude of the harmonic in the spectrum is larger, the estimation variance of the harmonic phase tends to be smaller. The projection orbit determined by MHSPE method can match the projection of RXTE in the direction of Crab pulsar, with the observed time increasing, the estimation accuracy converges rapidly to Cramer-Rao bound.
Characteristic analysis of power function type monostable stochastic resonance with Levy noise
Authors: Zhang Gang  , Hu Tao  , Zhang Tian-Qi  ,   School of Communication, Chongqing University of Posts and Telecommunications, Chongqing 400065, China;,   Key Laboratory of Signal and Information Processing of Chongqing, Chongqing 400065, China
Published: December 2020

In this paper, the Levy noise is combined with a power function type monostable stochastic resonance system for the first time. In order to ensure the reliability of the experimental data, the average signal-to-noise ratio gain is regarded as an index to investigate the stochastic resonance phenomenon stimulated by Levy noise. Potential function form of the monostable system and the method of generating Levy noise are presented in detail. The pulse characteristic and smear characteristic of Levy noise are also presented in detail. The laws for the resonant output of monostable system, governed by parameters a and b, the intensity amplification factor D of Levy noise, are explored under different values of characteristic index and symmetry parameter of Levy noise. Results show that no matter whether it is under any different characteristic index or symmetry parameter of Levy noise, the weak signal can be detected by adjusting the system parameters a and b. The intervals of a and b which can induce stochastic resonances are multiple, and do not change with nor . Moreover, the same rule is founded which by adjusting the intensity amplification factor D of Levy noise can also realize synergistic effect when studying the noise-induced stochastic resonance, and the interval of D does not change with nor ; the best value of characteristic index is =1 under any system parameter, and the best value of symmetry parameter is =1 under any system parameter. So, the system performance is best when =1 and =1. Finally, the interaction relationship between system parameters a and b is investigated, and it is found that the interval of a or b will change with b or a when characteristic index , symmetry parameter and the intensity amplification factor D of Levy noise are fixed. These results will contribute to reasonably choosing the system parameters and intensity amplification factor of power function type monostable stochastic resonance system under Levy noise, and provide a reliable basis for practical engineering application of weak signal detection by stochastic resonance.
Real-time polarization difference underwater imaging based on Stokes vector
Authors: Guan Jin-Ge  , Zhu Jing-Ping  , Tian Heng  , Hou Xun  ,   Key Laboratory for Physical Electronics and Devices of the Ministry of Education, Department of Electronic Science and Technology, Xi'an Jiaotong University, Xi'an 710049, China;,   Shaanxi Key Lab of Information Photonic Technique, Department of Electronic Science and Technology, Xi'an Jiaotong University, Xi'an 710049, China
Published: December 2020

Polarization difference imaging technique can effectively solve the underwater image deterioration problem that is caused by the interaction between light and water. Therefore, it has a significant application value in detecting and recognizing underwater target. In a traditional polarization difference imaging system, the object image is carried out by the common-mode rejection of background scattering light. However, the polarization state of the background scattering light is unknown, so the polarization difference imaging is realized by the irregular mechanical rotation of the optical polarization analyzer with two orthogonal polarization orientations. Therefore, it needs more time to determine the optimum detection angle of the polarization analyzer and cannot perform real-time underwater imaging, which restricts the rapid detecting function in the process of underwater imaging. In this paper, the detection principle of underwater polarization difference imaging is considered to exploit the difference in the polarization angle between background scattering light and target light. According to Marius's law, the physical model of polarization difference imaging is that the common-mode rejection of background scattering light will be achieved when the angles between the vibration direction of background and the two orthogonal polarization orientations are 45. Because the Stokes vector can be used to express the polarization angle of light, we propose the principle and construction of a computational polarization difference imaging system for real-time underwater imaging by incorporating the Stokes vector into the established model. It replaces the mechanical rotation of the polarization analyzer in a traditional polarization difference imaging system with the information processing of the Stokes vector. The experimental results show that the proposed method not only has the same effective performance as the conventional polarization difference imaging compared with the regular imaging, but also can improve the blurred imaging performance caused by an underwater scattering effect as well as increase the underwater detection distance. This method realizes rapid underwater target detection and recognition because it saves a large amount of time compared with the traditional one. Further, if we combine this method with the current polarization imaging instruments that capture the Stokes vector instantaneously, then a real-time automatic underwater polarization imaging can improve the efficiency of the underwater target detection and recognition. These findings are helpful for designing and developing the underwater polarization difference imaging systems.
Analysis of the clear aperture of Savart plates in polarization interference imaging spectrometer
Authors: Li Qi-Wei  , Zhang Chun-Min  , Wei Yu-Tong  , Chen Qing-Ying  ,   Institute of Space Optics, Xi'an Jiaotong University, Xi'an 710049, China;,   School of Science, Xi'an Jiaotong University, Xi'an 710049, China
Published: December 2020

