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Chin. Phys. B  
  Chin. Phys. B--2020, Vol.29, No.4
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TOPICAL REVIEW—Physics in neuromorphic devices

High-performance synaptic transistors for neuromorphic computing

Hai Zhong, Qin-Chao Sun, Guo Li, Jian-Yu Du, He-Yi Huang, Er-Jia Guo, Meng He, Can Wang, Guo-Zhen Yang, Chen Ge, Kui-Juan Jin
Chin. Phys. B 2020, 29 (4): 040703;  doi: 10.1088/1674-1056/ab7806
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The further development of traditional von Neumann-architecture computers is limited by the breaking of Moore's law and the von Neumann bottleneck, which make them unsuitable for future high-performance artificial intelligence (AI) systems. Therefore, new computing paradigms are desperately needed. Inspired by the human brain, neuromorphic computing is proposed to realize AI while reducing power consumption. As one of the basic hardware units for neuromorphic computing, artificial synapses have recently aroused worldwide research interests. Among various electronic devices that mimic biological synapses, synaptic transistors show promising properties, such as the ability to perform signal transmission and learning simultaneously, allowing dynamic spatiotemporal information processing applications. In this article, we provide a review of recent advances in electrolyte- and ferroelectric-gated synaptic transistors. Their structures, materials, working mechanisms, advantages, and disadvantages will be presented. In addition, the challenges of developing advanced synaptic transistors are discussed.

Optoelectronic memristor for neuromorphic computing

Wuhong Xue, Wenjuan Ci, Xiao-Hong Xu, Gang Liu
Chin. Phys. B 2020, 29 (4): 048401;  doi: 10.1088/1674-1056/ab75da
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With the need of the internet of things, big data, and artificial intelligence, creating new computing architecture is greatly desired for handling data-intensive tasks. Human brain can simultaneously process and store information, which would reduce the power consumption while improve the efficiency of computing. Therefore, the development of brain-like intelligent device and the construction of brain-like computation are important breakthroughs in the field of artificial intelligence. Memristor, as the fourth fundamental circuit element, is an ideal synaptic simulator due to its integration of storage and processing characteristics, and very similar activities and the working mechanism to synapses among neurons which are the most numerous components of the brains. In particular, memristive synaptic devices with optoelectronic responding capability have the benefits of storing and processing transmitted optical signals with wide bandwidth, ultrafast data operation speed, low power consumption, and low cross-talk, which is important for building efficient brain-like computing networks. Herein, we review recent progresses in optoelectronic memristor for neuromorphic computing, including the optoelectronic memristive materials, working principles, applications, as well as the current challenges and the future development of the optoelectronic memristor.
TOPICAL REVIEW—Magnetism, magnetic materials, and interdisciplinary research

Multicaloric and coupled-caloric effects

Jia-Zheng Hao, Feng-Xia Hu, Zi-Bing Yu, Fei-Ran Shen, Hou-Bo Zhou, Yi-Hong Gao, Kai-Ming Qiao, Jia Li, Cheng Zhang, Wen-Hui Liang, Jing Wang, Jun He, Ji-Rong Sun, Bao-Gen Shen
Chin. Phys. B 2020, 29 (4): 047504;  doi: 10.1088/1674-1056/ab7da7
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The multicaloric effect refers to the thermal response of a solid material driven by simultaneous or sequential application of more than one type of external field. For practical applications, the multicaloric effect is a potentially interesting strategy to improve the efficiency of refrigeration devices. Here, the state of the art in multi-field driven multicaloric effect is reviewed. The phenomenology and fundamental thermodynamics of the multicaloric effect are well established. A number of theoretical and experimental research approaches are covered. At present, the theoretical understanding of the multicaloric effect is thorough. However, due to the limitation of the current experimental technology, the experimental approach is still in progress. All these researches indicated that the thermal response and effective reversibility of multiferroic materials can be improved through multicaloric cycles to overcome the inherent limitations of the physical mechanisms behind single-field-induced caloric effects. Finally, the viewpoint of further developments is presented.
SPECIAL TOPIC—Recent advances in thermoelectric materials and devices
SPECIAL TOPIC—Optical field manipulation

Creation of topological vortices using Pancharatnam-Berry phase liquid crystal holographic plates

Xuyue Guo, Jinzhan Zhong, Peng Li, Bingyan Wei, Sheng Liu, Jianlin Zhao
Chin. Phys. B 2020, 29 (4): 040305;  doi: 10.1088/1674-1056/ab7805
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Recently, physical fields with topological configurations are evoking increasing attention due to their fascinating structures both in fundamental researches and practical applications. Therein, topological light fields, because of their unique opportunity of combining experimental and analytical studies, are attracting more interest. Here, based on the Pancharatnam-Berry (PB) phase, we report the creation of Hopf linked and Trefoil knotted optical vortices by using phase-only encoded liquid crystal (LC) holographic plates. Utilizing scanning measurement and the digital holographic interference method, we accurately locate the vortex singularities and map these topological nodal lines in three-dimensions. Compared with the common methods realized by the spatial light modulator (SLM), the phase-only LC plate is more efficient. Meanwhile, the smaller pixel size of the LC element reduces the imperfection induced by optical misalignment and pixellation. Moreover, we analyze the influence of the incident beam size on the topological configuration.
SPECIAL TOPIC—Terahertz physics

Hydrodynamic simulation of chaotic dynamics in InGaAs oscillator in terahertz region

Wei Feng
Chin. Phys. B 2020, 29 (4): 047302;  doi: 10.1088/1674-1056/ab696f
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Hydrodynamic calculations of the chaotic behaviors in n+nn+ In0.53Ga0.47As devices biased in terahertz (THz) electric field have been carried out. Their different transport characteristics have been carefully investigated by tuning the n-region parameters and the applied ac radiation. The oscillatory mode is found to transit between synchronization and chaos, as verified by the first return map. The transitions result from the mixture of the dc induced oscillation and the one driven by the ac radiation. Our findings will give further and thorough understanding of electron transport in In0.53Ga0.47As terahertz oscillator, which is a promising solid-state THz source.
GENERAL

Nonlocal symmetries and similarity reductions for Korteweg-de Vries-negative-order Korteweg-de Vries equation

Heng-Chun Hu, Fei-Yan Liu
Chin. Phys. B 2020, 29 (4): 040201;  doi: 10.1088/1674-1056/ab6dca
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The nonlocal symmetries are derived for the Korteweg-de Vries-negative-order Korteweg-de Vries equation from the Painlevé truncation method. The nonlocal symmetries are localized to the classical Lie point symmetries for the enlarged system by introducing new dependent variables. The corresponding similarity reduction equations are obtained with different constant selections. Many explicit solutions for the integrable equation can be presented from the similarity reduction.

Finite-time Mittag-Leffler synchronization of fractional-order delayed memristive neural networks with parameters uncertainty and discontinuous activation functions

Chong Chen, Zhixia Ding, Sai Li, Liheng Wang
Chin. Phys. B 2020, 29 (4): 040202;  doi: 10.1088/1674-1056/ab7803
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The finite-time Mittag-Leffler synchronization is investigated for fractional-order delayed memristive neural networks (FDMNN) with parameters uncertainty and discontinuous activation functions. The relevant results are obtained under the framework of Filippov for such systems. Firstly, the novel feedback controller, which includes the discontinuous functions and time delays, is proposed to investigate such systems. Secondly, the conditions on finite-time Mittag-Leffler synchronization of FDMNN are established according to the properties of fractional-order calculus and inequality analysis technique. At the same time, the upper bound of the settling time for Mittag-Leffler synchronization is accurately estimated. In addition, by selecting the appropriate parameters of the designed controller and utilizing the comparison theorem for fractional-order systems, the global asymptotic synchronization is achieved as a corollary. Finally, a numerical example is given to indicate the correctness of the obtained conclusions.

Reconciliation for CV-QKD using globally-coupled LDPC codes

Jin-Jing Shi, Bo-Peng Li, Duan Huang
Chin. Phys. B 2020, 29 (4): 040301;  doi: 10.1088/1674-1056/ab6d4f
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Reconciliation is a necessary step in postprocessing of continuous-variable quantum key distribution (CV-QKD) system. We use globally coupled low-density parity-check (GC-LDPC) codes in reconciliation to extract a precise secret key from the raw keys over the authenticated classical public channel between two users. GC-LDPC codes have excellent performance over both the additive Gaussian white noise and binary-erasure channels. The reconciliation based on GC-LDPC codes can improve the reconciliation efficiency to 95.42% and reduce the frame error rate to 3.25×10-3. Using distillation, the decoding speed can achieve 23.8 Mbits/s and decrease the cost of memory. Given decoding speed and low memory usage, this makes the proposed reconciliation method viable approach for high-speed CV-QKD system.

