Table of contents

    05 November 2020, Volume 29 Issue 11 Previous issue    Next issue
    Design of passive filters for time-delay neural networks with quantized output
    Jing Han(韩静), Zhi Zhang(章枝), Xuefeng Zhang(张学锋), and Jianping Zhou(周建平)
    Chin. Phys. B, 2020, 29 (11):  110201.  DOI: 10.1088/1674-1056/aba602
    Abstract ( 362 )   HTML ( 2 )   PDF (542KB) ( 101 )  

    Passive filtering of neural networks with time-invariant delay and quantized output is considered. A criterion on the passivity of a filtering error system is proposed by means of the Lyapunov–Krasovskii functional and the Bessel–Legendre inequality. Based on the criterion, a design approach for desired passive filters is developed in terms of the feasible solution of a set of linear matrix inequalities. Then, analyses and syntheses are extended to the time-variant delay situation using the reciprocally convex combination inequality. Finally, a numerical example with simulations is used to illustrate the applicability and reduced conservatism of the present passive filter design approaches.

    An efficient inverse approach for reconstructing time- and space-dependent heat flux of participating medium
    Shuang-Cheng Sun(孙双成), Guang-Jun Wang(王广军), and Hong Chen(陈红)$
    Chin. Phys. B, 2020, 29 (11):  110202.  DOI: 10.1088/1674-1056/aba608
    Abstract ( 275 )   HTML ( 2 )   PDF (1509KB) ( 40 )  

    The decentralized fuzzy inference method (DFIM) is employed as an optimization technique to reconstruct time- and space-dependent heat flux of two-dimensional (2D) participating medium. The forward coupled radiative and conductive heat transfer problem is solved by a combination of finite volume method and discrete ordinate method. The reconstruction task is formulated as an inverse problem, and the DFIM is used to reconstruct the unknown heat flux. No prior information on the heat flux distribution is required for the inverse analysis. All retrieval results illustrate that the time- and space-dependent heat flux of participating medium can be exactly recovered by the DFIM. The present method is proved to be more efficient and accurate than other optimization techniques. The effects of heat flux form, initial guess, medium property, and measurement error on reconstruction results are investigated. Simulated results indicate that the DFIM is robust to reconstruct different kinds of heat fluxes even with noisy data.

    Effect of weak measurement on quantum correlations
    L Jebli, M Amzioug, S E Ennadifi, N Habiballah, and M Nassik$
    Chin. Phys. B, 2020, 29 (11):  110301.  DOI: 10.1088/1674-1056/aba5fa
    Abstract ( 293 )   HTML ( 2 )   PDF (454KB) ( 140 )  

    We investigate the local quantum uncertainty (LQU ) in weak measurement. An expression of weak LQU is explicitly determined. Also, we consider some cases of three special X states, Werner state, circulant two-qubit states, and Heisenberg model via LQU in normal and weak measurements. We find that the LQU in weak measurement is weaker than the case of strong measurement.

    Zitterbewegung of Dirac quasiparticles emerged in a Su-Schrieffer–Heeger lattice
    Yue Hu(胡玥), Zheng-Xin Guo(郭政鑫), Ze-Ming Zhong(钟泽明), and Zhi Li(李志)
    Chin. Phys. B, 2020, 29 (11):  110302.  DOI: 10.1088/1674-1056/abb45b
    Abstract ( 463 )   HTML ( 2 )   PDF (1373KB) ( 211 )  

    We analytically and numerically investigate the dynamical properties of the tilted dispersion relativistic quasiparticles emerged in a cold atomic optical lattice system. By introducing the next nearest neighboring (NNN) hopping term into Su–Schrieffer–Heeger (SSH) model, the Dirac quasiparticles with tilted dispersion relation are realized. The results show that the tilted dispersion causes a drift in relativistic quasiparticles rather than affecting interference behavior between inner states. To be specific, the relativistic phenomena of the quasiparticles induced by the inner state interference (such as Zitterbewegung, Klein paradox, etc.) is completely unaffected by the tilted dispersion. In order to distinguish the drift induced by tilted dispersion and common initial velocity, we calculate the momentum distribution of the relativistic quasiparticles. We obtain the difference between the drift induced by initial velocity and tilted dispersion. The former affects the ZB, while the latter does not. By using this character, we propose a quench dynamics scheme to obtain a stable mono-spin state. The proposed cold atomic lattice system would provide a promising platform in exploring the intrinsic exotic physics of relativistic quasiparticles and the related systems.

    SPECIAL TOPIC—Quantum computation and quantum simulation
    A two-dimensional quantum walk driven by a single two-side coin
    Quan Lin(林泉), Hao Qin(秦豪) Kun-Kun Wang(王坤坤), Lei Xiao(肖磊), and Peng Xue(薛鹏)
    Chin. Phys. B, 2020, 29 (11):  110303.  DOI: 10.1088/1674-1056/abaee8
    Abstract ( 579 )   HTML ( 4 )   PDF (823KB) ( 247 )  

    We study a two-dimensional quantum walk with only one walker alternatively walking along the horizontal and vertical directions driven by a single two-side coin. We find the analytical expressions of the first two moments of the walker’s position distribution in the long-time limit, which indicates that the variance of the position distribution grows quadratically with walking steps, showing a ballistic spreading typically for quantum walks. Besides, we analyze the correlation by calculating the quantum mutual information and the measurement-induced disturbance respectively as the outcome of the walk in one dimension is correlated to the other with the coin as a bridge. It is shown that the quantum correlation between walker spaces increases gradually with the walking steps.

    New semi-quantum key agreement protocol based on high-dimensional single-particle states
    Huan-Huan Li(李欢欢), Li-Hua Gong(龚黎华), and Nan-Run Zhou(周南润)
    Chin. Phys. B, 2020, 29 (11):  110304.  DOI: 10.1088/1674-1056/abaedd
    Abstract ( 400 )   HTML ( 5 )   PDF (481KB) ( 237 )  

    A new efficient two-party semi-quantum key agreement protocol is proposed with high-dimensional single-particle states. Different from the previous semi-quantum key agreement protocols based on the two-level quantum system, the propounded protocol makes use of the advantage of the high-dimensional quantum system, which possesses higher efficiency and better robustness against eavesdropping. Besides, the protocol allows the classical participant to encode the secret key with qudit shifting operations without involving any quantum measurement abilities. The designed semi-quantum key agreement protocol could resist both participant attacks and outsider attacks. Meanwhile, the conjoint analysis of security and efficiency provides an appropriate choice for reference on the dimension of single-particle states and the number of decoy states.

    Systematic error suppression scheme of the weak equivalence principle test by dual atom interferometers in space based on spectral correlation
    Jian-Gong Hu(胡建功), Xi Chen(陈曦), Li-Yong Wang(王立勇), Qing-Hong Liao(廖庆洪), and Qing-Nian Wang(汪庆年)$
    Chin. Phys. B, 2020, 29 (11):  110305.  DOI: 10.1088/1674-1056/aba609
    Abstract ( 395 )   HTML ( 3 )   PDF (700KB) ( 168 )  

    Systematic error suppression and test data processing are very important in improving the accuracy and sensitivity of the atom interferometer (AI)-based weak-equivalence-principle (WEP) test in space. Here we present a spectrum correlation method to investigate the test data of the AI-based WEP test in space by analyzing the characteristics of systematic errors and noises. The power spectrum of the Eötvös coefficient η, systematic errors, and noises in AI-based WEP test in space are analyzed and calculated in detail. By using the method, the WEP violation signal is modulated from direct current (DC) frequency band to alternating current (AC) frequency band. We find that the signal can be effectively extracted and the influence of systematic errors can be greatly suppressed by analyzing the power spectrum of the test data when the spacecraft is in an inertial pointing mode. Furthermore, the relation between the Eötvös coefficient η and the number of measurements is obtained under certain simulated parameters. This method will be useful for both isotopic and nonisotopic AI-based WEP tests in space.

    Ferromagnetic transition of a spin–orbit coupled dipolar Fermi gas at finite temperature
    Xue-Jing Feng(冯雪景) and Lan Yin(尹澜)
    Chin. Phys. B, 2020, 29 (11):  110306.  DOI: 10.1088/1674-1056/aba9ca
    Abstract ( 454 )   HTML ( 2 )   PDF (375KB) ( 185 )  

    We study the ferromagnetic transition of a two-component homogeneous dipolar Fermi gas with 1D spin–orbit coupling (SOC) at finite temperature. The ferromagnetic transition temperature is obtained as functions of dipolar constant λd, spin–orbit coupling constant λSOC and contact interaction constant λs. It increases monotonically with these three parameters. In the ferromagnetic phase, the Fermi surfaces of different components can be deformed differently. The phase diagrams at finite temperature are obtained.

    Protecting the entanglement of two-qubit over quantum channels with memory via weak measurement and quantum measurement reversal
    Mei-Jiao Wang(王美姣), Yun-Jie Xia(夏云杰), Yang Yang(杨阳), Liao-Zhen Cao(曹连振), Qin-Wei Zhang(张钦伟), Ying-De Li(李英德), and Jia-Qiang Zhao(赵加强)
    Chin. Phys. B, 2020, 29 (11):  110307.  DOI: 10.1088/1674-1056/aba614
    Abstract ( 371 )   HTML ( 2 )   PDF (1701KB) ( 63 )  

    Based on the quantum technique of the weak measurement and quantum measurement reversal (WMR), we propose a scheme to protect entanglement for an entangled two-qubit pure state from four typical quantum noise channels with memory, i.e., the amplitude damping channel, the phase damping channel, the bit flip channel, and the depolarizing channel. For a given initial state | ψ 〉 = a| 00 〉 + d| 11 〉, it is found that the WMR operation indeed helps to protect entanglement from the above four quantum channels with memory, and the protection effect of WMR scheme is better when the coefficient a is small. For the other initial state | ϕ 〉 = b| 01 〉 + c| 10 〉, the effect of the protection scheme is the same regardless of the coefficient b and the WMR operation can protect entanglement in the amplitude damping channel with memory. Moreover, the protection of entanglement in quantum noise channels without memory in contrast to the results of the channels with memory is more effective. For | ψ 〉 or | ϕ 〉, we also find that the memory parameters play a significant role in the suppression of entanglement sudden death and the initial entanglement can be drastically amplified. Another more important result is that the relationship between the concurrence, the memory parameter, the weak measurement strength, and quantum measurement reversal strength is found through calculation and discussion. It provides a strong basis for the system to maintain maximum entanglement in the nosie channel.

    Thermal entanglement in a spin-1/2 Ising–Heisenberg butterfly-shaped chain with impurities Hot!
    Meng-Ru Ma(马梦如), Yi-Dan Zheng(郑一丹), Zhu Mao(毛竹), and Bin Zhou(周斌)
    Chin. Phys. B, 2020, 29 (11):  110308.  DOI: 10.1088/1674-1056/abbbde
    Abstract ( 581 )   HTML ( 2 )   PDF (2706KB) ( 269 )  

    We investigate the effect of impurities on the thermal entanglement in a spin-1/2 Ising–Heisenberg butterfly-shaped chain, where four interstitial Heisenberg spins are localized on the vertices of a rectangular plaquette in a unit block. By using the transfer-matrix approach, we numerically calculate the partition function and the reduced density matrix of this model. The bipartite thermal entanglement between different Heisenberg spin pairs is quantified by the concurrence. We also discuss the fluctuations caused by the impurities through the uniform distribution and the Gaussian distribution. Considering the effects of the external magnetic field, temperature, Heisenberg and Ising interactions as well as the parameter of anisotropy on the thermal entanglement, our results show that comparing with the case of the clean model, in both the two-impurity model and the impurity fluctuation model the entanglement is more robust within a certain range of anisotropic parameters and the region of the magnetic field where the entanglement occurred is also larger.