The principle of beam splitting of interference imaging spectrometer based on Savart plates is presented. The propagation track of light wave in crystal and the exit aperture is analysed by combining wave normal-tracing method with ray-tracing method at random incidence angle and azimuth angle. The relationship among exit aperture, incident angle, incident position and azimuth angle is deduced in detail. The requirements that the propagation track of light remains in Savart plates and would not exit from the top surface, bottom surface and side are discussed in detail. The area and the position of exit aperture are simulated by computer, which proves the correctness of the deduction for normal incidence of light. It is shown that the lateral shear of single Savart plate restricts the boundary of clear aperture, and the area of the clear aperture is only 85.73% against the total incident surface. The parameter of experimental prototype is introduced and the clear aperture is in detail analysed and discussed by the method mentioned above. The results show that the accurate and the approximate values of exit aperture area of experimental prototype are greatly different, and the position of exit aperture are shifted into the lower right, which may reduce the image quality and even cannot generate the double-beam interference fringes in some specific areas. The effective clear aperture as a function of azimuth angle is also presented. It shows that the used clear aperture area is between 0.8005 and 0.8547 while changes from 0 to 2, in order to match the conditions that the value of area availability decreases to 0.6976 when the light always propagates inside the Savart plates. The article shows that the change of clear aperture caused by crystal birefringence phenomenon cannot be ignored when selecting the instrument aperture stop and parameter of interference imaging spectrometer. The clear aperture of the two beams, o-light and e-light, which propagate in Savart plates should be calculated respectively and then they are used to determine the last clear apertures of plates. This study provides a theoretical and practical guidance for study, design, modulation, experiment and engineering of interference imaging spectrometers.
Analysis and alignment of the light path of Gauss beam matched to the fundamental mode ofan optical resonator
Authors: Cui Li-Hong  , Zhao Wei-Ning  , Yan Chang-Xiang  ,   Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

In order to reduce the influences of misalignment parameter and mismatch parameter on measurement based on optical resonator, the influence on the coupling efficiency of a source laser is stabilized to a fundamental cavity mode, and two limiting cases are analyzed and derived by using conversion of Gaussian beam, mode coupling theory and coordinate transformation theory, including the expression of coupling efficiency of fundamental cavity mode as two limiting cases emerge simultaneously. Analyses show that for mismatch parameter, only even-indexed Hermite-Gaussians beam is excited; for misalignment parameter, there exists an effect on the proportion of Hermite-Gaussians beam, which should bring about serious measurement error. These optical signals provide the error signals which are minimized. By taking the laser line width into account, we propose two methods for real time alignment of a Gaussian beam for an optical resonator perfectly coupled system: Fabry-Perot electro-optic sensors of a misadjusted system and control loops system depends on detecting emergent light of cavity via multi-dimensional quadrant detector. All of these will provide a theoretical direction for analyzing the measurement error and improving the measurement accuracy.
Precision measurement of single-atom trajectories in higher-order Laguerre-Gaussian transverse modes of a Fabry-Perot cavity
Authors: Wang Yan-Na  , Zhao Di  , Fang Ai-Ping  , Jiang Chen-Wei  , Gao Shao-Yan  , Li Fu-Li  ,   Department of Applied Physics, School of Science, Xi'an Jiaotong University, Xi'an 710049, China
Published: December 2020