Generating Kerr nonlinearity with an engineered non-Markovian environment

Fei-Lei Xiong, Wan-Li Yang, Mang Feng
Chin. Phys. B 2020, 29 (4): 040302;  doi: 10.1088/1674-1056/ab7741
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Kerr nonlinearity is an important resource for creating squeezing and entanglement in quantum technology. Here we propose a scheme for generating Kerr nonlinearity originated from an engineered non-Markovian environment, which is different from the previous efforts using nonlinear media or quantum systems with special energy structures. In the present work, the generation of Kerr nonlinearity depends on the system-environment interaction time, the energy spectrum of the environment, and the system-environment coupling strength, regardless of the environmental initial state. The scheme can be realized in systems originally containing no Kerr interaction, such as superconducting circuit systems, optomechanical systems, and cavity arrays connected by transmission lines.

Quantum coherence and correlation dynamics of two-qubit system in spin bath environment

Hao Yang, Li-Guo Qin, Li-Jun Tian, Hong-Yang Ma
Chin. Phys. B 2020, 29 (4): 040303;  doi: 10.1088/1674-1056/ab773e
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The quantum entanglement, discord, and coherence dynamics of two spins in the model of a spin coupled to a spin bath through an intermediate spin are studied. The effects of the important physical parameters including the coupling strength of two spins, the interaction strength between the intermediate spin and the spin bath, the number of bath spins and the temperature of the system on quantum coherence and correlation dynamics are discussed in different cases. The frozen quantum discord can be observed whereas coherence does not when the initial state is the Bell-diagonal state. At finite temperature, we find that coherence is more robust than quantum discord, which is better than entanglement, in terms of resisting the influence of environment. Therefore, quantum coherence is more tenacious than quantum correlation as an important resource.

Efficient scheme for remote preparation of arbitrary n-qubit equatorial states

Xin-Wei Zha, Min-Rui Wang, Ruo-Xu Jiang
Chin. Phys. B 2020, 29 (4): 040304;  doi: 10.1088/1674-1056/ab773d
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Recently, a scheme for deterministic remote preparation of arbitrary multi-qubit equatorial states was proposed by Wei et al. [Quantum Inf. Process. 17 70 (2018)]. It is worth mentioning that the construction of mutual orthogonal measurement basis plays a key role in quantum remote state preparation. In this paper, a simple and feasible remote preparation of arbitrary n-qubit equatorial states scheme is proposed. In our scheme, the success probability will reach unit. Moreover, there are no coefficient constraint and auxiliary qubits in this scheme. It means that the success probabilities are independent of the coefficients of the entangled channel. The advantage of our scheme is that the mutual orthogonal measurement basis is devised. To accomplish the quantum remote state preparation (RSP) schemes, some new sets of mutually orthogonal measurement basis are introduced.

Reduction of entropy uncertainty for qutrit system under non-Markov noisy environment

Xiong Xu, Mao-Fa Fang
Chin. Phys. B 2020, 29 (4): 040306;  doi: 10.1088/1674-1056/ab7ea5
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We explore the entropy uncertainty for qutrit system under non-Markov noisy environment and discuss the effects of the quantum memory system and the spontaneously generated interference (SGI) on the entropy uncertainty in detail. The results show that, the entropy uncertainty can be reduced by using the methods of quantum memory system and adjusting of SGI. Particularly, the entropy uncertainty can be decreased obviously when both the quantum memory system and the SGI are simultaneously applied.

Lump, lumpoff and predictable rogue wave solutions to a dimensionally reduced Hirota bilinear equation

Haifeng Wang, Yufeng Zhang
Chin. Phys. B 2020, 29 (4): 040501;  doi: 10.1088/1674-1056/ab75d7
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We study a simplified (3+1)-dimensional model equation and construct a lump solution for the special case of z=y using the Hirota bilinear method. Then, a more general form of lump solution is constructed, which contains more arbitrary autocephalous parameters. In addition, a lumpoff solution is also derived based on the general lump solutions and a stripe soliton. Furthermore, we figure out instanton/rogue wave solutions via introducing two stripe solitons. Finally, one can better illustrate these propagation phenomena of these solutions by analyzing images.

Nonlinear continuous bi-inductance electrical line with dissipative elements: Dynamics of the low frequency modulated waves

S M Ngounou, F B Pelap
Chin. Phys. B 2020, 29 (4): 040502;  doi: 10.1088/1674-1056/ab75d6
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The dynamics of modulated waves in a nonlinear bi-inductance transmission line with dissipative elements are examined. We show the existence of two frequency modes and carry out intensive investigations on the low frequency mode. Thanks to the multiple scales method, the behavior of these waves is investigated and the dissipative effects are analyzed. It appears that the dissipation coefficient increases with the carrier wave frequency. In the continuous approximation, we derive that the propagation of these waves is governed by the complex Ginzburg-Landau equation instead of the Korteweg-de-Vries equation as previously established. Asymptotic studies of the dynamics of plane waves in the line reveal the existence of three additional regions in the dispersion curve where the modulational phenomenon is observed. In the low frequency mode, we demonstrate that the network allows the propagation of dark and bright solitons. Numerical findings are in perfect agreement with the analytical predictions.

Novel Woods-Saxon stochastic resonance system for weak signal detection

Yong-Hui Zhou, Xue-Mei Xu, Lin-Zi Yin, Yi-Peng Ding, Jia-Feng Ding, Ke-Hui Sun
Chin. Phys. B 2020, 29 (4): 040503;  doi: 10.1088/1674-1056/ab75ca
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We propose a joint exponential function and Woods-Saxon stochastic resonance (EWSSR) model. Because change of a single parameter in the classical stochastic resonance model may cause a great change in the shape of the potential function, it is difficult to obtain the optimal output signal-to-noise ratio by adjusting one parameter. In the novel system, the influence of different parameters on the shape of the potential function has its own emphasis, making it easier for us to adjust the shape of the potential function. The system can obtain different widths of the potential well or barrier height by adjusting one of these parameters, so that the system can match different types of input signals adaptively. By adjusting the system parameters, the potential function model can be transformed between the bistable model and the monostable model. The potential function of EWSSR has richer shapes and geometric characteristics. The effects of parameters, such as the height of the barrier and the width of the potential well, on SNR are studied, and a set of relatively optimal parameters are determined. Moreover, the EWSSR model is compared with other classical stochastic resonance models. Numerical experiments show that the proposed EWSSR model has higher SNR and better noise immunity than other classical stochastic resonance models. Simultaneously, the EWSSR model is applied to the detection of actual bearing fault signals, and the detection effect is also superior to other models.

Energy cooperation in quantum thermoelectric systems withmultiple electric currents

Yefeng Liu, Jincheng Lu, Rongqian Wang, Chen Wang, Jian-Hua Jiang
Chin. Phys. B 2020, 29 (4): 040504;  doi: 10.1088/1674-1056/ab7da5
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The energy efficiency and output power of a quantum thermoelectric system with multiple electric currents and only one heat current are studied. The system is connected to the hot heat bath (cold bath) through one terminal (multiple terminals). In such configurations, there are multiple thermoelectric effects coexisting in the system. Using the Landauer-Büttiker formalism, we show that the cooperation between the two thermoelectric effects in the three-terminal thermoelectric systems can lead to markedly improved performance of the heat engine. Such improvement also occurs in four-terminal thermoelectric heat engines with three output electric currents. Cooperative effects in these multi-terminal thermoelectric systems can considerably enlarge the physical parameter region that realizes high energy efficiency and output power. For refrigeration, we find that the energy efficiency can also be substantially improved by exploiting the cooperative effects in multi-terminal thermoelectric systems. All these results reveal a useful approach toward high-performance thermoelectric energy conversion in multi-terminal mesoscopic systems.