    Jeans gravitational instability with κ-deformed Kaniadakis distribution in Eddington-inspired Born–Infield gravity
    Wei-Heng Yang(杨伟恒), Yu-Zhen Xiong(熊玉珍), Hui Chen(陈辉), and San-Qiu Liu(刘三秋)$
    Chin. Phys. B, 2020, 29 (11):  110401.  DOI: 10.1088/1674-1056/abb3f0
    Abstract ( 336 )   HTML ( 2 )   PDF (719KB) ( 111 )  

    Based on the framework of Kaniadakis’ statistics and its related kinetic theory, the Jeans instability for self-gravitational systems in the background of Eddington-inspired Born–Infield (EiBI) gravity is revisited. A dispersion relation generalizing the Jeans modes is derived by modifying the Maxwellian distribution to a family of power law distributions parameterized by the κ parameter. It is established that the κ-deformed Kaniadakis distribution has significant effects on the Jeans modes of the collisionless EiBI-gravitational systems. And as expected, in the limitation κ → 0, the corresponding results for Maxwellian case are recovered. The related result in the present work is valuable for the investigations involving the fields of astrophysics such as neutron stars, accretion disks, and relevant plasma physics, etc.

    Non-equilibrium atomic simulation for Frenkel–Kontorova model with moving dislocation at finite temperature
    Baiyili Liu(刘白伊郦) and Shaoqiang Tang(唐少强)
    Chin. Phys. B, 2020, 29 (11):  110501.  DOI: 10.1088/1674-1056/abaed4
    Abstract ( 340 )   HTML ( 1 )   PDF (798KB) ( 148 )  

    We apply the heat jet approach to realize atomic simulations at finite temperature for a Frenkel–Kontorova chain with moving dislocation. This approach accurately and efficiently controls the system temperature by injecting thermal fluctuations into the system from its boundaries, without modifying the governing equations for the interior domain. This guarantees the dislocation propagating in the atomic chain without nonphysical damping or deformation. In contrast to the non-equilibrium Nosé–Hoover heat bath, the heat jet approach efficiently suppresses boundary reflections while the moving dislocation and interior waves pass across the boundary. The system automatically returns back to the equilibrium state after all non-thermal motions pass away. We further apply this approach to study the impact of periodic potential and temperature field on the velocity of moving dislocation.

    Nonlinear dynamics of a classical rotating pendulum system with multiple excitations
    Ning Han(韩宁) and Pei-Pei Lu(鲁佩佩)
    Chin. Phys. B, 2020, 29 (11):  110502.  DOI: 10.1088/1674-1056/ab9df2
    Abstract ( 490 )   HTML ( 6 )   PDF (4386KB) ( 233 )  

    We report an attempt to reveal the nonlinear dynamic behavior of a classical rotating pendulum system subjected to combined excitations of constant force and periodic excitation. The unperturbed system characterized by strong irrational nonlinearity bears significant similarities to the coupling of a simple pendulum and a smooth and discontinuous (SD) oscillator, especially the phase trajectory with coexistence of Duffing-type and pendulum-type homoclinic orbits. In order to learn the effect of constant force on this pendulum system, all types of phase portraits are displayed by means of the Hamiltonian function with large constant excitation especially the transitions of complex singular closed orbits. Under sufficiently small perturbations of the viscous damping and constant excitation, the Melnikov method is used to analyze the global structure of the phase space and the feature of trajectories. It is shown, both theoretically and numerically, that this system undergoes a homoclinic bifurcation and then bifurcates a unique attracting rotating limit cycle. Finally, the estimation of the chaotic threshold of the rotating pendulum system with multiple excitations is calculated and the predicted periodic and chaotic motions can be shown by applying numerical simulations.

    Nonlinear dynamics in non-volatile locally-active memristor for periodic and chaotic oscillations
    Wen-Yu Gu(谷文玉), Guang-Yi Wang(王光义), Yu-Jiao Dong(董玉姣), and Jia-Jie Ying(应佳捷)
    Chin. Phys. B, 2020, 29 (11):  110503.  DOI: 10.1088/1674-1056/ab9ded
    Abstract ( 553 )   HTML ( 7 )   PDF (4613KB) ( 146 )  

    Complexity and abundant dynamics may arise in locally-active systems only, in which locally-active elements are essential to amplify infinitesimal fluctuation signals and maintain oscillating. It has been recently found that some memristors may act as locally-active elements under suitable biasing. A number of important engineering applications would benefit from locally-active memristors. The aim of this paper is to show that locally-active memristor-based circuits can generate periodic and chaotic oscillations. To this end, we propose a non-volatile locally-active memristor, which has two asymptotically stable equilibrium points (or two non-volatile memristances) and globally-passive but locally-active characteristic. At an operating point in the locally-active region, a small-signal equivalent circuit is derived for describing the characteristics of the memristor near the operating point. By using the small-signal equivalent circuit, we show that the memristor possesses an edge of chaos in a voltage range, and that the memristor, when connected in series with an inductor, can oscillate about a locally-active operating point in the edge of chaos. And the oscillating frequency and the external inductance are determined by the small-signal admittance Y(iω). Furthermore, if the parasitic capacitor in parallel with the memristor is considered in the periodic oscillating circuit, the circuit generates chaotic oscillations.

    Memristor-based hyper-chaotic circuit for image encryption
    Jiao-Jiao Chen(陈娇娇), Deng-Wei Yan(闫登卫), Shu-Kai Duan(段书凯), and Li-Dan Wang(王丽丹)
    Chin. Phys. B, 2020, 29 (11):  110504.  DOI: 10.1088/1674-1056/abbbfe
    Abstract ( 610 )   HTML ( 3 )   PDF (7096KB) ( 240 )  

    The memristor is a kind of non-linear element with memory function, which can be applied to chaotic systems to increase signal randomness and complexity. In this paper, a new four-dimensional hyper-chaotic system is designed based on a flux controlled memristor model, which can generate complex chaotic attractors. The basic dynamic theory analysis and numerical simulations of the system, such as the stability of equilibrium points, the Lyapunov exponents and dimension, Poincare maps, the power spectrum, and the waveform graph prove that it has rich dynamic behaviors. Then, the circuit implementation of this system is established. The consistency of simulation program with integrated circuit emphasis (SPICE) simulation and numerical analysis proves the ability to generate chaos. Finally, a new image encryption scheme is designed by using the memristor-based hyper-chaotic system proposed in this paper. The scheme involves a total of two encryptions. By using different security analysis factors, the proposed algorithm is compared with other image encryption schemes, including correlation analysis, information entropy, etc. The results show that the proposed image encryption scheme has a large key space and presents a better encryption effect.

    Dynamics of the two-SBT-memristor-based chaotic circuit
    Mei Guo(郭梅), Meng Zhang(张萌), Ming-Long Dou(窦明龙), Gang Dou(窦刚), and Yu-Xia Li(李玉霞)
    Chin. Phys. B, 2020, 29 (11):  110505.  DOI: 10.1088/1674-1056/abbbe3
    Abstract ( 438 )   HTML ( 5 )   PDF (24613KB) ( 207 )  

    A two-SBT-memristor-based chaotic circuit was proposed. The stability of the equilibrium point was studied by theoretical analysis. The close dependence of the circuit dynamic characteristics on its initial conditions and circuit parameters was investigated by utilizing Lyapunov exponents spectra, bifurcation diagrams, phase diagrams, and Poincaré maps. The analysis showed that the circuit system had complex dynamic behaviors, such as stable points, period, chaos, limit cycles, and so on. In particular, the chaotic circuit produced the multistability phenomenon, such as coexisting attractors and coexisting periods.

    Sensitivity enhancement of WS2-coated SPR-based optical fiber biosensor for detecting glucose concentration
    Yun Cai(蔡云), Wei Li(李卫), Ye Feng(冯烨), Jian-Sheng Zhao(赵建胜), Gang Bai(白刚), Jie Xu(许杰), and Jin-Ze Li(李金泽)$
    Chin. Phys. B, 2020, 29 (11):  110701.  DOI: 10.1088/1674-1056/aba601
    Abstract ( 548 )   HTML ( 6 )   PDF (1139KB) ( 271 )  

    In this paper, we propose a theoretical model of the surface plasmon resonance-based optical fiber biosensor for detecting glucose concentration. The Au/ZnO/WS2 multilayer film is coated around optical fiber. Compared with the conventional surface plasmon resonance sensor, WS2 material can increase the sensitivity of the biosensor. The absorption capacity of WS2 is used to load glucose oxidase by forming a sensitive area to recognize glucose. Refractive index of the solution is calculated and then the concentration of the glucose can be obtained by the correspondence between refractive index and glucose concentration. The highest sensitivity of the SPR biosensor with a structure of 40-nm Au/5-nm ZnO/14 layers of WS2 is 4310 nm/RIU. The proposed WS2-based SPR fiber biosensor has a unique effect on the detection of glucose concentration. It is expected to have potential applications in future medical blood glucose concentration detection.

    Tuning the type of charge carriers in N-heterocyclic carbene-based molecular junctions through electrodes
    Ming-Lang Wang(王明郎) and Chuan-Kui Wang(王传奎)
    Chin. Phys. B, 2020, 29 (11):  113101.  DOI: 10.1088/1674-1056/aba277
    Abstract ( 380 )   HTML ( 3 )   PDF (1700KB) ( 78 )  

    Designing tunable molecular devices with different charge carriers in single-molecule junctions is crucial to the next-generation electronic technology. Recently, it has been demonstrated that the type of charge carriers depends on and can be tuned by controlling the molecular length and the number of interfacial covalent bonds. In this study, we show that the type of charge carriers can also be tuned by controlling the material and shape of electrodes. N-heterocyclic carbenes (NHCs) have attracted attention because of their ability to form strong, substitutional inert bonds in a variety of metals. Also, NHCs are more stable than the widely used thiol group. Therefore, we use electrodes to tune the type of charge carriers in a series of NHCs with different side groups. The ab initio calculations based on non-equilibrium Green’s formalism combined with density functional theory show that the dominant charge carrier switches from electrons to holes when gold electrodes are changed into platinum ones. The nature of the charge carriers can be identified by variations in the transport spectra at the Fermi level (EF), which are caused by the side groups. The projections of transport spectra onto the central molecules further validate our inferences. In addition, the transmission coefficient at EF is found to be dependent on the atomic interface structure. In particular, for the NHC without methyl or ethyl side groups, connecting a protruding atom on the electrode surface significantly enhances the transportability of both electrode materials. Overall, this study presents an effective approach to modifying transport properties, which has potential applications in designing functional molecular devices based on NHCs.

    Ionization of two-electron atom (xenon) studied by Bohmian mechanics theory
    Yang Song(宋阳), Shu Han(韩姝), Yu-Jun Yang(杨玉军), Fu-Ming Guo(郭福明), and Su-Yu Li(李苏宇)
    Chin. Phys. B, 2020, 29 (11):  113201.  DOI: 10.1088/1674-1056/abb22e
    Abstract ( 459 )   HTML ( 2 )   PDF (1646KB) ( 77 )  

    The ionization dynamics of two-electron atom in an intense laser field is studied by the Bohmian mechanics (BM) theory, and the xenon atomic potential function is used as a model. The single ionization process and double ionization process are calculated by the BM theory and their results are in good agreement with those calculated by numerically solving the time-dependent Schrödinger equation. The analyses of the types, trajectories, and forces of Bohmian particles (BPs) undergoing the single and double ionizations indicate that the re-collision process accounts for a considerable proportion in the singly ionized cases. Furthermore, the analysis of the work done by the external force acting on the BPs shows that the quantum force plays an important role in the re-collision process. This work is helpful in understanding the ionization of two-electron atom in an intense laser field.

    Nonadiabatic molecular dynamics simulation of ${{\rm{C}}}_{2}{{\rm{H}}}_{2}^{2+}$ in a strong laser field
    Ji-Gen Chen(陈基根), Gang-Tai Zhang(张刚台), Ting-Ting Bai(白婷婷), Jun Wang(王俊), Ping-Ping Chen(陈平平), Wei-Wei Yu(于伟威)§, and Xi Zhao(赵曦)¶
    Chin. Phys. B, 2020, 29 (11):  113202.  DOI: 10.1088/1674-1056/abb3de
    Abstract ( 408 )   HTML ( 2 )   PDF (656KB) ( 160 )  

    We investigate the alignment dependence of the strong laser dissociation dynamics of molecule ${{\rm{C}}}_{2}{{\rm{H}}}_{2}^{2+}$ in the frame of real-time and real-space time-dependent density function theory coupled with nonadiabatic quantum molecular dynamics (TDDFT-MD) simulation. This work is based on a recent experiment study “ultrafast electron diffraction imaging of bond breaking in di-ionized acetylene” [Wolter et al, Science 354, 308–312 (2016)]. Our simulations are in excellent agreement with the experimental data and the analysis confirms that the alignment dependence of the proton dissociation dynamics comes from the electron response of the driving laser pulse. Our results validate the ability of the TDDFT-MD method to reveal the underlying mechanism of experimentally observed and control molecular dissociation dynamics.