A coupled quantum system composed of cavity field and atoms is one of the main research contents of cavity quantum electrodynamics. It can be used to realize single atom manipulation and measurement, and has important significance for studying the interaction between light and the atom, preparing quantum states and quantum entanglement. Current research work mainly focuses on two aspects. One is to achieve the atom trapping via the feedback control of the trapping laser intensity. The other is to measure the single atomic motion in a Fabry-Perot cavity by using Hermite-Gaussian transverse modes. The detection of the atomic trajectories has been realized via the observation of transmission spectra of the strong coupling system composed of cold atoms and Hermite-Gaussian transverse modes in a Fabry-Perot cavity. In order to observe the atomic motion trajectories in the cavity, we theoretically study the transmission spectrum of a strong coupling system composed of cold atoms and Laguerre-Gaussian transverse modes in a Fabry-Perot cavity in this paper. We calculate the relationship between the coupling coefficient and the mode number of Laguerre-Gaussian transverse modes. The result shows that with the increase of Laguerre-Gaussian transverse mode number, the maximum coupling coefficient between the atoms and cavity fields is almost unchanged, so the contrast of the detected spectrum is nearly independent of the mode number. Analysis shows that Laguerre-Gaussian transverse mode provides more abundant information about atomic motion trajectory than Hermite-Gaussian transverse mode. The field distribution of Laguerre-Gaussian transverse mode is ring-shaped. Owing to the ring shape, the atoms dropped at different positions experience different electric field intensities, and the detected transmission spectra are changed. Therefore, we can implement the high precision distinguishment of the atomic trajectories by observing the features of the transmission spectra such as the number of the transmission peaks and their positions. Furthermore, a small deviation of the atomic motion trajectories, on the edges of the rings of the electric field, may induce great change in transmission spectrum, and then we can very accurately detect the atomic motion around these positions.
Experimental investigation of the outer scale in atmospheric optical turbulence near the ground
Authors: Wang Qian  , Mei Hai-Ping  , Qian Xian-Mei  , Rao Rui-Zhong  ,   Key Laboratory of Atmospheric Composition and Optical Radiation, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China;,   University of Chinese Academy of Sciences, Beijing 100084, China
Published: December 2020

In this article, the Von-Karman model of turbulence spatial correlation function which contains the parameter of outer scale is analyzed. Then, the experimental data of air refractive index variation obtained from a high quality fiber optical turbulence sensing array are used to evaluate the outer scale of atmospheric optical turbulence as well as its diurnal variation through the algorithm of nonlinear fitting. The results validate the suitability of the Von-Karman model. By combining the theory of correlation function with the spatially distributed and simultaneously measured data, three kinds of turbulence spatial scales described by correlation function are revealed as clearly as possible. Results show that the values of outer scale in atmospheric optical turbulence 1.8 m above the grassland tend to be larger in the daytime and smaller in the night. The mean value around noon is 0.44 m, while in the night it becomes 0.3 m. Here, three of the important views should be noted. Firstly, when the displacement of two points is just equal to the outer scale, their correlation coefficient is 0.26, and when it exceeds the outer scale, there is still a certain value of correlation coefficient. Secondly, the integral scale represents the averaged value of scale in the vortex of optical turbulence. And, it is slightly smaller than the outer scale. Thirdly, when the distance of two points equals the biggest scale of vortex in optical turbulence, the correlation coefficient tends to zero, and the value of biggest scale is slightly bigger than the outer scale. It is easy to find that the diurnal variation tendencies of the three spatial scales are similar to that of intensity in optical turbulence. The method of obtaining the characteristic scales by spatially arranged and simultaneously measured optical turbulence is direct, and the results can be considered as the evidence to prove the models of correlation function including the Von-Karman model. So, it promotes the research on the property of spatial structure to a certain extent.
Electronic structure and optical properties of C doped rutile TiO2: the first-principles calculations
Authors: Pan Feng-Chun  , Lin Xue-Ling  , Chen Huan-Ming  ,   School of Physics and Electric Information Engineering, Ningxia University, Yinchuan 750021, China
Published: December 2020