Preliminary abnormal electrocardiogram segment screening method for Holter data based on long short-term memory networks

Siying Chen, Hongxing Liu
Chin. Phys. B 2020, 29 (4): 040701;  doi: 10.1088/1674-1056/ab6d52
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Holter usually monitors electrocardiogram (ECG) signals for more than 24 hours to capture short-lived cardiac abnormalities. In view of the large amount of Holter data and the fact that the normal part accounts for the majority, it is reasonable to design an algorithm that can automatically eliminate normal data segments as much as possible without missing any abnormal data segments, and then take the left segments to the doctors or the computer programs for further diagnosis. In this paper, we propose a preliminary abnormal segment screening method for Holter data. Based on long short-term memory (LSTM) networks, the prediction model is established and trained with the normal data of a monitored object. Then, on the basis of kernel density estimation, we learn the distribution law of prediction errors after applying the trained LSTM model to the regular data. Based on these, the preliminary abnormal ECG segment screening analysis is carried out without R wave detection. Experiments on the MIT-BIH arrhythmia database show that, under the condition of ensuring that no abnormal point is missed, 53.89% of normal segments can be effectively obviated. This work can greatly reduce the workload of subsequent further processing.

A synthetic optically pumped gradiometer for magnetocardiography measurements

Shu-Lin Zhang, Ning Cao
Chin. Phys. B 2020, 29 (4): 040702;  doi: 10.1088/1674-1056/ab7801
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Magnetocardiography (MCG) measurement is important for investigating the cardiac biological activities. Traditionally, the extremely weak MCG signal was detected by using superconducting quantum interference devices (SQUIDs). As a room-temperature magnetic-field sensor, optically pumped magnetometer (OPM) has shown to have comparable sensitivity to that of SQUIDs, which is very suitable for biomagnetic measurements. In this paper, a synthetic gradiometer was constructed by using two OPMs under spin-exchange relaxation-free (SERF) conditions within a moderate magnetically shielded room (MSR). The magnetic noise of the OPM was measured to less than 70 fT/Hz1/2. Under a baseline of 100 mm, noise cancellation of about 30 dB was achieved. MCG was successfully measured with a signal to noise ratio (SNR) of about 37 dB. The synthetic gradiometer technique was very effective to suppress the residual environmental fields, demonstrating the OPM gradiometer technique for highly cost-effective biomagnetic measurements.
ATOMIC AND MOLECULAR PHYSICS

Analytical expressions of non-relativistic static multipole polarizabilities for hydrogen-like ions

Xuesong Mei, Wanping Zhou, Zhenxiang Zhong, Haoxue Qiao
Chin. Phys. B 2020, 29 (4): 043101;  doi: 10.1088/1674-1056/ab75d0
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Analytical formulas for the static multipole polarizabilities of hydrogen-like ions are derived by using the analytical wave functions and the reduced Green function and by applying a numerical fitting procedure. Our results are then applied to the studies of blackbody radiation shifts to atomic energy levels at different temperatures. Our analytical results can be served as a benchmark for other theoretical methods.

Non-Born-Oppenheimer study of the muonic molecule ion 4Heμ+

Hang Yang, Meng-Shan Wu, Yi Zhang, Ting-Yun Shi, Kalman Varga, Jun-Yi Zhang
Chin. Phys. B 2020, 29 (4): 043102;  doi: 10.1088/1674-1056/ab7908
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Accurate non-Born-Oppenheimer variational calculations of all bound states of the positive muon molecular ion 4Heμ+ have been performed using explicitly correlated Gaussian functions in conjunction with the global vectors. All the energies obtained are accurate in the order of 10-6 Hartree (1 Hartree = 27.2114 eV). Compared with the binding energies obtained from calculations based on the Born-Oppenheimer potential with the mass-weighted adiabatic corrections (Chem. Phys. Lett. 110 487 (1984)), the largest relative deviation is up to 15%. By analyzing the average interparticle distances and possibility distributions of interparticle distances of this system, it is confirmed that the Born-Oppenheimer approximation is reasonable for this system and that 4Heμ+ can be regarded as a system of positive muon bound to a slightly distorted helium atom.

Re effects in model Ni-based superalloys investigated with first-principles calculations and atom probe tomography

Dianwu Wang, Chongyu Wang, Tao Yu, Wenqing Liu
Chin. Phys. B 2020, 29 (4): 043103;  doi: 10.1088/1674-1056/ab6587
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The phase partition and site preference of Re atoms in a ternary Ni-Al-Re model alloy, including the electronic structure of different Re configurations, are investigated with first-principles calculations and atom probe tomography. The Re distribution of single, nearest neighbor (NN), next-nearest neighbor (NNN), and cluster configurations are respectively designed in the models with γ and γ' phases. The results show that the Re atoms tend to entering γ' phase and the Re atoms prefer to occupy the Al sites in γ' phase. The Re cluster with a combination of NN and NNN Re-Re pair configuration is not preferred than the isolated Re atom in the Ni-based superalloys, and the configuration with isolated Re atom is more preferred in the system. Especially, the electronic states are analyzed and the energetic parameters are calculated. The electronic structure analyses show there exists strong Ni-Re electronic interaction and it is mainly contributed by the d-d hybridization. The characteristic features of the electronic states of the Re doping effects are also given. It is also found that Re atoms prefer the Al sites in γ' side at the interface. The density of states at or near the Fermi level and the d-d hybridizations of NN Ni-Re are found to be important in the systems.

Controlling paths of high-order harmonic generation by orthogonal two-color fields

Ze-Hui Ma, Cai-Ping Zhang, Jun-Lin Ma, Xiang-Yang Miao
Chin. Phys. B 2020, 29 (4): 043201;  doi: 10.1088/1674-1056/ab7189
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Controlling paths of high-order harmonic generation from H2+ is theoretically investigated by numerically solving the time-dependent Schrödinger equation based on the Born-Oppenheimer approximation in orthogonal two-color fields. Our simulations show that the change of harmonic emission paths is dependent on time-dependent distribution of electrons. Compared with one-dimensional linearly polarized long wavelength laser, multiple returns are suppressed and short paths are dominant in the process of harmonic emission by two-dimensional orthogonal ω/2ω laser fields. Furthermore, not only are multiple returns weaken, but also the harmonic emission varies from twice to once in an optical cycle by orthogonal ω/1.5ω laser fields. Combining the time-frequency distributions and the time-dependent electron wave packets probability density, the mechanism of controlling paths is further explained. As a result, a 68-as isolated attosecond pulse is obtained by superposing a proper range of the harmonics.

Controlling electron collision by counterrotating circular two-color laser fields

Baoqin Li, Xianghe Ren, Jingtao Zhang
Chin. Phys. B 2020, 29 (4): 043202;  doi: 10.1088/1674-1056/ab7907
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Electron collision as well as its controlling lies in the core of study on nonsequential double ionization (NSDI). A single collision occurred in a convergent time is important to disclose the essential features of the electron correlation. However, it is difficult to form such a collision. By using counterrotating circular two-color (CRTC) laser fields, we show that a single electron collision can be achieved in a convergent time and a net electron correlation is set up within the sub-femtosecond time scale in the NSDI process of Ar atoms. The proposed method is also valid for other atoms, provided that one chooses the frequency and intensity of the CRTC field according to a scaling law.

Coherent 420 nm laser beam generated by four-wave mixing in Rb vapor with a single continuous-wave laser

Hao Liu, Jin-Peng Yuan, Li-Rong Wang, Lian-Tuan Xiao, Suo-Tang Jia
Chin. Phys. B 2020, 29 (4): 043203;  doi: 10.1088/1674-1056/ab75d9
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We demonstrate the generation of the coherent 420 nm laser via parametric four-wave mixing process in Rb vapor. A single 778 nm laser with circular polarization is directly injected into a high-density atomic vapor, which drives the atoms from the 5S1/2 state to the 5D5/2 state with monochromatic two-photon transition. The frequency up-conversion laser is generated by the parametric four-wave mixing process under the phase matching condition. This coherent laser is firstly certified by the knife-edge method and a narrow range grating spectrometer. Then the generated laser power is investigated in terms of the power and frequency of the incoming beam as well as the density of the atoms. Finally, a 420 nm coherent laser with power of 19 μW and beam quality of Mx2 =1.32, My2 =1.37 is obtained with optimal experimental parameters. This novel laser shows potential prospects in the measurement of material properties, information storage, and underwater optical communication.