    Effect of patterned hydrodynamic slip on electromagnetohydrodynamic flow in parallel plate microchannel
    Chun-Hong Yang(杨春红) and Yong-Jun Jian(菅永军)
    Chin. Phys. B, 2020, 29 (11):  114101.  DOI: 10.1088/1674-1056/abab71
    Abstract ( 282 )   HTML ( 3 )   PDF (891KB) ( 69 )  

    A fully developed electromagnetohydrodynamic (EMHD) flow through a microchannel with patterned hydrodynamic slippage on the channel wall is studied. The flow is driven by the Lorentz force which originates from the interaction between an externally imposed lateral electric field and a perpendicular magnetic field. The governing equations for the velocity with patterned slip boundary conditions are solved analytically by perturbation techniques under the assumption of small Reynolds number Re. In addition, the numerical solutions for the velocity are obtained by using the finite-difference method, and they are found to be in good agreement with the analytical solutions within admissible parameter range. The effects of different parameters on the velocity and volume flow rate due to patterned hydrodynamic slippage are discussed in detail, including wave-number K, Hartmann number Ha, amplitude δ of the patterned slip length, and normalized electric field strength S. The results show that patterned slippage over microchannel walls can induce transverse flows, which will increase the mixing rates in microfluidic devices. In addition, we also find that precise flow control can be achieved by controlling the magnetic flux and the wave-number and also by well choosing the electric field intensity. Our analysis can be used for designing the efficient micro-fluidic mixers.

    Near 100% spectral-purity photons from reconfigurable micro-rings Hot!
    Pingyu Zhu(朱枰谕), Yingwen Liu(刘英文), Chao Wu(吴超), Shichuan Xue(薛诗川), Xinyao Yu(于馨瑶), Qilin Zheng(郑骑林), Yang Wang(王洋), Xiaogang Qiang(强晓刚), Junjie Wu(吴俊杰), and Ping Xu(徐平)
    Chin. Phys. B, 2020, 29 (11):  114201.  DOI: 10.1088/1674-1056/abbb28
    Abstract ( 583 )   HTML ( 3 )   PDF (910KB) ( 351 )  

    We propose an on-chip reconfigurable micro-ring to engineer the spectral-purity of photons. The micro-ring resonator is designed to be coupled by one or two asymmetric Mach–Zehnder interferometers and the coupling coefficients hence the quality-factors of the pump and the converted photons can be dynamically changed by the interferometer’s internal phase-shifter. We calculate the joint-spectrum function and obtain the spectral-purity of photons and Schmidt number under different phases. We show that it is a dynamical method to adjust the spectral-purity and can optimize the spectral-purity of photons up to near 100%. The condition for high-spectral-purity photons is ensured by the micro-ring itself, so it overcomes the trade-off between spectral purity and brightness in the traditional post-filtering method. This scheme is robust to fabrication variations and can be successfully applied in different fabrication labs and different materials. Such high-spectral-purity photons will be beneficial for quantum information processing like Boson sampling and other quantum algorithms.

    Pseudo complementary measurement for traditional single-pixel cameras
    Qian Dong(董乾), Xu-Ri Yao(姚旭日), Xin Liu(刘歆), Bing Liu(刘冰), and Guang-Jie Zhai(翟光杰)$
    Chin. Phys. B, 2020, 29 (11):  114202.  DOI: 10.1088/1674-1056/abb22b
    Abstract ( 318 )   HTML ( 2 )   PDF (3247KB) ( 141 )  

    A traditional single-pixel camera needs a large number of measurements to reconstruct the object with compressive sensing computation. Compared with the 1/0 matrices in classical measurement, the 1/−1 matrices in the complementary measurement has better property for reconstruction computation and returns better reconstruction results. However, each row of the 1/−1 matrices needs two measurements with the traditional single-pixel camera which results into double measurements compared with the 1/0 matrices. In this paper, we consider the pseudo complementary measurement which only takes the same amount of measurements with the row number of some properly designed 1/0 matrix to compute the total luminous flux of the objective and derives the measurement data of the corresponding 1/−1 matrix in a mathematical way. The numerical simulation and experimental result show that the pseudo complementary measurement is an efficient tool for the traditional single-pixel camera imaging under low measurement rate, which can combine the advantages of the classical and complementary measurements and significantly improve the peak signal-to-noise ratio.

    Polarization-insensitive complementary metamaterial structure based on graphene for independently tuning multiple transparency windows
    Hailong Huang(黄海龙), Hui Xia(夏辉), and Hongjian Li(李宏建)
    Chin. Phys. B, 2020, 29 (11):  114203.  DOI: 10.1088/1674-1056/abb307
    Abstract ( 445 )   HTML ( 1 )   PDF (1118KB) ( 70 )  

    Polarization-insensitive multiple transparency windows are obtained with a graphene-based complementary metamaterial structure in terahertz regions, which is composed of two kinds of monolayer graphene perforated in shapes of a cross and four identical split rings that construct a resonator. The geometric parameters of resonators are different from each other. Numerical and theoretical results show that the quantum effect of Autler–Townes splitting is the key factor for appearance of transparency windows within the resonant dips. Further investigation demonstrates that by employing the fourfold-symmetry graphene complementary structure, polarization-independent transparency windows can be achieved. Moreover, multiple transparency windows can be separately manipulated over a broad frequency range via adjusting the chemical potential of the corresponding graphene resonators, and the bandwidth as well as resonance strength can also be tuned by changing the relative displacement between resonators each consisting of a cross and four split rings. The proposed metamaterial structure may be utilized in some practical applications with requirements of no polarization-varied loss and slowing the light speed.

    Ultra wide sensing range plasmonic refractive index sensor based on nano-array with rhombus particles
    Jiankai Zhu(朱剑凯), Xiangxian Wang(王向贤), Xiaoxiong Wu(吴枭雄), Yingwen Su(苏盈文), Yueqi Xu(徐月奇), Yunping Qi(祁云平), Liping Zhang(张丽萍), and Hua Yang(杨华)$
    Chin. Phys. B, 2020, 29 (11):  114204.  DOI: 10.1088/1674-1056/abb229
    Abstract ( 492 )   HTML ( 4 )   PDF (1120KB) ( 201 )  

    We propose a two-dimensional metal grating with rhombus particles on a gold film structure for refractive index sensing due to its perfect absorption at near-infrared wavelength. Via two-dimensional metal grating coupling, the incident light energy is effectively transformed into the surface plasmons which propagate along the upper surface of the gold film and interact with the surrounding environment in a wide range. The plasmonic resonance mechanism of the structure is discussed in detail by theoretical analysis and finite-difference time-domain method. After optimizing the geometrical parameters, the designed structure shows the sensing performance with a refractive index sensitivity of 1006 nm/RIU. More importantly, this plasmonic refractive index sensor achieves an ultra wide refractive index sensing range from 1.0 to 2.4 with a stable sensing performance. The promising simulation results of the structure show that the sensor has a broad application prospect in the field of biology and chemistry.

    Actively tunable polarization-sensitive multiband absorber based on graphene
    Ai-Li Cao(曹爱利), Kun Zhang(张昆), Jia-Rui Zhang(张佳瑞), Yan Liu(刘燕), and Wei-Jin Kong(孔伟金)
    Chin. Phys. B, 2020, 29 (11):  114205.  DOI: 10.1088/1674-1056/abb663
    Abstract ( 315 )   HTML ( 1 )   PDF (3790KB) ( 133 )  

    We design an actively tunable polarization-sensitive multiband absorber in the mid-infrared region, which consists of stacked graphene multilayers separated by dielectric layers on a metal mirror. Benefiting from the anisotropic structure, the absorber has dual absorption bands with almost perfect absorption at different wavelengths under the x and y polarizations. Analyzing the electric field amplitude distributions and the surface currents, we find that the absorption peaks under the same polarization are excited in the graphene layers independently. Therefore, more absorption bands can be achieved by increasing the graphene layers. Adjusting the Fermi energy of the graphene layers, the working wavelengths of the polarization-sensitive multiband absorbers can be tuned actively, and thus achieving a wide band regulation range. Besides, the peak number and the peak strength of the multiband absorber can be actively controlled by the polarization angle as well. We also propose a method to design an actively tunable polarization-sensitive multiband absorber, which may have potential applications in mid-infrared devices, such as polarization-sensitive filters and detectors.

    All-fiberized very-large-mode-area Yb-doped fiber based high-peak-power narrow-linewidth nanosecond amplifier with tunable pulse width and repetition rate
    Min Yang(杨敏), Ping-Xue Li(李平雪), Dong-Sheng Wang(王东生), Ke-Xin Yu(于可新), Xue-Yan Dong(董雪岩), Ting-Ting Wang(王婷婷), Chuan-Fei Yao(姚传飞), and Wei-Xin Yang(杨卫鑫)
    Chin. Phys. B, 2020, 29 (11):  114206.  DOI: 10.1088/1674-1056/abb3e9
    Abstract ( 343 )   HTML ( 2 )   PDF (938KB) ( 101 )  

    We demonstrate an all-fiberized narrow-linewidth nanosecond amplifier with high peak power, tunable pulse width, and repetition rate. A fiber-coupled narrow-linewidth laser diode operating at 1064.1 nm is employed as the seed source, which is gain-switched to generate nanosecond pulses with tunable pulse widths of 1–200 ns and tunable repetition rates of 10 Hz–100 kHz. By utilizing a very-large-mode-area Yb-doped fiber with a core diameter of 50 μm in the power amplifier, thresholds of the stimulated Brillouin scattering at different pulse widths and repetition rates are increased. The maximum average power reaches 30.8 W at the pulse width of 4 ns and a repetition rate of 100 kHz, corresponding to an optical-to-optical conversion efficiency of ∼55.2%. Pulse energy and peak power are calculated to be 0.2 mJ and 50 kW, respectively, which are limited by stimulated Brillouin scattering. The 3-dB spectral linewidth remains around 0.05 nm during the power scaling process. The stimulated Brillouin scattering limited output powers at different pulse widths and repetition rates are investigated. Peak power of 47.5 kW (0.19 mJ) is obtained for the 4 ns pulses at a repetition rate of 50 kHz, which is nearly the same as that of 4 ns pulses at 100 kHz. When the pulse width of the seed source is increased to 8 ns, peak powers/pulse energies are decreased to 19.6 kW/0.11 mJ and 13.3 kW/0.08 mJ at repetition rates of 50 kHz and 100 kHz, respectively.

    Variable optical chirality in atomic assisted microcavity
    Hao Zhang(张浩), Wen-Xiu Li (李文秀), Peng Han(韩鹏), Xiao-Yang Chang(常晓阳), Shuo Jiang(蒋硕), An-Ping Huang(黄安平), and Zhi-Song Xiao(肖志松)
    Chin. Phys. B, 2020, 29 (11):  114207.  DOI: 10.1088/1674-1056/abb660
    Abstract ( 397 )   HTML ( 2 )   PDF (756KB) ( 69 )  

    The manipulating of optical waves in a microcavity is essential to developing the integrated optical devices. Generally, the two eigenmodes in a whispering-gallery-mode (WGM) microcavity possess chiral symmetry. Here we show the chiral symmetry breaking is induced by the asymmetric backscattering of counter-propagating optical waves in a whispering-gallery-mode (WGM) microcavity with a cavity-made slot filled with atomic vapor. Through tuning the dispersion relation of the atomic vapor in the cavity-made slot, the chiral modes are continuously steered. The mode frequency splitting in the transmission and reflection spectra stem from the chiral symmetry breaking of the two eigenmodes. The displacement sensitivity of the proposed system in response to the length variation of cavity-made slot exhibits a high sensitivity value of 15.22 THz/nm.