The lattice parameters and band-gap of native rutile TiO2 are investigated by the first-principles calculations of local density approximation+U method with different U values for Ti-3d (0 U 9 eV). The electronic structures and optical properties of different content C doped rutile TiO2 systems are also studied by the same method with appropriate U values. The calculations results show that the lattice parameters and band-gaps of TiO2 increase with the increase of U and the U =3 eV is fitted for the corrected band-gap. For the doped systems, the impurity energy level is introduced due to the coupling between O-2p and C-2p, which can increase the TiO2 absorption edge to the visible region, and therefore enlarge the absorption region of TiO2. Moreover, the 8.3% C is an optimal doped density, which can lead to the red-shift of optical absorption edge obviously and increase the coefficient of light absorption, therefore facilitate the enhancement of the photocatalytic efficiency.
A novel circularly polarized patch antenna with low radar cross section and high-gain
Authors: Cong Li-Li  , Fu Qiang  , Cao Xiang-Yu  , Gao Jun  , Song Tao  , Li Wen-Qiang  , Zhao Yi  , Zheng Yue-Jun  ,   Information and Navigation College, Air Force Engineering University, Xi'an 710077, China;,   Air and Missile Defense College, Air Force Engineering University, Xi'an 710077, China
Published: December 2020

A novel circularly polarized patch antenna, which can achieve low radar cross section (RCS) and high gain performance simultaneously, is designed on the basis of metamaterial superstrate. The novelty of the design is that this antenna can possess the absorbing characteristic and the partially reflective characteristic simultaneously in an integrated structure. The proposed superstrate is composed of two metallic layers with different periodic patterns on both sides of a dielectric substrate. Through constructing different metallic patterns on the two sides of the superstrate, the upper and bottom surfaces of the superstrate will have different transmission and reflection performances when illuminated by an incident plane wave. The low RCS characteristic is dependent on the upper surface, while the gain enhancement of the resonator antenna relies on the reflection coefficient of the bottom surface. The upper surface consisting of a periodic metallic square loop with four lumped resistances on the four sides of the loop is of low reflection and transmission, and the bottom surface composed of a metallic plane with periodic slots is of high reflection and low transmission. When the superstrate is located at approximately half a wavelength above the ground plane of the circularly polarized patch antenna, the upper surface will absorb most of the incident wave by converting the electromagnetic wave into heat as Ohm loss to reduce the antenna RCS, and the bottom surface will form a Fabry-Perot resonance cavity with the ground plane of the antenna to achieve high gain and high directivity by multiple reflections between the bottom surface and the ground plane. The measured results show that with using the superstrate, the relative axial ratio bandwidth of the circularly polarized patch antenna extends from 5.9% to 7.1%, and the high gain performance is achieved in the whole working frequency band, which can be enhanced by 6.61 dB at most. Meanwhile, the RCS of the proposed antenna is dramatically reduced in a wide angle range and a broad frequency band covering a range from 2 to 14 GHz. The measured results are in good agreement with the simulated ones, which further verifies the correctness and effectiveness of the proposed method.
D-shaped photonic crystal fiber refractive index and temperature sensor based on surface plasmon resonance and directional coupling
Authors: Shi Wei-Hua  , You Cheng-Jie  , Wu Jing  ,   School of Optoelectronic Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
Published: December 2020