Spin-exchange relaxation of naturally abundant Rb in a K-Rb-21Ne self-compensated atomic comagnetometer

Yan Lu, Yueyang Zhai, Yong Zhang, Wenfeng Fan, Li Xing, Wei Quan
Chin. Phys. B 2020, 29 (4): 043204;  doi: 10.1088/1674-1056/ab75d3
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The total effective spin-exchange relaxation of naturally abundant Rb in a K-Rb-21Ne comagnetometer is analyzed, and the results show that the coexistence of 87Rb and 85Rb isotopes in the same volume can lead to a large extra spin-exchange broadening compared to pure 87Rb. This broadening mainly comes from the contribution of the equivalent reduction in the Rb spin-exchange rate. On this basis, an approximate relaxation model is proposed and experimentally demonstrated to be more accurate than that from a previous work. This study also provides a method for determining the properties of alkali-metal vapor cells.

Filling gap of combination of gauge and analytical method in KFR-like theory

Jian Li, Feng-Cai Ma
Chin. Phys. B 2020, 29 (4): 043205;  doi: 10.1088/1674-1056/ab75ce
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Bauer recently presented a formula for the ionization rate of a hydrogen atom in a strong linearly polarized laser field [J. Phys. B 49 145601 (2016)]. He started from the Keldysh probability amplitude in the length gauge and utilized Reiss's method in the velocity gauge. Instead, according to the Reiss probability amplitude in the velocity gauge and Keldysh's derivation for the length gauge, we derive a formula for the ionization rate of a ground-state hydrogen atom or a hydrogen-like atom in a strong linearly polarized laser field. We compare the numerical results of the total ionization rate and the photoelectron energy distribution calculated from our formula with the results from Keldysh, Reiss, and Bauer. We find that the apparent discrepancies in the ionization rate are caused not only by different gauges, but also by different analytical methods used to derive the ionization rate.

Polarization and fundamental sensitivity of 39K (133Cs)-85Rb-21Neco-magnetometers

Jian-Hua Liu, Dong-Yang Jing, Lin Zhuang, Wei Quan, Jiancheng Fang, Wu-Ming Liu
Chin. Phys. B 2020, 29 (4): 043206;  doi: 10.1088/1674-1056/ab7d94
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The hybrid optical pumping spin exchange relaxation free (HOPSERF) atomic co-magnetometers make ultrahigh sensitivity measurement of inertia achievable. The wall relaxation rate has a big effect on the polarization and fundamental sensitivity for the co-magnetometer, but it is often neglected in the experiments. However, there is almost no work about the systematic analysis of the influence factors on the polarization and the fundamental sensitivity of the HOPSERF co-magnetometers. Here we systematically study the polarization and the fundamental sensitivity of 39K-85Rb-21Ne and 133Cs-85Rb-21Ne HOPSERF co-magnetometers with low polarization limit and the wall relaxation rate. The 21Ne number density, the power density and wavelength of pump beam will affect the polarization greatly by affecting the pumping rate of the pump beam. We obtain a general formula on the fundamental sensitivity of the HOPSERF co-magnetometers due to shot-noise and the fundamental sensitivity changes with multiple systemic parameters, where the suitable number density of buffer gas and quench gas make the fundamental sensitivity highest. The fundamental sensitivity 7.5355×10-11 rad·-1·Hz-1/2 of 133Cs-85Rb-21Ne co-magnetometer is higher than the ultimate theoretical sensitivity 2×10-10 rad·-1·Hz-1/2 of K-21Ne co-magnetometer.

Two types of highly efficient electrostatic traps for single loading or multi-loading of polar molecules

Bin Wei, Hengjiao Guo, Yabing Ji, Shunyong Hou, Jianping Yin
Chin. Phys. B 2020, 29 (4): 043701;  doi: 10.1088/1674-1056/ab7905
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Two novel electrostatic traps named octopole-based disk electrostatic trap (ODET) and tubular-based disk electrostatic trap (TDET) are proposed for trapping cold polar molecules in low-field-seeking states. Using MgF as the target molecule, single loading and multi-loading methods are numerically simulated with varied incident velocities of slow molecular beams in the two types of traps, respectively. In ODET, with an incident velocity of 10 m/s, a highest loading efficiency of 78.4% or 99.9% has been achieved under the single loading or multi-loading operation mode. In TDET, with an incident velocity of 11 m/s, a highest loading efficiency of 81.6% or 106.5% has been achieved using the two loading methods, respectively. With such high loading efficiencies, the trapped cold molecules can be applied in the researches of cold collisions, high precision spectroscopy, and precision measurements. Especially, together with a blue-detuned hollow beam, the new electrostatic traps proposed here offer a new platform for the following gradient-intensity cooling of MgF molecules, which may provide a new way to produce high density ultracold molecules.

Influence of driving ways on measurement of relative phase in a two-atoms cavity system

Daqiang Bao, Jingping Xu, Yaping Yang
Chin. Phys. B 2020, 29 (4): 043702;  doi: 10.1088/1674-1056/ab7904
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We study the influence of driving ways on the interaction in a two-atoms cavity quantum electrodynamics system. The results show that driving ways can induce different excitation pathways. We show two kinds of significantly different excitation spectrums under two ways: driving cavity and driving atoms. We demonstrate that driving atoms can be considered as a method to obtain the position information of atoms. This research has very practical application values on obtaining the position information of atoms in a cavity.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

Dissipative quantum phase transition in a biased Tavis-Cummings model

Zhen Chen, Yueyin Qiu, Guo-Qiang Zhang, Jian-Qiang You
Chin. Phys. B 2020, 29 (4): 044201;  doi: 10.1088/1674-1056/ab7b55
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We study the dissipative quantum phase transition (QPT) in a biased Tavis-Cummings model consisting of an ensemble of two-level systems (TLSs) interacting with a cavity mode, where the TLSs are pumped by a drive field. In our proposal, we use a dissipative TLS ensemble and an active cavity with effective gain. In the weak drive-field limit, the QPT can occur under the combined actions of the loss and gain of the system. Owing to the active cavity, the QPT behavior can be much differentiated even for a finite strength of the drive field on the TLS ensemble. Also, we propose to implement our scheme based on the dissipative nitrogen-vacancy (NV) centers coupled to an active optical cavity made from the gain-medium-doped silica. Furthermore, we show that the QPT can be measured by probing the transmission spectrum of the cavity embedding the ensemble of the NV centers.

Conserved quantities and adiabatic invariants of fractional Birkhoffian system of Herglotz type

Juan-Juan Ding, Yi Zhang
Chin. Phys. B 2020, 29 (4): 044501;  doi: 10.1088/1674-1056/ab6d51
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In order to further study the dynamical behavior of nonconservative systems, we study the conserved quantities and the adiabatic invariants of fractional Brikhoffian systems with four kinds of different fractional derivatives based on Herglotz differential variational principle. Firstly, the conserved quantities of Herglotz type for the fractional Brikhoffian systems based on Riemann-Liouville derivatives and their existence conditions are established by using the fractional Pfaff-Birkhoff-d'Alembert principle of Herglotz type. Secondly, the effects of small perturbations on fractional Birkhoffian systems are studied, the conditions for the existence of adiabatic invariants for the Birkhoffian systems of Herglotz type based on Riemann-Liouville derivatives are established, and the adiabatic invariants of Herglotz type are obtained. Thirdly, the conserved quantities and adiabatic invariants for the fractional Birkhoffian systems of Herglotz type under other three kinds of fractional derivatives are established, namely Caputo derivative, Riesz-Riemann-Liouville derivative and Riesz-Caputo derivative. Finally, an example is given to illustrate the application of the results.

Discharge flow of granular particles through an orifice on a horizontal hopper: Effect of the hopper angle

Xin Wang, Hong-Wei Zhu, Qing-Fan Shi, Ning Zheng
Chin. Phys. B 2020, 29 (4): 044502;  doi: 10.1088/1674-1056/ab7b51
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We experimentally investigate the effect of the hopper angle on the flow rate of grains discharged from a two-dimensional horizontal hopper on a conveyor belt. The flow rate grows with the hopper angle, and finally reaches a plateau. The curve feature appears to be similar for different orifice widths and conveyor belt-driven velocities. On the basis of an empirical law of flow rate for a flat-bottom hopper, we propose a modified equation to describe the relation between the flow rate and hopper angle, which is in a good agreement with the experimental results.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

Hybrid-PIC/PIC simulations on ion extraction by electric field in laser-induced plasma

Xiao-Yong Lu, Cheng Yuan, Xiao-Zhang Zhang, Zhi-Zhong Zhang
Chin. Phys. B 2020, 29 (4): 045201;  doi: 10.1088/1674-1056/ab75d8
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The processes of electric ion extraction from plasma induced by pulse lasers are simulated by particle-in-cell (PIC) method and hybrid-PIC method. A new calculation scheme named preprocessing hybrid-PIC is presented because neither of the two methods above is omnipotent, especially under the circumstance of high initial plasma density. The new scheme provides credible results with less computational consumption than PIC method in both one- and two-dimensional simulations, except for Π-type electrode configuration. The simulation results show that the M-type performs best in all electrode configurations in both high-density and low-density plasma conditions.