    Effect of recombination process in femtosecond laser-induced modification on Ge crystal
    Jia-Qi Ju(居家奇), Zi-Yao Qin(秦子尧), Ju-Kun Liu(刘聚坤), Hong-Wei Zhao(赵宏伟), Yao-Qing Huang(黄耀清), Rong-Rong Hu(胡蓉蓉), and Hua Wu(吴华)$
    Chin. Phys. B, 2020, 29 (11):  114208.  DOI: 10.1088/1674-1056/abbbe9
    Abstract ( 383 )   HTML ( 2 )   PDF (487KB) ( 53 )  

    The dynamics of produced excited carriers under the irradiation of Ge crystal is investigated theoretically by using femtosecond laser pulse. A two-temperature model combined with the Drude model is also used to study the nonequilibrium carrier density, carrier and lattice temperatures, and optical properties of the crystal. The properties of the surface plasmon wave when excited are also studied. The influences of non-radiation and radiative recombination process on the photoexcitation of the semiconductor during pulse and the relaxation after the pulse are described in detail. The results show that the effects of Auger recombination on the nonequilibrium carrier density and optical properties of the crystal and the properties of the surface plasmon polariton are great, whereas the effect of radiative recombination is extremely small.

    Efficient and multifunctional terahertz polarization control device based on metamaterials
    Xiao-Fei Jiao(焦晓飞), Zi-Heng Zhang(张子恒), Yun Xu(徐云), and Guo-Feng Song(宋国峰)
    Chin. Phys. B, 2020, 29 (11):  114209.  DOI: 10.1088/1674-1056/abb661
    Abstract ( 343 )   HTML ( 1 )   PDF (517KB) ( 122 )  

    Terahertz polarization devices are an important part of terahertz optical systems. Traditional terahertz polarization devices rely on birefringent crystals, and their performances are limited by the material structures. In this work, we theoretically demonstrate that the metamaterial consisting of the medium and the periodic metal band embedded in the medium can control broadband polarization effectively. The transmission length of the subwavelength waveguide mode gives rise to a broadband transmission peak. The resonant cavity structure formed by the dielectric layer and the waveguide layer possesses a high transmission efficiency. By optimizing the metamaterial structure parameters, we design a high-efficient (>90%) quarter-wave plate over a frequency range of 0.90 THz–1.10 THz and a high-efficient (>90%) half-wave plate over a frequency range of 0.92 THz–1.02 THz. Besides, due to the anisotropy of the structure, the metamaterials with the same structural parameters can achieve the function of the polarized beam splitting with an efficiency of up to 99% over a frequency range of 0.10 THz–0.55 THz. Therefore, the designed metamaterial has a multifunctional polarization control effect, which has potential applications in the terahertz integrated polarization optical system.

    Linear and nonlinear propagation characteristics of multi-Gaussian laser beams
    Naveen Gupta and Sandeep Kumar
    Chin. Phys. B, 2020, 29 (11):  114210.  DOI: 10.1088/1674-1056/abb306
    Abstract ( 489 )   HTML ( 3 )   PDF (2999KB) ( 92 )  

    Theoretical investigation on the propagation characteristics of a new class of laser beams known as multi Gaussian (M.G) laser beams has been presented. To investigate the linear characteristics, propagation of the laser beam in vacuum has been considered. Whereas, the nonlinear characteristics have been investigated in plasmas. Optical nonlinearity of the plasma has been modeled by relativistic mass nonlinearity of the plasma electrons in the field of laser beam. Formulation is based on finding the semi analytical solution of the wave equation for the slowly varying envelope of the laser beam. Particularly, the dynamical evolutions of the beam width and longitudinal phase of the laser beam have been investigated in detail.

    Propagation of shaped beam through uniaxially anisotropic chiral slab
    Ming-Jun Wang(王明军), Jia-Lin Zhang(张佳琳), Hua-Yong Zhang(张华永), and Zi-Han Wang(王梓涵)$
    Chin. Phys. B, 2020, 29 (11):  114211.  DOI: 10.1088/1674-1056/abab6c
    Abstract ( 457 )   HTML ( 4 )   PDF (1337KB) ( 90 )  

    A general solution is obtained to a canonical problem of the reflection and refraction of an arbitrary shaped beam by using a uniaxially anisotropic chiral slab. The reflected, internal as well as refracted shaped beams are expanded in terms of cylindrical vector wave functions, and the expansion coefficients are determined by using the boundary conditions and method of moments procedure. As two typical examples, the normalized field intensity distributions are evaluated for a fundamental Gaussian beam and Hermite–Gaussian beam, and some propagation properties, especially the negative refraction phenomenon, are discussed briefly.

    Study on dispersion characteristics of terahertz waves in helical waveguides
    Jin-Hai Sun(孙金海), Shao-Hua Zhang(张少华), Xu-Tao Zhang(张旭涛), He Cai(蔡禾), Yong-Qiang Liu(刘永强), and Zeng-Ming Chao(巢增明)$
    Chin. Phys. B, 2020, 29 (11):  114301.  DOI: 10.1088/1674-1056/aba098
    Abstract ( 347 )   HTML ( 2 )   PDF (1353KB) ( 53 )  

    Corresponding to the atmospheric transmission windows of the electromagnetic spectrum in the low terahertz range, the mode coupling and dispersion characteristics of two helically corrugated waveguides (HCW) in the frequency ranges of 90 GHz–100 GHz and 260 GHz–265 GHz are studied. Through analytic calculations and numerical simulations, dispersion curves and structural parameters of the two frequency ranges waveguides are obtained. A novel method was proposed to obtain the dispersion of the HCW from the eigenwave solution using a periodic boundary condition. The HCW in a frequency range of 90 GHz–100 GHz was fabricated and its dispersion performance was measured. By comparing the measured results with the theoretical and the simulated results, the validity of the analytical and simulation method is verified. Limited to our machining capability, the dispersion of the 260 GHz–265 GHz HCW was only simulated and calculated and it was found that the results agree well with each other.

    Acoustic radiation force and torque on a lossless eccentric layered fluid cylinder
    F G Mitri
    Chin. Phys. B, 2020, 29 (11):  114302.  DOI: 10.1088/1674-1056/aba27a
    Abstract ( 548 )   HTML ( 2 )   PDF (3021KB) ( 764 )  

    Exact analytical equations and computations for the longitudinal and transverse acoustic radiation force and axial torque components for a lossless eccentric liquid cylinder submerged in a nonviscous fluid and insonified by plane waves progressive waves (of arbitrary incidence in the polar plane) are established and computed numerically. The modal matching method and the translational addition theorem in cylindrical coordinates are used to derive exact mathematical expressions applicable to any inner and outer cylinder sizes without any approximations, and taking into account the interaction effects between the waves propagating in the layer and those scattered from the cylindrical core. The results show that longitudinal and transverse radiation force components arise, in addition to the emergence of an axial radiation torque component acting on the non-absorptive compound cylinder due to geometrical asymmetry as the eccentricity increases. The computations demonstrate that the axial torque component, which arises due to a geometrical asymmetry, can be positive (causing counter-clockwise rotation in the polar plane), negative (clockwise rotation) or neutral (rotation cancellation) depending on the size parameter of the cylinder and the amount of eccentricity. Furthermore, verification and validation of the results have been accomplished from the standpoint of energy conservation law applied to scattering, and based on the reciprocity theorem.

    Dynamical interactions between higher-order rogue waves and various forms of n-soliton solutions (n → ∞) of the (2+1)-dimensional ANNV equation
    Md Fazlul Hoque, Harun-Or-Roshid, and Fahad Sameer Alshammari
    Chin. Phys. B, 2020, 29 (11):  114701.  DOI: 10.1088/1674-1056/aba612
    Abstract ( 408 )   HTML ( 1 )   PDF (10690KB) ( 212 )  

    We present new lemmas, theorem and corollaries to construct interactions among higher-order rogue waves, n-periodic waves and n-solitons solutions (n → ∞) to the (2+1)-dimensional asymmetric Nizhnik–Novikov–Veselov (ANNV) equation. Several examples for theories are given by choosing definite interactions of the wave solutions for the model. In particular, we exhibit dynamical interactions between a rogue and a cross bright-dark bell wave, a rogue and a cross-bright bell wave, a rogue and a one-, two-, three-, four-periodic wave. In addition, we also present multi-types interactions between a rogue and a periodic cross-bright bell wave, a rogue and a periodic cross-bright-bark bell wave. Finally, we physically explain such interaction solutions of the model in the 3D and density plots.

    The role of velocity derivative skewness in understanding non-equilibrium turbulence
    Feng Liu(刘锋), Le Fang(方乐), and Liang Shao(邵亮)$
    Chin. Phys. B, 2020, 29 (11):  114702.  DOI: 10.1088/1674-1056/abbbdc
    Abstract ( 394 )   HTML ( 1 )   PDF (1073KB) ( 268 )  

    The turbulence governed by the Navier–Stokes equation is paramount in many physical processes. However, it has been considered as a challenging problem due to its inherent nonlinearity, non-equilibrium, and complexity. Herein, we review the connections between the velocity derivative skewness Sk and the non-equilibrium properties of turbulence. Sk, a reasonable candidate for describing the non-equilibrium turbulence, which varies during the non-equilibrium procedure. A lot of experimental or numerical evidences have shown that the perturbation of energy spectrum, which associated with the excitation of large scales, results in an obvious variation of Sk, and Sk is a negative value in this rapid energy decay process. The variation of positive Sk is closely related to the perturbation of transfer spectrum, and this corresponds to the backward energy transfer process. In addition, the skewness characterizes the production (or reduction) rate of enstrophy due to vortex stretching (or compression). Using the transport equation of turbulent energy dissipation rate and enstrophy, it is possible to establish a theoretical connection between skewness and the non-equilibrium turbulence. It is expected that this work could trigger the rapid advancement of the future studies of non-equilibrium turbulence, and also the improvement of turbulence models.

    SPECIAL TOPIC—Water at molecular level
    Energy stored in nanoscale water capillary bridges formed between chemically heterogeneous surfaces with circular patches
    Bin-Ze Tang(唐宾泽), Xue-Jia Yu(余雪佳), Sergey V. Buldyrev, Nicolas Giovambattista§, and Li-Mei Xu(徐莉梅)¶
    Chin. Phys. B, 2020, 29 (11):  114703.  DOI: 10.1088/1674-1056/abb664
    Abstract ( 480 )   HTML ( 6 )   PDF (955KB) ( 237 )  

    The formation of nanoscale water capillary bridges (WCBs) between chemically heterogeneous (patchy) surfaces plays an important role in different scientific and engineering applications, including nanolithography, colloidal aggregation, and bioinspired adhesion. However, the properties of WCB of nanoscale dimensions remain unclear. Using molecular dynamics simulations, we investigate the geometrical and thermodynamic properties of WCB confined between chemically heterogeneous surfaces composed of circular hydrophilic patches on a hydrophobic background. We find that macroscopic capillary theory provides a good description of the WCB geometry and forces induced by the WCB on the confining surfaces even in the case of surface patches with diameters of only 4 nm. Upon stretching, the WCB contact angle changes from hydrophobic-like values (θ > 90°) to hydrophilic-like values (θ < 90°) until it finally breaks down into two droplets at wall separations of ∼ 9–10 nm. We also show that the studied nanoscale WCB can be used to store relevant amounts of energy EP and explore how the walls patch geometry can be improved in order to maximize EP. Our findings show that nanoscale WCB can, in principle, be exploited for the design of clean energy storage devices as well as actuators that respond to changes in relative humidity. The present results can also be of crucial importance for the understanding of water transport in nanoporous media and nanoscale engineering systems.

    Investigation on plasma structure evolution and discharge characteristics of a single-stage planar-pulsed-inductive accelerator under ambient fill condition
    Xiao-Kang Li(李小康), Bi-Xuan Che(车碧轩), Mou-Sen Cheng(程谋森), Da-Wei Guo(郭大伟), Mo-Ge Wang(王墨戈), and Yun-Tian Yang(杨云天)
    Chin. Phys. B, 2020, 29 (11):  115201.  DOI: 10.1088/1674-1056/ab9f2a
    Abstract ( 556 )   HTML ( 1 )   PDF (1186KB) ( 185 )  

    The physical process of a single-stage planar-pulsed-inductive accelerator is investigated. Measurements include the waveforms of circuit current, capacitor voltage, plasma radiation intensity, and temporal plasma structure photos captured by a high-speed camera. Experiments are conducted under static ambient fill condition using argon as propellant. Varied values of capacitor voltage and gas pressure are compared. Further discussions quantify the EM interaction between circuit and plasma, as well as their energy deposition and current sheet acceleration. Based on the results of experiments, physical mechanisms of the initial ionization phase and the following acceleration phase are analyzed theoretically.