The photonic crystal fiber has received the widespread attention in the sensing field because of its flexible structure and unique features. A refractive index and temperature sensor based on the D-shaped photonic crystal fiber is designed and analyzed. In the side section of the D-shaped photonic crystal fiber, a coat with a gold film is used as a surface plasmon resonance (SPR) sensing channel for measuring the refractive index of liquid determinand. Temperature sensitive liquid-toluene is filled in an air hole A as a directional coupling sensing channel to realize the temperature measurement. When the SPR mode and guided mode satisfy the phase matching condition, the SPR effect is produced. Most of the core energy is transferred to the metal film layer in the surface, and then the loss of guided mode in the fiber core will grow. Therefore, the shift of the SPR peak position can be used to measure the refractive index of the determinand indirectly. When the wave mode in the thermosensitive liquid-toluene can achieve phase matching with the guided mode, the directional coupling effect occurs, and then the wavelength of the absorption peak position can be used to measure the change of temperature indirectly. Based on further numerical simulation, the peak position of directional coupling is not changed by the refractive index of the determinand, and the SPR peak position is not shifted by the temperature change either. As these two sensing mechanisms can be distinguished easily, the refractive index and temperature sensing are simultaneously realized. The characteristics of the sensor are simulated numerically by using a full vector finite element method under the boundary condition of anisotropic perfectly matched layer. From the analysis of the D-shaped photonic crystal fiber structure parameters, we find that the diameter d of air hole plays an important role in the directional coupling absorption peak position and temperature sensitivity. For the SPR peak, its position is only affected by the thickness t of gold film, and its relative intensity is changed with the diameter d of air hole and grinding depth d1. The results show that when the temperature ranges from -10 ℃ to 80 ℃, the temperature sensitivity reaches 11.6 nm/℃, and when the refractive index is in a range from 1.32 to 1.44, its sensitivity reaches 26000 nm/RIU.
Influence of the interfacial properties on guided circumferential wave propagation in the circular tube structure
Authors: Gao Guang-Jian  , Deng Ming-Xi  , Li Ming-Liang  , Liu Chang  ,   Logistics Engineering University, Chongqing 401331, China
Published: December 2020

The mathematical expressions both of displacement and stress fields of circumferential wave propagation in circular tube structure with a weak interface are derived on condition that the interfacial properties between the two circular tubes are characterized by the interfacial spring model. Based on the said displacement and stress expressions derived, the dispersion equation of ultrasonic guided circumferential wave (UGCW) modes is formally presented by using the corresponding mechanical boundary conditions. According to the technique of modal expansion analysis for waveguide excitation, for a given excitation source used to generate circumferential wave in circular tube structure, the corresponding field of circumferential wave propagation can be decomposed into a series of UGCW modes. Using the reciprocity relations and mode orthogonality, the analytical expression of UGCW mode expansion coefficient is derived, which is closely related to the given excitation source for UGCW generation and the interfacial properties between the two tubes. The influences of change in the interfacial property on dispersion and acoustic field of the UGCW propagation are numerically analyzed. In the cases of perfect and sliding interfaces, for a given UGCW mode, the relative change rate of phase velocity is defined, and then its curve versus frequency is calculated, through which the specific frequency can be determined where the UGCW phase velocity appears to be most sensitive to the change in the interfacial property. For a given UGCW mode and driving frequency, it is numerically found that the displacement field on the outside surface of the circular tube structure changes sensitively and monotonically with change in interfacial property between the tubes. Clearly, through choosing the appropriate driving frequency and the mode of UGCW propagation, both the UGCW phase velocity and the displacement field on the outside surface of the circular tube structure will be monotonic and sensitive to change in interfacial property. It is expected that the results obtained in this paper will be of significance for accurately characterizing the interfacial properties of composite circular tube structures by using the UGCW technique.