Tunability of Fano resonance in cylindrical core-shell nanorods

Ben-Li Wang
Chin. Phys. B 2020, 29 (4): 045202;  doi: 10.1088/1674-1056/ab75d1
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The optical properties of cylindrical core-shell nanorods (CCSNs) are theoretically investigated in this paper. The results show that Fano resonance can be generated in CCSNs, and the wavelength and the intensity at Fano dip can be tuned respectively by adjusting the field coupling of cavity mode inside and near field on gold surface. The high tuning sensitivity which is about 400 nm per refractive-index unit can be obtained, and an easy-to-realize tunable parameter is also proposed. A two-oscillator model is also introduced to describe the generation of Fano resonance in CCSNs, and the results from this model are in good agreement with theoretical results. The CCSNs investigated in this work may have promising applications in optical devices.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

Fundamental band gap and alignment of two-dimensional semiconductors explored by machine learning Hot!

Zhen Zhu, Baojuan Dong, Huaihong Guo, Teng Yang, Zhidong Zhang
Chin. Phys. B 2020, 29 (4): 046101;  doi: 10.1088/1674-1056/ab75d5
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Two-dimensional (2D) semiconductors isoelectronic to phosphorene have been drawing much attention recently due to their promising applications for next-generation (opt)electronics. This family of 2D materials contains more than 400 members, including (a) elemental group-V materials, (b) binary III-VII and IV-VI compounds, (c) ternary III-VI-VII and IV-V-VII compounds, making materials design with targeted functionality unprecedentedly rich and extremely challenging. To shed light on rational functionality design with this family of materials, we systemically explore their fundamental band gaps and alignments using hybrid density functional theory (DFT) in combination with machine learning. First, calculations are performed using both the Perdew-Burke-Ernzerhof exchange-correlation functional within the general-gradient-density approximation (GGA-PBE) and Heyd-Scuseria-Ernzerhof hybrid functional (HSE) as a reference. We find this family of materials share similar crystalline structures, but possess largely distributed band-gap values ranging approximately from 0 eV to 8 eV. Then, we apply machine learning methods, including linear regression (LR), random forest regression (RFR), and support vector machine regression (SVR), to build models for the prediction of electronic properties. Among these models, SVR is found to have the best performance, yielding the root mean square error (RMSE) less than 0.15 eV for the predicted band gaps, valence-band maximums (VBMs), and conduction-band minimums (CBMs) when both PBE results and elemental information are used as features. Thus, we demonstrate that the machine learning models are universally suitable for screening 2D isoelectronic systems with targeted functionality, and especially valuable for the design of alloys and heterogeneous systems.

Irradiation hardening behaviors of tungsten-potassium alloy studied by accelerated 3-MeV W2+ ions

Xiao-Liang Yang, Long-Qing Chen, Wen-Bin Qiu, Yang-Yi-Peng Song, Yi Tang, Xu-Dong Cui, Chang-Song Liu, Yan Jiang, Tao Zhang, Jun Tang
Chin. Phys. B 2020, 29 (4): 046102;  doi: 10.1088/1674-1056/ab75cf
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Tungsten-potassium (WK) alloy with ultrafine/fine grains and nano-K bubbles is fabricated through spark plasma sintering (SPS) and rolling process. In this study, 3-MeV W2+ ion irradiation with a tandem accelerator is adopted to simulate the displacement damage caused by neutrons. As the depth of irradiation damage layer is limited to only 500 nm, the hardening behaviors of WK alloy and ITER (International Thermonuclear Experimental Reactor)-W under several damage levels are investigated through Bercovich tip nanoindentation test and other morphological characterizations. The indenter size effect (ISE), soft substrate effect (SSE), and damage gradient effect (DGE) are found to influence the measurement of nano-hardness. Few or no pop-ins in irradiated samples are observed while visible pop-in events take place in unirradiated metals. Extensive pile-up with different morphology features around the indentation exists in both WK and ITER-W. The WK shows a smaller hardness increment than ITER-W under the same condition of displacement damage. This study provides beneficial information for WK alloy serving as a promising plasma facing materials (PFMs) candidate.

Electrical properties of Ca3-xSmxCo4O9+δ ceramics preparedunder magnetic field

Xiu-Rong Qu, Yan-Yan Xu, Shu-Chen Lü, Jian-Min Hu
Chin. Phys. B 2020, 29 (4): 046103;  doi: 10.1088/1674-1056/ab75cb
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We fabricate Sm-doped Ca3Co4O9+δ (CCO) bulk materials in magnetic field during both processes of chemical synthesis and cold pressing. The structure and electrical performance of the samples are investigated. With the increasing Sm concentration, the electrical conductivity 1/ρ decreases and the Seebeck coefficient α increases. As a result, the power factor (PF=α2/ρ) is raised slightly. After applying magnetic field, the extent of texture, grain size and density of all the bulk materials are improved obviously, thereby an enhanced electrical conductivity can be gained. Additionally, the degeneracy of Co4+ state in the CoO2 layer of CCO is also increased as the magnetic field is used in the preparing process, which results in an enhanced α. The Ca2.85Sm0.15Co4O9+δ prepared in magnetic field shows the largest power factor (0.20 mW·m-1·K-2 at 1073 K).

Improved carrier transport in Mn:ZnSe quantum dots sensitized La-doped nano-TiO2 thin film

Shao Li, Gang Li, Li-Shuang Yang, Kui-Ying Li
Chin. Phys. B 2020, 29 (4): 046104;  doi: 10.1088/1674-1056/ab7742
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Mn:ZnSe/ZnS/L-Cys core-shell quantum dots (QDs) sensitized La-doped nano-TiO2 thin film (QDSTF) was prepared. X-ray photoelectron spectroscopy (XPS), nanosecond transient photovoltaic (TPV), and steady state surface photovoltaic (SPV) technologies were used for probing the photoelectron behaviors in the Mn-doped QDSTF. The results revealed that the Mn-doped QDSTF had a p-type TPV characteristic. The bottom of the conduction band of the QDs as a sensitizer was just 0.86 eV above that of the La-doped nano-TiO2 thin film, while the acceptor level of the doped Mn2+ ions was located at about 0.39 eV below and near the bottom of the conduction band of the QDs. The intensity of the SPV response of the Mn-doped QDSTF at a specific wavelength was ~2.1 times higher than that of the undoped QDSTF. The region of the SPV response of the Mn-doped QDSTF was extended by 191 nm to almost the whole visible region as compared with the undoped QDSTF one. And the region of the TPV response of the Mn-doped QDSTF was also obviously wider than that of the undoped QDSTF. These PV characteristics of the Mn-doped QDSTF may be due to the prolonged lifetime and extended diffusion length of photogenerated free charge carriers injected into the sensitized La-doped nano-TiO2 thin film.

Nearly golden-ratio order in Ta metallic glass

Yuan-Qi Jiang, Ping Peng
Chin. Phys. B 2020, 29 (4): 046105;  doi: 10.1088/1674-1056/ab773f
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The formation of mono-atomic tantalum (Ta) metallic glass (MG) through ultrafast liquid cooling is investigated by ab-initio molecular dynamics (MD) simulations. It is found that there exists nearly golden ratio order (NGRO) between the nearest and second nearest atoms in Ta MG, which has been indirectly confirmed by Khmich et al. and Liang et al.. The NGRO is another universal structural feature in metallic glass besides the local five-fold symmetry (LFFS). Further analyzing of electronic structure shows that the obvious orientation of covalent bond could be attributed to the NGRO in amorphous Ta at 300 K.