    Weakly nonlinear multi-mode Bell–Plesset growth in cylindrical geometry
    Hong-Yu Guo(郭宏宇), Tao Cheng(程涛), and Ying-Jun Li(李英骏)
    Chin. Phys. B, 2020, 29 (11):  115202.  DOI: 10.1088/1674-1056/ab9c14
    Abstract ( 475 )   HTML ( 2 )   PDF (587KB) ( 50 )  

    Bell–Plesset (BP) effect caused perturbation growth plays an important role in better understanding of characteristics of the convergence effect. Governing equations for multi-mode perturbation growth on a cylindrically convergent interface are derived. The second-order weakly nonlinear (WN) solutions for two-mode perturbations at the interface which is subject to uniformly radical motion are obtained. Our WN theory is consistent with the numerical result in terms of mode-coupling effect in converging Richtmyer–Meshkov instability. Nonlinear mode-coupling effects will cause irregular deformation of the convergent interface. The mode-coupling behavior in convergent geometry depends on the mode number, Atwood number A and convergence ratio Cr. The A = –1.0 at the interface results in larger perturbation growth than A = 1.0. The growth of generated perturbation modes from two similar modes at the initial stage are smaller than that from two dissimilar modes.

    Structure and tribological properties of Si/a-C:H(Ag) multilayer film in stimulated body fluid
    Yan-Xia Wu(吴艳霞), Yun-Lin Liu(刘云琳), Ying Liu(刘颖), Bing Zhou(周兵), Hong-Jun Hei(黑鸿君), Yong Ma(马永), Sheng-Wang Yu(于盛旺), and Yu-Cheng Wu(吴玉程)
    Chin. Phys. B, 2020, 29 (11):  116101.  DOI: 10.1088/1674-1056/ab9c10
    Abstract ( 283 )   HTML ( 2 )   PDF (1583KB) ( 63 )  

    Si/a-C:H(Ag) multilayer films with different modulation periods are prepared to test their potential applications in human body. The composition, microstructure, mechanical and tribological properties in the simulated body fluid are investigated. The results show the concentration of Ag first decreases and then increases with the modulation period decreasing from 984 nm to 250 nm. Whereas the C content has an opposite variation trend. Notably, the concentration of Ag plays a more important role than the modulation period in the properties of the multilayer film. The a-C:H sublayer of the film with an appropriate Ag concentration (8.97 at.%) (modulation period of 512 nm) maintains the highest sp3/sp2 ratio, surface roughness and hardness, and excellent tribological property in the stimulated body fluid. An appropriate number of Ag atoms and size of Ag atom allow the Ag atoms to easily enter into the contact interface for load bearing and lubricating. This work proves that the Ag nanoparticles in the a-C:H sublayer plays a more important role in the tribological properties of the composite-multilayer film in stimulated body fluid condition.

    Microwave-assisted synthesis of Mg:PbI2 nanostructures and their structural, morphological, optical, dielectric and electrical properties for optoelectronic technology
    Mohd. Shkir, Ziaul Raza Khan, T Alshahrani, Kamlesh V. Chandekar, M Aslam Manthrammel, Ashwani Kumar, and S AlFaify$
    Chin. Phys. B, 2020, 29 (11):  116102.  DOI: 10.1088/1674-1056/aba60e
    Abstract ( 349 )   HTML ( 1 )   PDF (2434KB) ( 208 )  

    This work reports the cost-effective growth of Mg:PbI2 nanostructures with 0, 1, 2.5 and 5.0 wt.% Mg doping concentrations. Structural, vibrational, morphological properties are analyzed using x-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). XRD and Raman studies confirm the monophasic hexagonal system of Mg:PbI2, and no additional impurity peaks are detected. The Scherrer formula is used to determine sizes of crystallites to be in the range of 47–52 nm. EDX/SEM e-mapping analyses confirm the incorporation of Mg in PbI2 matrix and its uniform distribution throughout the sample. The hexagonal nanosheet- and nanoplate-like morphologies are detected in SEM images for pure and Mg-doped PbI2. An optical band gap of nanostructures is obtained from Tauc’s relation to be in the range 3.0–3.25 eV. Dielectric and electrical properties are found in significant enhancement as Mg doping in PbI2 matrix, also the conduction mechanism is discussed.

    TOPICAL REVIEW—Machine learning in condensed matter physics
    Machine learning in materials design: Algorithm and application
    Zhilong Song(宋志龙), Xiwen Chen(陈曦雯), Fanbin Meng(孟繁斌), Guanjian Cheng(程观剑), Chen Wang(王陈), Zhongti Sun(孙中体), and Wan-Jian Yin(尹万健)
    Chin. Phys. B, 2020, 29 (11):  116103.  DOI: 10.1088/1674-1056/abc0e3
    Abstract ( 1044 )   HTML ( 16 )   PDF (4567KB) ( 989 )  

    Traditional materials discovery is in ‘trial-and-error’ mode, leading to the issues of low-efficiency, high-cost, and unsustainability in materials design. Meanwhile, numerous experimental and computational trials accumulate enormous quantities of data with multi-dimensionality and complexity, which might bury critical ‘structure–properties’ rules yet unfortunately not well explored. Machine learning (ML), as a burgeoning approach in materials science, may dig out the hidden structure–properties relationship from materials bigdata, therefore, has recently garnered much attention in materials science. In this review, we try to shortly summarize recent research progress in this field, following the ML paradigm: (i) data acquisition → (ii) feature engineering → (iii) algorithm → (iv) ML model → (v) model evaluation → (vi) application. In section of application, we summarize recent work by following the ‘material science tetrahedron’: (i) structure and composition → (ii) property → (iii) synthesis → (iv) characterization, in order to reveal the quantitative structure–property relationship and provide inverse design countermeasures. In addition, the concurrent challenges encompassing data quality and quantity, model interpretability and generalizability, have also been discussed. This review intends to provide a preliminary overview of ML from basic algorithms to applications.

    Plastic deformation mechanism transition of Ti/Ni nanolaminate with pre-existing crack: Molecular dynamics study
    Meng-Jia Su(宿梦嘉), Qiong Deng(邓琼)†, Min-Rong An(安敏荣), and Lan-Ting Liu(刘兰亭)
    Chin. Phys. B, 2020, 29 (11):  116201.  DOI: 10.1088/1674-1056/aba2e5
    Abstract ( 592 )   HTML ( 4 )   PDF (7301KB) ( 94 )  

    Tensile behaviors of Ti/Ni nanolaminate with model-I crack are investigated by molecular dynamics simulations. The Ti/Ni nanolaminates with center crack either in Ti layer or in Ni layer under different loading directions are utilized to systematically study the mechanical performance of the cracked material. The results indicate that pre-existing crack dramatically changes the plastic deformation mechanism of the Ti/Ni nanolaminate. Unlike the initial plastic deformation originating from the interface or weak Ti layer of the crack-free samples, the plastic behavior of cracked Ti/Ni nanolaminate first occurs at the crack tip due to the local stress concentration. Subsequent plastic deformation is dominated by the interaction between the crack and interface. The Ti/Ni interface not only impedes the movement of the initial plastic deformation carriers (dislocation, slip band, and deformation twinning) from the crack tip, but also promotes the movement of interfacial dislocations in the tension process. Microstructure evolution analysis further confirms that the plastic deformation mechanism transition is ascribed to the orientation-dependent tensile behavior at the crack tip, which is intrinsically attributed to the anisotropy of the certain crystal structure and loading direction of the cracked Ti/Ni nanolaminate. In addition, by analyzing the effects of different plastic deformation carriers on crack propagation in specific crystal, it can be discovered that the interfacial dislocations moving towards the crack tip can further promote the crack growth.

    Sintering reaction and microstructure of MAl (M = Ni, Fe, and Mg) nanoparticles through molecular dynamics simulation
    Yuwen Zhang(张宇文), Yonghe Deng(邓永和), Qingfeng Zeng(曾庆丰), Dadong Wen(文大东), Heping Zhao(赵鹤平), Ming Gao(高明), Xiongying Dai(戴雄英), and Anru Wu(吴安如)$
    Chin. Phys. B, 2020, 29 (11):  116601.  DOI: 10.1088/1674-1056/aba2dd
    Abstract ( 535 )   HTML ( 2 )   PDF (1013KB) ( 62 )  

    The sintering–alloying processes of nickel (Ni), iron (Fe), and magnesium (Mg) with aluminum (Al) nanoparticles were studied by molecular dynamics simulation with the analytic embedded-atom model (AEAM) potential. Potential energy, mean heterogeneous coordination number ${N}_{{\rm{A}}}^{{\rm{B}}}$, and surface atomic number Nsurf–A were used to monitor the sintering–reaction processes. The effects of surface segregation, heat of formation, and melting point on the sintering–alloying processes were discussed. Results revealed that sintering proceeded in two stages. First, atoms with low surface energy diffused onto the surface of atoms with high surface energy; second, metal atoms diffused with one another with increased system temperature to a threshold value. Under the same initial conditions, the sintering reaction rate of the three systems increased in the order MgAl < FeAl < NiAl. Depending on the initial reaction temperature, the final core–shell (FeAl and MgAl) and alloyed (NiAl and FeAl) nanoconfigurations can be observed.

    Ground-state phases and spin textures of spin–orbit-coupled dipolar Bose–Einstein condensates in a rotating toroidal trap
    Qing-Bo Wang(王庆波), Hui Yang(杨慧), Ning Su(苏宁), and Ling-Hua Wen(文灵华)
    Chin. Phys. B, 2020, 29 (11):  116701.  DOI: 10.1088/1674-1056/abbbe8
    Abstract ( 504 )   HTML ( 5 )   PDF (2535KB) ( 171 )  

    We investigate the ground-state phases and spin textures of spin–orbit-coupled dipolar pseudo-spin-1/2 Bose–Einstein condensates in a rotating two-dimensional toroidal potential. The combined effects of dipole–dipole interaction (DDI), spin–orbit coupling (SOC), rotation, and interatomic interactions on the ground-state structures and topological defects of the system are analyzed systematically. For fixed SOC strength and rotation frequency, we provide a set of phase diagrams as a function of the DDI strength and the ratio between inter- and intra-species interactions. The system can show rich quantum phases including a half-quantum vortex, symmetrical (asymmetrical) phase with quantum droplets (QDs), asymmetrical segregated phase with hidden vortices (ASH phase), annular condensates with giant vortices, triangular (square) vortex lattice with QDs, and criss-cross vortex string lattice, depending on the competition between DDI and contact interaction. For given DDI strength and rotation frequency, the increase of the SOC strength leads to a structural phase transition from an ASH phase to a tetragonal vortex lattice then to a pentagonal vortex lattice and finally to a vortex necklace, which is also demonstrated by the momentum distributions. Without rotation, the interplay of DDI and SOC may result in the formation of a unique trumpet-shaped Bloch domain wall. In addition, the rotation effect is discussed. Furthermore, the system supports exotic topological excitations, such as a half-skyrmion (meron) string, triangular skyrmion lattice, skyrmion–half-skyrmion lattice, skyrmion–meron cluster, skyrmion–meron layered necklace, skyrmion–giant-skyrmion necklace lattice, and half-skyrmion–half-antiskyrmion necklace.

    Energy storage performances regulated by BiMnO3 proportion in limited solid solution films
    Fei Guo(郭飞), Zhifeng Shi(史智锋), Yaping Liu(刘亚平), and Shifeng Zhao(赵世峰)
    Chin. Phys. B, 2020, 29 (11):  116801.  DOI: 10.1088/1674-1056/aba604
    Abstract ( 452 )   HTML ( 1 )   PDF (2495KB) ( 173 )  

    Na0.5Bi0.5TiO3–BiMnO3 (NBT–BM) limited solid solution films were fabricated to investigate the lattice modification on the energy storage performances. The introduction of the BM solute lattice induces the NBT solvent lattices undergoing the transition from the pure phase, solid solution, solubility limit to precipitation. Correspondingly, the polarization states transfer from the macroscopic ferroelectric domains to nanodomains then to compound ferroelectric domains. The introduction of BiMnO3 generates great lattice changes including the local lattice fluctuation and the large lattice stretching, which enhance the energy storage performances, with the energy storage efficiency being enhanced from 39.2% to 53.2% and 51.7% and the energy density being enhanced from 33.1 J/cm3 to 76.5 J/cm3 and 83.8 J/cm3 for the BM components of 2% and 4%, respectively. The lattice modifications play a key role in the energy storage performances for limited solid solution films, which provides an alternative strategy for energy storage material.