Anisotropic plasticity of nanocrystalline Ti: A molecular dynamics simulation

Minrong An, Mengjia Su, Qiong Deng, Haiyang Song, Chen Wang, Yu Shang
Chin. Phys. B 2020, 29 (4): 046201;  doi: 10.1088/1674-1056/ab7188
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Using molecular dynamics simulations, the plastic deformation behavior of nanocrytalline Ti has been investigated under tension and compression normal to the {0001}, {1010}, and {1210} planes. The results indicate that the plastic deformation strongly depends on crystal orientation and loading directions. Under tension normal to basal plane, the deformation mechanism is mainly the grain reorientation and the subsequent deformation twinning. Under compression, the transformation of hexagonal-close packed (HCP)-Ti to face-centered cubic (FCC)-Ti dominates the deformation. When loading is normal to the prismatic planes (both {1010} and {1210}), the deformation mechanism is primarily the phase transformation among HCP, body-centered cubic (BCC), and FCC structures, regardless of loading mode. The orientation relations (OR) of {0001}HCP||{111}FCC and <1210>HCP||<110>FCC, and {1010}HCP||{110}FCC and <0001>HCP||<010>FCC between the HCP and FCC phases have been observed in the present work. For the transformation of HCP→BCC→HCP, the OR is {0001}α1||{110}β||{1010}α2 (HCP phase before the critical strain is defined as α1-Ti, BCC phase is defined as β-Ti, and the HCP phase after the critical strain is defined as α2-Ti). Energy evolution during the various loading processes further shows the plastic anisotropy of nanocrystalline Ti is determined by the stacking order of the atoms. The results in the present work will promote the in-depth study of the plastic deformation mechanism of HCP materials.

Improvement of high-frequency properties of Co2FeSi Heusler films by ultrathin Ru underlayer

Cuiling Wang, Shouheng Zhang, Shandong Li, Honglei Du, Guoxia Zhao, Derang Cao
Chin. Phys. B 2020, 29 (4): 046202;  doi: 10.1088/1674-1056/ab7802
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Heusler Co2FeSi films with a uniaxial magnetic anisotropy and high ferromagnetic resonance frequency fr were deposited by an oblique sputtering technique on Ru underlayers with various thicknesses tRu from 0 nm to 5 nm. It is revealed that the Ru underlayers reduce the grain size of Co2FeSi, dramatically enhance the magnetic anisotropy field HK induced by the internal stress from 242 Oe (1 Oe=79.5775 A·m-1) to 582 Oe with an increment ratio of 2.4, while a low damping coefficient remains. The result of damping implies that the continuous interface between Ru and Co2FeSi induces a large in-plane anisotropic field without introducing additional external damping. As a result, excellent high-frequency soft magnetic properties with fr up to 6.69 GHz are achieved.

Molecular dynamics simulation of thermal conductivity of silicone rubber

Wenxue Xu, Yanyan Wu, Yuan Zhu, Xin-Gang Liang
Chin. Phys. B 2020, 29 (4): 046601;  doi: 10.1088/1674-1056/ab7743
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Silicone rubber is widely used as a kind of thermal interface material (TIM) in electronic devices. However few studies have been carried out on the thermal conductivity mechanism of silicone rubber. This paper investigates the thermal conductivity mechanism by non-equilibrium molecular dynamics (NEMD) in three aspects: chain length, morphology, and temperature. It is found that the effect of chain length on thermal conductivity varies with morphologies. In crystalline state where the chains are aligned, the thermal conductivity increases apparently with the length of the silicone-oxygen chain, the thermal conductivity of 79 nm-long crystalline silicone rubber could reach 1.49 W/(m·K). The thermal conductivity of amorphous silicone rubber is less affected by the chain length. The temperature dependence of thermal conductivity of silicone rubbers with different morphologies is trivial. The phonon density of states (DOS) is calculated and analyzed. The results indicate that crystalline silicone rubber with aligned orientation has more low frequency phonons, longer phonon MFP, and shorter conducting path, which contribute to a larger thermal conductivity.

Effect of initial crystallization temperature and surface diffusion on formation of GaAs multiple concentric nanoring structures by droplet epitaxy

Yi Wang, Xiang Guo, Jiemin Wei, Chen Yang, Zijiang Luo, Jihong Wang, Zhao Ding
Chin. Phys. B 2020, 29 (4): 046801;  doi: 10.1088/1674-1056/ab790b
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GaAs multiple concentric nano-ring structures (CNRs) are prepared with multistep crystallization procedures by droplets epitaxy on GaAs (001) to explore the influence of different initial crystallization temperatures on CNRs morphology. Atomic force microscope (AFM) images show that GaAs nanostructures are more likely to form elliptical rings due to diffusion anisotropy. Meanwhile, with the increase of initial crystallization temperature, the inner ring height and density of CNRs are increased, and outer rings are harder to form. In addition, the mechanism of formation of CNRs is discussed by classical nucleation theory and diffusion theory. The method can be used to calculate the diffusion activation energy of gallium atoms (0.7±0.1 eV) on the GaAs (001) surface conveniently.

Influence of external load on friction coefficient of Fe-polytetrafluoroethylene

Xiu-Hong Hao, Deng Pan, Ze-Yang Zhang, Shu-Qiang Wang, Yu-Jin Gao, Da-Peng Gu
Chin. Phys. B 2020, 29 (4): 046802;  doi: 10.1088/1674-1056/ab7da0
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A coarse-grained molecular dynamics simulation model was developed in this study to investigate the friction process occurring between Fe and polytetrafluoroethylene (PTFE). We investigated the effect of an external load on the friction coefficient of Fe-PTFE using the molecular dynamics simulations and experimental methods. The simulation results show that the friction coefficient decreases with the external load increasing, which is in a good agreement with the experimental results. The high external load could result in a larger contact area between the Fe and PTFE layers, severer springback as a consequence of the deformed PTFE molecules, and faster motion of the PTFE molecules, thereby affecting the friction force and normal force during friction and consequently varying the friction coefficient.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

Ab initio study of structural, electronic, thermo-elastic and optical properties of Pt3Zr intermetallic compound

Wahiba Metiri, Khaled Cheikh
Chin. Phys. B 2020, 29 (4): 047101;  doi: 10.1088/1674-1056/ab7223
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Structural, elastic, electronic and optical properties of the Pt3Zr intermetallic compound are investigated using first principles calculations based on the density functional theory (DFT) within the generalized gradient approximation (GGA) and the local density approximation (LDA). The Pt3Zr compound is predicted to be of cubic L12 and hexagonal D024 structures. The calculated equilibrium ground-state properties (lattice parameters a and c, bulk modulus B and its pressure derivative B', formation enthalpy ΔH) of the Pt3Zr compound, for both cubic and hexagonal phases, show good agreement with the experimental results and other theoretical data. Elastic constants (C11, C12, C13, C33, C44, and C55) are calculated. The predicted elastic properties such as Young's modulus E and shear modulus GH, Poisson ratio ν, anisotropic ratio A, Kleinman parameter ξ, Cauchy pressure (C12-C44), ratios B/C44 and B/G, and Vickers hardness Hv indicate the stiffness, hardness and ductility of the compound. Thermal characteristic parameters such as Debye temperature θD and melting temperature Tm are computed. Electronic properties such as density of states (DOS) and electronic specific heat γ are also reported. The calculated results reveal that the Fermi level is on the psedogap for the D024 structure and on the antibonding side for the L12 structure. The optical property functions (real part ε1(ω) and imaginary part ε2(ω) of dielectric function), optical conductivity σ (ω), refraction index n(ω), reflectivity R(ω), absorption α (ω) and extinction coefficients k(ω) and loss function L(ω)) are also investigated for the first time for Pt3Zr in a large gamme of energy from 0 to 70 eV.

Surface potential-based analytical model for InGaZnO thin-film transistors with independent dual-gates

Yi-Ni He, Lian-Wen Deng, Ting Qin, Cong-Wei Liao, Heng Luo, Sheng-Xiang Huang
Chin. Phys. B 2020, 29 (4): 047102;  doi: 10.1088/1674-1056/ab75d2
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An analytical drain current model on the basis of the surface potential is proposed for indium-gallium zinc oxide (InGaZnO) thin-film transistors (TFTs) with an independent dual-gate (IDG) structure. For a unified expression of carriers' distribution for the sub-threshold region and the conduction region, the concept of equivalent flat-band voltage and the Lambert W function are introduced to solve the Poisson equation, and to derive the potential distribution of the active layer. In addition, the regional integration approach is used to develop a compact analytical current-voltage model. Although only two fitting parameters are required, a good agreement is obtained between the calculated results by the proposed model and the simulation results by TCAD. The proposed current-voltage model is then implemented by using Verilog-A for SPICE simulations of a dual-gate InGaZnO TFT integrated inverter circuit.

Ab initio calculations on oxygen vacancy defects in strained amorphous silica

Bao-Hua Zhou, Fu-Jie Zhang, Xiao Liu, Yu Song, Xu Zuo
Chin. Phys. B 2020, 29 (4): 047103;  doi: 10.1088/1674-1056/ab75db
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The effects of uniaxial tensile strain on the structural and electronic properties of positively charged oxygen vacancy defects in amorphous silica (a-SiO2) are systematically investigated using ab-initio calculation based on density functional theory. Four types of positively charged oxygen vacancy defects, namely the dimer, unpuckered, and puckered four-fold (4×), and puckered five-fold (5×) configurations have been investigated. It is shown by the calculations that applying uniaxial tensile strain can lead to irreversible transitions of defect structures, which can be identified from the fluctuations of the curves of relative total energy versus strain. Driven by strain, a positively charged dimer configuration may relax into a puckered 5×configuration, and an unpuckered configuration may relax into either a puckered 4×configuration or a forward-oriented configuration. Accordingly, the Fermi contacts of the defects remarkably increase and the defect levels shift under strain. The Fermi contacts of the puckered configurations also increase under strain to the values close to that of Eα' center in a-SiO2. In addition, it is shown by the calculations that the relaxation channels of the puckered configurations after electron recombination are sensitive to strain, that is, those configurations are more likely to relax into a two-fold coordinated Si structure or to hold a puckered structure under strain, both of which may raise up the thermodynamic charge-state transition levels of the defects into Si band gap. As strain induces more puckered configurations with the transition levels in Si band gap, it may facilitate directly the development of oxide charge accumulation and indirectly that of interface charge accumulation by promoting proton generation under ionization radiation. This work sheds a light on understanding the strain effect on ionization damage at an atomic scale.

In-situ SiN combined with etch-stop barrier structure for high-frequency AlGaN/GaN HEMT

Min-Han Mi, Sheng Wu, Ling Yang, Yun-Long He, Bin Hou, Meng Zhang, Li-Xin Guo, Xiao-Hua Ma, Yue Hao
Chin. Phys. B 2020, 29 (4): 047104;  doi: 10.1088/1674-1056/ab7746
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The etch-stop structure including the in-situ SiN and AlGaN/GaN barrier is proposed for high frequency applications. The etch-stop process is realized by placing an in-situ SiN layer on the top of the thin AlGaN barrier. F-based etching can be self-terminated after removing SiN, leaving the AlGaN barrier in the gate region. With this in-situ SiN and thin barrier etch-stop structure, the short channel effect can be suppressed, meanwhile achieving highly precisely controlled and low damage etching process. The device shows a maximum drain current of 1022 mA/mm, a peak transconductance of 459 mS/mm, and a maximum oscillation frequency (fmax) of 248 GHz.

Identifying anomalous Floquet edge modes via bulk-edge correspondence

Huanyu Wang, Wuming Liu
Chin. Phys. B 2020, 29 (4): 047301;  doi: 10.1088/1674-1056/ab6dcb
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Floquet engineering appears as a new protocol for designing topological states of matter, and features anomalous edge modes pinned at quasi-energy π/T with vanished topological index. We propose how to predict the anomalous edge modes via the bulk Hamiltonian in frequency space, and use Zak phase to quantitatively index the topological properties. The above methods are clarified by the example of time periodic Kitaev chain with chemical potential of harmonic driving and pulse driving, and topological phase transitions are manifested at different driving frequencies.

Negative bias-induced threshold voltage instability and zener/interface trapping mechanism in GaN-based MIS-HEMTs

Qing Zhu, Xiao-Hua Ma, Yi-Lin Chen, Bin Hou, Jie-Jie Zhu, Meng Zhang, Mei Wu, Ling Yang, Yue Hao
Chin. Phys. B 2020, 29 (4): 047304;  doi: 10.1088/1674-1056/ab7809
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We investigate the instability of threshold voltage in D-mode MIS-HEMT with in-situ SiN as gate dielectric under different negative gate stresses. The complex non-monotonic evolution of threshold voltage under the negative stress and during the recovery process is induced by the combination effect of two mechanisms. The effect of trapping behavior of interface state at SiN/AlGaN interface and the effect of zener traps in AlGaN barrier layer on the threshold voltage instability are opposite to each other. The threshold voltage shifts negatively under the negative stress due to the detrapping of the electrons at SiN/AlGaN interface, and shifts positively due to zener trapping in AlGaN barrier layer. As the stress is removed, the threshold voltage shifts positively for the retrapping of interface states and negatively for the thermal detrapping in AlGaN. However, it is the trapping behavior in the AlGaN rather than the interface state that results in the change of transconductance in the D-mode MIS-HEMT.

Fabrication and characterization of vertical GaN Schottky barrier diodes with boron-implanted termination

Wei-Fan Wang, Jian-Feng Wang, Yu-Min Zhang, Teng-Kun Li, Rui Xiong, Ke Xu
Chin. Phys. B 2020, 29 (4): 047305;  doi: 10.1088/1674-1056/ab7909
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The vertical GaN-on-GaN Schottky barrier diode with boron-implanted termination was fabricated and characterized. Compared with the Schottky barrier diode (SBD) without boron-implanted termination, this SBD effectively improved the breakdown voltage from 189 V to 585 V and significantly reduced the reverse leakage current by 105 times. In addition, a high Ion/Ioff ratio of ~108 was achieved by the boron-implanted technology. We used Technology Computer Aided Design (TCAD) to analyze reasons for the improved performance of the SBD with boron-implanted termination. The improved performance of diodes may be attributed to that B+ could confine free carriers to suppress electron field crowding at the edge of the diode, which could improve the breakdown voltage and suppress the reverse leakage current.

Nanofabrication of 50 nm zone plates through e-beam lithography with local proximity effect correction for x-ray imaging Hot!

Jingyuan Zhu, Sichao Zhang, Shanshan Xie, Chen Xu, Lijuan Zhang, Xulei Tao, Yuqi Ren, Yudan Wang, Biao Deng, Renzhong Tai, Yifang Chen
Chin. Phys. B 2020, 29 (4): 047501;  doi: 10.1088/1674-1056/ab7800
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High resolution Fresnel zone plates for nanoscale three-dimensional imaging of materials by both soft and hard x-rays are increasingly needed by the broad applications in nanoscience and nanotechnology. When the outmost zone-width is shrinking down to 50 nm or even below, patterning the zone plates with high aspect ratio by electron beam lithography still remains a challenge because of the proximity effect. The uneven charge distribution in the exposed resist is still frequently observed even after standard proximity effect correction (PEC), because of the large variety in the line width. This work develops a new strategy, nicknamed as local proximity effect correction (LPEC), efficiently modifying the deposited energy over the whole zone plate on the top of proximity effect correction. By this way, 50 nm zone plates with the aspect ratio from 4:1 up to 15:1 and the duty cycle close to 0.5 have been fabricated. Their imaging capability in soft (1.3 keV) and hard (9 keV) x-ray, respectively, has been demonstrated in Shanghai Synchrotron Radiation Facility (SSRF) with the resolution of 50 nm. The local proximity effect correction developed in this work should also be generally significant for the generation of zone plates with high resolutions beyond 50 nm.