    Interfaces between MoOx and MoX2 (X = S, Se, and Te)
    Fengming Chen(陈凤鸣), Jinxin Liu(刘金鑫), Xiaoming Zheng(郑晓明), Longhui Liu(刘龙慧), Haipeng Xie(谢海鹏), Fei Song(宋飞), Yongli Gao(高永立), and Han Huang(黄寒)
    Chin. Phys. B, 2020, 29 (11):  116802.  DOI: 10.1088/1674-1056/abb310
    Abstract ( 322 )   HTML ( 3 )   PDF (3894KB) ( 272 )  

    In the past decades there have been many breakthroughs in low-dimensional materials, especially in two-dimensional (2D) atomically thin crystals like graphene. As structural analogues of graphene but with a sizeable band gap, monolayers of atomically thin transition metal dichalcogenides (with formula of MX2, M = Mo, W; X = S, Se, Te, etc.) have emerged as the ideal 2D prototypes for exploring fundamentals in physics such as valleytronics due to the quantum confinement effects, and for engineering a wide range of nanoelectronic, optoelectronic, and photocatalytic applications. Transition metal trioxides as promising materials with low evaporation temperature, high work function, and inertness to air have been widely used in the fabrication and modification of MX2. In this review, we reported the fabrications of one-dimensional MoS2 wrapped MoO2 single crystals with varied crystal direction via atmospheric pressure chemical vapor deposition method and of 2D MoOx covered MoX2 by means of exposing MoX2 to ultraviolet ozone. The prototype devices show good performances. The approaches are common to other transition metal dichalcogenides and transition metal oxides.

    TOPICAL REVIEW—Water at molecular level
    Atomic-level characterization of liquid/solid interface
    Jiani Hong(洪嘉妮) and Ying Jiang(江颖)
    Chin. Phys. B, 2020, 29 (11):  116803.  DOI: 10.1088/1674-1056/aba9d0
    Abstract ( 498 )   HTML ( 15 )   PDF (2242KB) ( 426 )  

    The detailed understanding of various underlying processes at liquid/solid interfaces requires the development of interface-sensitive and high-resolution experimental techniques with atomic precision. In this perspective, we review the recent advances in studying the liquid/solid interfaces at atomic level by electrochemical scanning tunneling microscope (EC-STM), non-contact atomic force microscopy (NC-AFM), and surface-sensitive vibrational spectroscopies. Different from the ultrahigh vacuum and cryogenic experiments, these techniques are all operated in situ under ambient condition, making the measurements close to the native state of the liquid/solid interface. In the end, we present some perspectives on emerging techniques, which can defeat the limitation of existing imaging and spectroscopic methods in the characterization of liquid/solid interfaces.

    Water on surfaces from first-principles molecular dynamics
    Peiwei You(游佩桅), Jiyu Xu(徐纪玉), Cui Zhang(张萃), and Sheng Meng(孟胜)$
    Chin. Phys. B, 2020, 29 (11):  116804.  DOI: 10.1088/1674-1056/aba279
    Abstract ( 548 )   HTML ( 4 )   PDF (1229KB) ( 370 )  

    Water is ubiquitous and so is its presence in the proximity of surfaces. To determine and control the properties of interfacial water molecules at nanoscale is essential for its successful applications in environmental and energy-related fields. It is very challenging to explore the atomic structure and electronic properties of water under various conditions, especially at the surfaces. Here we review recent progress and open challenges in describing physicochemical properties of water on surfaces for solar water splitting, water corrosion, and desalination using first-principles approaches, and highlight the key role of these methods in understanding the complex electronic and dynamic interplay between water and surfaces. We aim at showing the importance of unraveling fundamental mechanisms and providing physical insights into the behavior of water on surfaces, in order to pave the way to water-related material design.

    Machine learning identification of impurities in the STM images
    Ce Wang(王策), Haiwei Li(李海威), Zhenqi Hao(郝镇齐), Xintong Li(李昕彤), Changwei Zou(邹昌炜), Peng Cai(蔡鹏), Yayu Wang(王亚愚), Yi-Zhuang You(尤亦庄), and Hui Zhai(翟荟)
    Chin. Phys. B, 2020, 29 (11):  116805.  DOI: 10.1088/1674-1056/abc0d5
    Abstract ( 645 )   HTML ( 10 )   PDF (1270KB) ( 334 )  

    We train a neural network to identify impurities in the experimental images obtained by the scanning tunneling microscope (STM) measurements. The neural network is first trained with a large number of simulated data and then the trained neural network is applied to identify a set of experimental images taken at different voltages. We use the convolutional neural network to extract features from the images and also implement the attention mechanism to capture the correlations between images taken at different voltages. We note that the simulated data can capture the universal Friedel oscillation but cannot properly describe the non-universal physics short-range physics nearby an impurity, as well as noises in the experimental data. And we emphasize that the key of this approach is to properly deal with these differences between simulated data and experimental data. Here we show that even by including uncorrelated white noises in the simulated data, the performance of the neural network on experimental data can be significantly improved. To prevent the neural network from learning unphysical short-range physics, we also develop another method to evaluate the confidence of the neural network prediction on experimental data and to add this confidence measure into the loss function. We show that adding such an extra loss function can also improve the performance on experimental data. Our research can inspire future similar applications of machine learning on experimental data analysis.

    Dynamic recombination of triplet excitons in polymer heterojunctions
    Ya-Dong Wang(王亚东), Jian-Jun Liu(刘建军), Xi-Ru Wang(王溪如), Yan-Xia Liu(刘艳霞), and Yan Meng(孟艳)
    Chin. Phys. B, 2020, 29 (11):  117101.  DOI: 10.1088/1674-1056/ab9c13
    Abstract ( 376 )   HTML ( 1 )   PDF (673KB) ( 39 )  

    The dynamic recombination of two triplet excitons with opposite spins in the heterojunction structure has been investigated using a nonadiabatic evolution method. We demonstrate that luminous composite states including the excited polaron and the biexciton can be formed efficiently via the triplet exciton–triplet exciton reaction in the heterojunction and therefore this reaction can enhance the electroluminescence efficiency considerably, which is consistent qualitatively with experimental observations. Meanwhile, we find that, although the heterojunctions are beneficial to the generation of luminescent particles, large band offset caused by the heterojunction structure is not helpful to improve the electroluminescence efficiency. In addition, the mechanism of the triplet exciton–triplet exciton reaction in heterojunction is different from that of two similar coupling chains. Our results may deepen the understanding of the electroluminescence mechanism in polymer light-emitting devices.

    Recent progress on excitation and manipulation of spin-waves in spin Hall nano-oscillators
    Liyuan Li(李丽媛), Lina Chen(陈丽娜), Ronghua Liu(刘荣华), and Youwei Du(都有为)
    Chin. Phys. B, 2020, 29 (11):  117102.  DOI: 10.1088/1674-1056/abaed5
    Abstract ( 696 )   HTML ( 3 )   PDF (2606KB) ( 401 )  

    Spin Hall nano oscillator (SHNO), a new type spintronic nano-device, can electrically excite and control spin waves in both nanoscale magnetic metals and insulators with low damping by the spin current due to spin Hall effect and interfacial Rashba effect. Several spin-wave modes have been excited successfully and investigated substantially in SHNOs based on dozens of different ferromagnetic/nonmagnetic (FM/NM) bilayer systems (e.g., FM = Py, [Co/Ni], Fe, CoFeB, Y3Fe5O12; NM = Pt, Ta, W). Here, we will review recent progress about spin-wave excitation and experimental parameters dependent dynamics in SHNOs. The nanogap SHNOs with in-plane magnetization exhibit a nonlinear self-localized bullet soliton localized at the center of the gap between the electrodes and a secondary high-frequency mode which coexists with the primary bullet mode at higher currents. While in the nanogap SHNOs with out of plane magnetization, besides both nonlinear bullet soliton and propagating spin-wave mode are achieved and controlled by varying the external magnetic field and current, the magnetic bubble skyrmion mode also can be excited at a low in-plane magnetic field. These spin-wave modes show thermal-induced mode hopping behavior at high temperature due to the coupling between the modes mediated by thermal magnon mediated scattering. Moreover, thanks to the perpendicular magnetic anisotropy induced effective field, the single coherent mode also can be achieved without applying an external magnetic field. The strong nonlinear effect of spin waves makes SHNOs easy to achieve synchronization with external microwave signals or mutual synchronization between multiple oscillators which improve the coherence and power of oscillation modes significantly. Spin waves in SHNOs with an external free magnetic layer have a wide range of applications from as a nanoscale signal source of low power consumption magnonic devices to spin-based neuromorphic computing systems in the field of artificial intelligence.

    Collective modes of Weyl fermions with repulsive S-wave interaction Hot!
    Xun-Gao Wang(王勋高), Huan-Yu Wang(王寰宇), Jiang-Min Zhang(张江敏), and Wu-Ming Liu(刘伍明)
    Chin. Phys. B, 2020, 29 (11):  117201.  DOI: 10.1088/1674-1056/abbbdb
    Abstract ( 663 )   HTML ( 2 )   PDF (731KB) ( 518 )  

    We calculate the spin and density susceptibility of Weyl fermions with repulsive S-wave interaction in ultracold gases. Weyl fermions have a linear dispersion, which is qualitatively different from the parabolic dispersion of conventional materials. We find that there are different collective modes for the different strengths of repulsive interaction by solving the poles equations of the susceptibility in the random-phase approximation. In the long-wavelength limit, the sound velocity and the energy gaps vary with the different strengths of the interaction in the zero sound mode and the gapped modes, respectively. The particle–hole continuum is obtained as well, where the imaginary part of the susceptibility is nonzero.

    Short-wavelength infrared InAs/GaSb superlattice hole avalanche photodiode
    Jia-Feng Liu(刘家丰), Ning-Tao Zhang(张宁涛), Yan Teng(滕), Xiu-Jun Hao(郝修军), Yu Zhao(赵宇), Ying Chen(陈影), He Zhu(朱赫), Hong Zhu(朱虹), Qi-Hua Wu(吴启花), Xin Li(李欣), Bai-Le Chen(陈佰乐)§, and Yong Huang(黄勇)
    Chin. Phys. B, 2020, 29 (11):  117301.  DOI: 10.1088/1674-1056/aba2e0
    Abstract ( 441 )   HTML ( 1 )   PDF (536KB) ( 88 )  

    We demonstrate two short-wavelength infrared avalanche photodiodes based on InAs/GaSb superlattice grown by metal-organic chemical vapor deposition. The difference between the two devices, namely, p+nn+ and p+nnn+, is that the p+nnn+ device possesses an additional middle-doped layer to separate the multiplication region from the absorption region. By properly controlling the electric field distribution in the p+nnn+ device, an electric field of 906 kV/cm has been achieved, which is 2.6 times higher than that in the p+nn+ device. At a reverse bias of –0.1 V at 77 K, both devices show a 100% cut-off wavelength of 2.25 μm. The p+nn+ and p+nnn+ show a dark current density of 1.5 × 10−7 A/cm2 and 1.8 × 10−8 A/cm2, and a peak responsivity about 0.35 A/W and 0.40 A/W at 1.5 μm, respectively. A maximum multiplication gain of 55 is achieved in the p+nnn+ device while the value is only less than 2 in the p+nn+ device. Exponential nature of the gain characteristic as a function of reverse bias confirms a single carrier hole dominated impact ionization.

    Surface states modulated exchange interaction in Bi2Se3/thulium iron garnet heterostructures Hot!
    Hai-Bin Shi(石海滨), Li-Qin Yan(闫丽琴), Yang-Tao Su(苏仰涛), Li Wang(王力), Xin-Yu Cao(曹昕宇), Lin-Zhu Bi(毕林竹), Yang Meng(孟洋), Yang Sun(孙阳), and Hong-Wu Zhao(赵宏武)
    Chin. Phys. B, 2020, 29 (11):  117302.  DOI: 10.1088/1674-1056/abbbdf
    Abstract ( 607 )   HTML ( 11 )   PDF (773KB) ( 406 )  

    We investigate the modulation of magnetic anisotropy of thulium iron garnet (TmIG) films by interfaced Bi2Se3 thin films. High quality epitaxial growth of Bi2Se3 films has been achieved by molecular beam epitaxy on TmIG films. By the method of ferromagnetic resonance, we find that the perpendicular magnetic anisotropy (PMA) of TmIG can be greatly strengthened by the adjacent Bi2Se3 layer. Moreover, the competition between topological surface states and thickness dependent bulk states of Bi2Se3 gives rise to the modulation of PMA of the Bi2Se3/TmIG heterostructures. The interfacial interaction can be attributed to the enhanced exchange coupling between Fe3+ ions of TmIG mediated by topological surface electrons of Bi2Se3.

    TOPICAL REVIEW—Twistronics
    Twistronics in graphene-based van der Waals structures
    Ya-Ning Ren(任雅宁), Yu Zhang(张钰), Yi-Wen Liu(刘亦文), and Lin He(何林)
    Chin. Phys. B, 2020, 29 (11):  117303.  DOI: 10.1088/1674-1056/abbbe2
    Abstract ( 807 )   HTML ( 28 )   PDF (2345KB) ( 656 )  

    The electronic properties of van der Waals (vdW) structures can be substantially modified by the moiré superlattice potential, which strongly depends on the twist angle among the compounds. In twisted bilayer graphene (TBG), two low-energy Van Hove singularities (VHSs) move closer with decreasing twist angles and finally become highly non-dispersive flat bands at the magic angle (∼ 1.1°). When the Fermi level lies within the flat bands of the TBG near the magic angle, Coulomb interaction is supposed to exceed the kinetic energy of the electrons, which can drive the system into various strongly correlated phases. Moreover, the strongly correlated states of flat bands are also realized in other graphene-based vdW structures with an interlayer twist. In this article, we mainly review the recent scanning tunneling microscopy (STM) advances on the strongly correlated physics of the magic-angle TBG (MATBG) and the small-angle twisted multilayer graphene. Lastly we will give out a perspective of this field.

    SPECIAL TOPIC—Machine learning in condensed matter physics
    Artificial neural network potential for gold clusters
    Ling-Zhi Cao(曹凌志), Peng-Ju Wang(王鹏举), Lin-Wei Sai(赛琳伟), Jie Fu(付洁), and Xiang-Mei Duan(段香梅)
    Chin. Phys. B, 2020, 29 (11):  117304.  DOI: 10.1088/1674-1056/abc15d
    Abstract ( 522 )   HTML ( 5 )   PDF (966KB) ( 229 )  

    In cluster science, it is challenging to identify the ground state structures (GSS) of gold (Au) clusters. Among different search approaches, first-principles method based on density functional theory (DFT) is the most reliable one with high precision. However, as the cluster size increases, it requires more expensive computational cost and becomes impracticable. In this paper, we have developed an artificial neural network (ANN) potential for Au clusters, which is trained to the DFT binding energies and forces of 9000 AuN clusters (11 ≤ N ≤ 100). The root mean square errors of energy and force are 13.4 meV/atom and 0.4 eV/Å, respectively. We demonstrate that the ANN potential has the capacity to differentiate the energy level of Au clusters and their isomers and highlight the need to further improve the accuracy. Given its excellent transferability, we emphasis that ANN potential is a promising tool to breakthrough computational bottleneck of DFT method and effectively accelerate the pre-screening of Au clusters’ GSS.

    Electronic and thermoelectric properties of alkali metal-based perovskites CsYbF3 and RbYbF3
    Q Mahmood, N A Noor, T Ghrib, Nessrin A Kattan, Asif Mahmood, and Shahid M Ramay
    Chin. Phys. B, 2020, 29 (11):  117305.  DOI: 10.1088/1674-1056/ab9de3
    Abstract ( 397 )   HTML ( 2 )   PDF (5547KB) ( 116 )  

    The electronic and thermoelectric properties of alkali metal-based fluorides CsYbF3 and RbYbF3 are studied by using Wien2k and BoltzTraP codes. The structural and thermodynamic stability of these materials are confirmed by tolerance factor (0.94 and 0.99 for RbYbF3 and CsYbF3) and negative formation energy. The optimized lattice constants and bulk moduli are consistent with the results reported in the literature. The reported band gap for RbYbF3 is 0.86 eV which decreases to 0.83 eV by the replacement of Cs with Rb. The electrical and thermal conductivities along with Seebeck coefficients decrease with temperature rising from 0 K to 800 K. The large values of thermoelectric parameters for positive chemical potentials show that the character is dominated by electrons. The studied materials have figures of merit 0.82 and 0.81 at room temperature respectively, for RbYbF3 and CsYbF3 and increase with temperature rising. Therefore, the materials under study may have potential application values in thermoelectric generators and refrigerators.

    TOPICAL REVIEW—Twistronics
    Superconductivity in twisted multilayer graphene: A smoking gun in recent condensed matter physics
    Yonghuan Chu(楚永唤), Fangduo Zhu(朱方铎), Lingzhi Wen(温凌志), Wanying Chen(陈婉莹), Qiaoni Chen(陈巧妮), and Tianxing Ma(马天星)
    Chin. Phys. B, 2020, 29 (11):  117401.  DOI: 10.1088/1674-1056/abbbea
    Abstract ( 666 )   HTML ( 16 )   PDF (998KB) ( 444 )  

    We review the recent discoveries of exotic phenomena in graphene, especially superconductivity. It has been theoretically suggested for more than one decade that superconductivity may emerge in doped graphene-based materials. For single-layer pristine graphene, there are theoretical predictions that spin-singlet d + id pairing superconductivity is present when the filling is around the Dirac point. If the Fermi level is doped to the Van Hove singularity where the density of states diverges, then unconventional superconductivity with other pairing symmetry would appear. However, the experimental perspective was a bit disappointing. Despite extensive experimental efforts, superconductivity was not found in monolayer graphene. Recently, unconventional superconductivity was found in magic-angle twisted bilayer graphene. Superconductivity was also found in ABC stacked trilayer graphene and other systems. In this article, we review the unique properties of superconducting states in graphene, experimentally controlling the superconductivity in twisted bilayer graphene, as well as a gate-tunable Mott insulator, and the superconductivity in trilayer graphene. These discoveries have attracted the attention of a large number of physicists. The study of the electronic correlated states in twisted multilayer graphene serves as a smoking gun in recent condensed matter physics.

    Magnetoelastic coupling effect of Fe10Co90 films grown on different flexible substrates
    Jiapeng Zhao(赵佳鹏), Qinhuang Guo(郭勤皇), Huizhong Yin(尹慧中), Jintang Zou(邹锦堂), Zhenjie Zhao(赵振杰), Wenjuan Cheng(程文娟), Dongmei Jiang(蒋冬梅), and Qingfeng Zhan(詹清峰)
    Chin. Phys. B, 2020, 29 (11):  117501.  DOI: 10.1088/1674-1056/abb22d
    Abstract ( 591 )   HTML ( 1 )   PDF (998KB) ( 156 )  

    The magneto–mechanical coupling effect and magnetic anisotropy of Fe10Co90 (FeCo) films deposited on silicon wafer (Si), flexible polyethylene terephthalate (PET), freestanding polydimethylsiloxane (PDMS), and pre-stretched 20% PDMS substrates were studied in detail. The loop squareness ratio Mr/Ms and the coercive Hc of the FeCo film grown on a PET substrate can be obviously tuned by applying a small tensile-bending strain, and those of the FeCo film grown on a freestanding PDMS substrate can only be slightly changed when applying a relatively large tensile bending strain. For the FeCo film prepared on a 20% pre-stretched PDMS, a wrinkled morphology is obtained after removing the pre-strain. The wrinkled FeCo film can keep the magnetic properties unchanged when applying a relatively large tensile bending strain perpendicular to the wrinkles. This reveals that PDMS is an ideal substrate for magnetic films to realize flexible immutability. Our results may help for developing flexible magnetic devices.

    Influence of transition metals (Sc, Ti, V, Cr, and Mn) doping on magnetism of CdS
    Zhongqiang Suo(索忠强), Jianfeng Dai(戴剑锋), Shanshan Gao(高姗姗), and Haoran Gao(高浩然)$
    Chin. Phys. B, 2020, 29 (11):  117502.  DOI: 10.1088/1674-1056/aba2e6
    Abstract ( 391 )   HTML ( 3 )   PDF (1653KB) ( 252 )  

    The influence of transition metals (Sc, Ti, V, Cr, and Mn) doping at different distances on the magnetism of CdS is studied by using generalized gradient approximation combined with Hubbard U in the VASP package. The results show that the doping systems are more stable, easy to form, and the wurtzite structure of CdS is not changed. It is found that the systems are antiferromagnetic (AFM) when nearest neighbor doping, which is attributed to the direct charge transfers between two impurity ions. The systems are ferromagnetic (FM) when the doping distance increases further, since the double exchange interactions are observed among the 3d orbital of the transition metal, the Cd-5s and the S-3p orbitals are at conduction band minimum. We also found that the total magnetic moment of each ferromagnetic system increases with the order of SC to Mn-doping, the spin polarizability of Cr-doping system is 100%. The estimated Curie temperature indicates that the Cr- and Mn-doped CdS in this paper can achieve room-temperature ferromagnetic characteristics, especially the Cr doping is the most prominent. And TM-doping does not destroy the semiconductor characteristics of the system. Therefore, the TM-doped CdS can be used as an ideal dilute magnetic semiconductor functional material.

    Investigation of the magnetoresistance in EuS/Nb:SrTiO3 junction
    Jia Lu(芦佳), Yu-Lin Gan(甘渝林), Yun-Lin Lei(雷蕴麟), Lei Yan(颜雷), and Hong Ding(丁洪)$
    Chin. Phys. B, 2020, 29 (11):  117503.  DOI: 10.1088/1674-1056/abbbf2
    Abstract ( 471 )   HTML ( 4 )   PDF (368KB) ( 188 )  

    EuS is one of typical ferromagnetic semiconductor using as spin filter in spintronic devices, and the doped one could be a good spin injector. Herein, we fabricate a spin-functional tunnel junction by epitaxially growing the ferromagnetic EuS film on Nb-doped SrTiO3. The improvement of Curie temperature up to 35 K is associated with indirect exchange through additional charge carriers at the interface of EuS/Nb:STO junction. Its magnetic field controlled current–voltage curves indicate the large magnetoresistance (MR) effect in EuS barriers as a highly spin-polarized injector. The negative MR is up to 60% in 10-nm EuS/Nb:STO at 4 T and 30 K. The MR is enhanced with increasing thickness of EuS barrier. The large negative MR effect over a wide temperature range makes this junction into a potential candidate for spintronic devices.

    Magnetization reorientation induced by spin–orbit torque in YIG/Pt bilayers Hot!
    Ying-Yi Tian(田颖异), Shuan-Hu Wang(王拴虎), Gang Li(李刚), Hao Li(李豪), Shu-Qin Li(李书琴), Yang Zhao(赵阳), Xiao-Min Cui(崔晓敏), Jian-Yuan Wang(王建元), Lv-Kuan Zou(邹吕宽), and Ke-Xin Jin(金克新)
    Chin. Phys. B, 2020, 29 (11):  117504.  DOI: 10.1088/1674-1056/abb666
    Abstract ( 721 )   HTML ( 7 )   PDF (735KB) ( 319 )  

    In this work, we report the reorientation of magnetization by spin–orbit torque (SOT) in YIG/Pt bilayers. The SOT is investigated by measuring the spin Hall magnetoresistance (SMR), which is highly sensitive to the direction of magnetic moment of YIG. An external in-plane rotating magnetic field which is applied to the YIG/Pt bilayers, and the evolutions of SMR under different injected currents in the Pt layer, result in deviation of SMR curve from the standard shape. We conclude that the SOT caused by spin accumulation near the interface between YIG and Pt can effectively reorient the in-plane magnetic moment of YIG. This discovery provides an effective way to modulate YIG magnetic moments by electrical methods.

    Surface termination effects on the electrical characteristics of La2O3/Al2O3 nanolaminates deposited by atomic layer deposition
    Ji-Bin Fan(樊继斌), Shan-Ya Ling(凌山雅), Hong-Xia Liu(刘红侠), Li Duan(段理), Yan Zhang(张研), Ting-Ting Guo(郭婷婷), Xing Wei(魏星), and Qing He(何清)$
    Chin. Phys. B, 2020, 29 (11):  117701.  DOI: 10.1088/1674-1056/abc0d4
    Abstract ( 458 )   HTML ( 2 )   PDF (770KB) ( 76 )  

    Effects of initial surface termination on electrical characteristics of La2O3/Al2O3 nanolaminates deposited by atomic layer deposition are studied by conductive atomic force microscopy working in contact mode and standard electrical characterization methods. It is found that, compared with La2O3/Al2O3 nanolaminates with LaOx as termination, lower interface trap density, less current leakage spots, and higher breakdown voltage are obtained in the La2O3/Al2O3 nanolaminates with AlOx as termination after annealing. A clear promotion of interface silicate layer is observed for La2O3/Al2O3 nanolaminates with AlOx as termination compared with LaOx as termination under the same annealing condition. In addition, the current conduction mechanism in La2O3/Al2O3 nanolaminates is considered as the Poole–Frenkel conduction. All results indicate that the AlOx is a more appropriate termination to deposit La2O3/Al2O3 nanolaminates on Si substrate, which is useful for the high-κ process development.

    A systematic study of light dependency of persistent photoconductivity in a-InGaZnO thin-film transistors
    Yalan Wang(王雅兰), Mingxiang Wang(王明湘), Dongli Zhang(张冬利), and Huaisheng Wang(王槐生)
    Chin. Phys. B, 2020, 29 (11):  118101.  DOI: 10.1088/1674-1056/abb222
    Abstract ( 516 )   HTML ( 6 )   PDF (951KB) ( 182 )  

    Persistent photoconductivity (PPC) effect and its light-intensity dependence of both enhancement and depletion (E-/D-) mode amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) are systematically investigated. Density of oxygen vacancy (VO) defects of E-mode TFTs is relatively small, in which formation of the photo-induced metastable defects is thermally activated, and the activation energy (Ea) decreases continuously with increasing light-intensity. Density of VO defects of D-mode TFTs is much larger, in which the formation of photo-induced metastable defects is found to be spontaneous instead of thermally activated. Furthermore, for the first time it is found that a threshold dose of light-exposure is required to form fully developed photo-induced metastable defects. Under low light-exposure below the threshold, only a low PPC barrier is formed and the PPC recovery is fast. With increasing the light-exposure to the threshold, the lattice relaxation of metal cations adjacent to the doubly ionized oxygen vacancies (${{\rm{V}}}_{{\rm{O}}}^{2+}$) is fully developed, and the PPC barrier increases to ∼ 0.25 eV, which remains basically unchanged under higher light-exposure. Based on the density of VO defects in the channel and the condition of light illumination, a unified model of formation of photo-induced metastable defects in a-IGZO TFTs is proposed to explain the experimental observations.

    Hydrothermal synthesis and characterization of carbon-doped TiO2 nanoparticles
    Zafar Ali, Javaid Ismail, Rafaqat Hussain, A. Shah, Arshad Mahmood, Arbab Mohammad Toufiq, and Shams ur Rahman
    Chin. Phys. B, 2020, 29 (11):  118102.  DOI: 10.1088/1674-1056/ab9f29
    Abstract ( 485 )   HTML ( 2 )   PDF (1836KB) ( 204 )  

    We report the hydrothermal growth of pure and doped TiO2 nanoparticles with different concentrations of carbon. The microstructure of the as-synthesized samples is characterized by x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive x-ray spectroscopy (EDX), and Raman spectroscopy to understand the structure and composition. The XRD patterns confirm the formation of anatase phase of TiO2 with the average crystallite size is calculated to be in the range of 13 nm to 14.7 nm. The functional groups of these nanostructures are characterized by Fourier transformed infrared (FT-IR) spectroscopy, which further confirms the single anatase phase of the synthesized nanostructures. UV-visible absorption spectroscopy is used to understand the absorption behavior, which shows modification in the optical bandgap from 3.13 eV (pure TiO2) to 3.74 eV (1.2 mol% C-doped TiO2). Furthermore, the Ti3+ centers associated with oxygen vacancies are identified using electron paramagnetic resonance spectroscopy (EPR).

    Twisted and coiled bamboo artificial muscles for moisture responsive torsional and tensile actuation Hot!
    Xiaoyu Hu(胡晓宇), Xueqi Leng(冷雪琪), Tianjiao Jia(贾天娇), and Zunfeng Liu(刘遵峰)
    Chin. Phys. B, 2020, 29 (11):  118103.  DOI: 10.1088/1674-1056/abbbda
    Abstract ( 1039 )   HTML ( 4 )   PDF (1197KB) ( 1402 )  

    Smart textiles responding to the ambient environment like temperature, humidity, and light are highly desirable to improve the comfortability and realize multifunctions. The bamboo yarn has merits like air permeability, biodegradability, and excellent heat dissipation performance, but it has not been prepared for responsive materials and smart textiles. In this paper, the moisture-responsive twisted bamboo yarns were plied to form a self-balanced torsional actuator and wrapped around a mandrel to form a coil, followed by water immersion and evaporation to fix the shape and serve as a tensile actuator. A torsional actuation of 64.4°⋅ mm−1 was realized for the twisted actuator in 4.2 s; a maximum elongation of 133% or contraction of 50% was achieved for a coiled tensile actuator with good cyclability. The porous structure of bamboo yarns helped improve the water absorbance speed and decrease the response time of moisture. The self-balanced two-ply physical structure and reversible generation of chemical phase after soaking in aqueous solution fixed internal stress and provided good cyclability. With the unique properties including aqueous water-induced shape fixation and moisture-induced actuation, the application of tensile actuation of bamboo yarns was demonstrated, showing promising prospects on smart textiles.

    Thermoelectric properties of orthorhombic silicon allotrope Si (oP32) from first-principles calculations
    Pei Zhang(张培), Tao Ouyang(欧阳滔), Chao Tang(唐超), Chao-Yu He(何朝宇), Jin Li(李金), Chun-Xiao Zhang(张春小), and Jian-Xin Zhong(钟建新)
    Chin. Phys. B, 2020, 29 (11):  118401.  DOI: 10.1088/1674-1056/aba2e1
    Abstract ( 362 )   HTML ( 3 )   PDF (9472KB) ( 76 )  

    The diamond-like cubic silicon (d-Si) is widely used in modern electronics and solar cell industries. However, it is not an optimal candidate for thermoelectric application due to its high lattice thermal conductivity. Si (oP32) is a recently predicted orthorhombic silicon allotrope, whose total energy is close to that of d-Si. Using first-principles calculations and Boltzmann transport theory, we systematically investigate the thermoelectric properties of Si (oP32). The lower phonon thermal conductivity and higher power factor are obtained in Si (oP32) than those in diamond silicon. The low phonon thermal conductivity (33.77 W/mK at 300 K) is mainly due to the reduction of the phonon group velocity and enhancement of phonon–phonon scattering (including scattering phase space and strength). Meanwhile, the results also show that the thermoelectric performance along the zz lattice direction is better than that along the xx and yy lattice directions, and the figure of merit (700 K) along the zz lattice direction could approach to 2.45 and 1.75 for p-type and n-type Si (oP32), respectively. The values are much higher than those of d-Si (about 0.06)) and Si24 (0.6), indicating that the Si (oP32) is a promising candidate for thermoelectric applications. Our theoretical studies shed light on the thermoelectric properties of Si (oP32) and could stimulate further experimental studies.

    A compact dual-band radiation system
    Yuan-Qiang Yu(于元强), Yu-Wei Fan(樊玉伟), and Xiao-Yu Wang(王晓玉)$
    Chin. Phys. B, 2020, 29 (11):  118402.  DOI: 10.1088/1674-1056/aba607
    Abstract ( 581 )   HTML ( 4 )   PDF (1187KB) ( 77 )  

    Complex magnetically insulated transmission line oscillator (MILO), as an important development direction, can enhance the power efficiency and generate dual-band high power microwaves (HPMs). A complex MILO and a preliminary dual-band radiation system have been proposed in our previous studies. However, the axial length of the dual-band radiation system is too long to meet the compact requirements. In this paper, a compact dual-band radiation system is presented and investigated numerically. The compact dual-band radiation system comprises a dual-band cross-shaped mode converter and a dual-band coaxial conical horn antenna. It can convert two coaxial TEM mode microwaves (1.717 GHz and 4.167 GHz) generated by the complex MILO into the coaxial TE11 mode microwaves, and then radiate them into the air. At 1.717 GHz, the gain of the antenna is 17.9 dB, and the total return loss and diffraction loss are 1.50% and 0, respectively. At 4.167 GHz, the gain is 19.4 dB, and the total return loss and diffraction loss are 1.17% and 0.78%, respectively. The power handling capacity of the antenna is 5.1 GW at 1.717 GHz and 2.0 GW at 4.167 GHz. Comparing with the original structure, the length of the dual-band radiation system is reduced by 45.2%.

    Electrostatic gating of solid-ion-conductor on InSe flakes and InSe/h-BN heterostructures
    Zhang Zhou(周璋), Liangmei Wu(吴良妹), Jiancui Chen(陈建翠), Jiajun Ma(马佳俊), Yuan Huang(黄元), Chengmin Shen(申承民), Lihong Bao(鲍丽宏), and Hong-Jun Gao(高鸿钧)
    Chin. Phys. B, 2020, 29 (11):  118501.  DOI: 10.1088/1674-1056/aba60a
    Abstract ( 402 )   HTML ( 3 )   PDF (1998KB) ( 69 )  

    We report the electrical transport properties of InSe flakes electrostatically gated by a solid ion conductor. The large tuning capability of the solid ion conductor as gating dielectric is confirmed by the saturation gate voltage as low as ∼1 V and steep subthreshold swing (83 mV/dec). The p-type conduction behavior of InSe is obtained when negative gate voltages are biased. Chemical doping of the solid ion conductor is suppressed by inserting a buffer layer of hexagonal boron nitride (h-BN) between InSe and the solid-ion-conductor substrate. By comparing the performance of devices with and without h-BN, the capacitance of solid ion conductors is extracted to be the same as that of ∼2 nm h-BN, and the mobility of InSe on solid ion conductors is comparable to that on the SiO2 substrate. Our results show that solid ion conductors provide a facile and powerful method for electrostatic doping.

    Improving the performance of crystalline Si solar cell by high-pressure hydrogenation
    Xi-Yuan Dai(戴希远), Yu-Chen Zhang(张宇宸), Liang-Xin Wang(王亮兴), Fei Hu(胡斐), Zhi-Yuan Yu(于志远), Shuai Li(李帅), Shu-Jie Li(李树杰), Xin-Ju Yang(杨新菊), and Ming Lu(陆明)
    Chin. Phys. B, 2020, 29 (11):  118801.  DOI: 10.1088/1674-1056/abb3e3
    Abstract ( 346 )   HTML ( 2 )   PDF (817KB) ( 29 )  

    We report an approach of high-pressure hydrogenation to improve the performance of crystalline Si (c-Si) solar cells. As-received p-type c-Si wafer-based PN junctions were subjected to high-pressure (2.5 MPa) hydrogen atmosphere at 200 °C, followed by evaporating antireflection layers, passivation layers, and front and rear electrodes. The efficiency of the so prepared c-Si solar cell was found to increase evidently after high-pressure hydrogenation, with a maximal enhancement of 10%. The incorporation of hydrogen by Si solar cells was identified, and hydrogen passivation of dangling bonds in Si was confirmed. Compared to the regular approach of hydrogen plasma passivation, the approach of high-pressure hydrogenation reported here needs no post-hydrogenation treatment, and can be more convenient and efficient to use in improving the performances of the c-Si and other solar cells.

ISSN 1674-1056   CN 11-5639/O4
, Vol. 29, No. 11

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