Magnetocaloric effect and critical behavior of the Mn-rich itinerant material Mn3GaC with enhanced ferromagnetic interaction

Pengfei Liu, Jie Peng, Mianqi Xue, Bosen Wang
Chin. Phys. B 2020, 29 (4): 047503;  doi: 10.1088/1674-1056/ab7da1
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We revisit the reversible magnetocaloric effect of itinerant ferromagnet Mn3GaC near the ferromagnetic to paramagnetic phase transition by adopting the experimental and theoretical methods and critical behavior of Mn-rich Mn3GaC with an enhanced FM interaction. Landau theory model cannot account for temperature dependent magnetic entropy change which is estimated from thermal magnetic methods only considering magnetoelastic coupling and the electron-electron interaction, apart from molecular mean-field model. Critical behavior is studied by adopting the modified Arrott plot, Kouvel-Fisher plot, and critical isotherm analysis. With these critical exponents, experimental data below and above Tc collapse into two universal branches, fulfilling the single scaling equation m=f±(h), where m and h are renormalized magnetization and field. Critical exponents are confirmed by Widom scaling law and just between mean-field model and three-dimensional Heisenberg model, as the evidence for the existence of long-range ferromagnetic interaction. With increasing the Mn content, Tc increases monotonously and critical exponents increases accordingly. The exchange distance changes from J(r) ~ r-4.68 for x = 0 to J(r) ~ r-4.71 for x = 0.08, respectively, which suggests the competition of the Mn-Mn direct interaction and the itinerant Mn-C-Mn hybridization. The possible mechanism is proposed.

Improvement of memory characteristics by employing a charge trapping layer with combining bent and flat energy bands

Zhen-Jie Tang, Rong Li, Xi-Wei Zhang
Chin. Phys. B 2020, 29 (4): 047701;  doi: 10.1088/1674-1056/ab7224
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Designed ZrxSi1-xO2 films with combining bent and flat energy bands are employed as a charge trapping layer for memory capacitors. Compared to a single bent energy band, the bandgap structure with combining bent and flat energy bands exhibits larger memory window, faster program/erase speed, lower charge loss even at 200℃ for 104 s, and wider temperature insensitive regions. The tunneling thickness together with electron recaptured efficiency in the trapping layer, and the balance of two competing electron loss mechanisms in the bent and flat energy band regions collectively contribute to the improved memory characteristics. Therefore, the proposed ZrxSi1-xO2 with combining bent and flat energy bands should be a promising candidate for future nonvolatile memory applications, taking into consideration of the trade-off between the operation speed and retention characteristics.

Refractive index of ionic liquids under electric field: Methyl propyl imidazole iodide and several derivatives

Ji Zhou, Shi-Kui Dong, Zhi-Hong He, Yan-Hu Zhang
Chin. Phys. B 2020, 29 (4): 047801;  doi: 10.1088/1674-1056/ab75cd
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Ionic liquids have received wide attention due to their novel optoelectronic structures and devices as an optical means of regulating electricity. However, the quantitative testing and analysis of refractive index of ionic liquids under electric field are rarely carried out. In the present study, an experimental apparatus including a hollow prism is designed to measure the refractive indices of ionic liquids under different electric fields. Five groups of imidazole ionic liquids are experimentally investigated and an inversion is performed to determine the refractive indices under electric fields. The error propagation analysis of the apex angle and the minimum deflection angle are conducted, and the machining accuracy requirements of the hollow prism are determined. The results show that the refractive indices of imidazole ionic liquids change with the light wavelength, following a downward convex parabola. Furthermore, the refractive index decreases with the carbon chain length of ionic liquid at a given wavelength, presenting an order of C3MImI > C4MImI > C5MImI > C3MImBr > C3MImBF4. Notably, the refractive index of imidazole ionic liquid exhibits a nonlinear change with the applied voltage at 546 nm and a monotonical decrease at 1529 nm. Besides, the variation of refractive index at 1529 nm with the applied voltage is larger than that at 546 nm and 1013 nm. Importantly, the variation of refractive index is contrary to that of absorption coefficient under electric field. This study illustrates that the theory of electrode and carrier transport can be used to explain the law of variation of n-k value of ionic liquid under the electric field, and provides the support for the evaluation of physical properties of ionic liquids, the measurement of optical functional parameters and the regulation of electric-optic performances of optical devices.

Effect of AlGaN interlayer on luminous efficiency and reliability of GaN-based green LEDs on silicon substrate

Jiao-Xin Guo, Jie Ding, Chun-Lan Mo, Chang-Da Zheng, Shuan Pan, Feng-Yi Jiang
Chin. Phys. B 2020, 29 (4): 047303;  doi: 10.1088/1674-1056/ab7903
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The effect of AlGaN interlayer in quantum barrier on the electroluminescence characteristics of GaN-based green light emitting diodes (LEDs) grown on silicon substrate was investigated. The results show that AlGaN interlayer is beneficial to improve the luminous efficiency of LED devices and restrain the phase separation of InGaN. The former is ascribed to the inserted AlGaN layers can play a key role in determining the carrier distribution and screening dislocations in the active region, and the latter is attributed to the increased compressive stress in the quantum well. However, when the electrical stress aging tests were performed at a current density of 100 A/cm2, LED devices with AlGaN interlayers are more likely to induce the generation/proliferation of defects in the active region under the effect of electrical stress, resulting in the reduced light output power at low current density.

Dependence of limited radiative recombination rate of InGaN-based light-emitting diode on lattice temperature with high injection

Jiang-Dong Gao, Jian-Li Zhang, Zhi-Jue Quan, Jun-Lin Liu, Feng-Yi Jiang
Chin. Phys. B 2020, 29 (4): 047802;  doi: 10.1088/1674-1056/ab790a
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It is observed that the radiative recombination rate in InGaN-based light-emitting diode decreases with lattice temperature increasing. The effect of lattice temperature on the radiative recombination rate tends to be stable at high injection. Thus, there should be an upper limit for the radiative recombination rate in the quantum well with the carrier concentration increasing, even under the same lattice temperature. A modified and easily used ABC-model is proposed. It describes that the slope of the radiative recombination rate gradually decreases to zero, and further reaches a negative value in a small range of lattice temperature increasing. These provide a new insight into understanding the dependence of the radiative recombination rate on lattice temperature and carrier concentration in InGaN-based light-emitting diode.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

A numerical study of dynamics in thin hopper flow and granular jet

Meng-Ke Wang, Guang-Hui Yang, Sheng Zhang, Han-Jie Cai, Ping Lin, Liang-Wen Chen, Lei Yang
Chin. Phys. B 2020, 29 (4): 048102;  doi: 10.1088/1674-1056/ab7807
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The dynamics of granular material discharging from a cuboid but thin hopper, including the hopper flow and granular jet, are investigated via discrete element method (DEM) simulations. The slot width is varied to study its influence on the flow. It is found the flow in the cuboid hopper has similarity with the flow in two-dimensional (2D) hopper. When the slot width is large, the flowrate is higher than the predicted value from Beverloo's law and the velocity distribution is not Gaussian-like. For granular jet, there is a transition with varying slot width. For large slot width, there is a dense core in the jet and the variations of velocities and density are relatively small. Finally, the availability of continuum model is assessed and the results show that the performance with large slot width is better than that with small slot width.

Moisture-sensitive torsional cotton artificial muscle and textile Hot!

Yuanyuan Li, Xueqi Leng, Jinkun Sun, Xiang Zhou, Wei Wu, Hong Chen, Zunfeng Liu
Chin. Phys. B 2020, 29 (4): 048103;  doi: 10.1088/1674-1056/ab7745
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Developing moisture-sensitive artificial muscles from industrialized natural fibers with large abundance is highly desired for smart textiles that can respond to humidity or temperature change. However, currently most of fiber artificial muscles are based on non-common industrial textile materials or of a small portion of global textile fiber market. In this paper, we developed moisture-sensitive torsional artificial muscles and textiles based on cotton yarns. It was prepared by twisting the cotton yarn followed by folding in the middle point to form a self-balanced structure. The cotton yarn muscle showed a torsional stroke of 42.55°/mm and a rotational speed of 720 rpm upon exposure to water moisture. Good reversibility and retention of stroke during cyclic exposure and removal of water moisture were obtained. A moisture-sensitive smart window that can close when it rains was demonstrated based on the torsional cotton yarn muscles. This twist-based technique combining natural textile fibers provides a new insight for construction of smart textile materials.

Characteristics of AlGaN/GaN high electron mobility transistors on metallic substrate

Minglong Zhao, Xiansheng Tang, Wenxue Huo, Lili Han, Zhen Deng, Yang Jiang, Wenxin Wang, Hong Chen, Chunhua Du, Haiqiang Jia
Chin. Phys. B 2020, 29 (4): 048104;  doi: 10.1088/1674-1056/ab7d9c
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We have successfully prepared GaN based high electron mobility transistors (HEMTs) on metallic substrates transferred from silicon substrates by electroplating technique. GaN HEMTs on Cu substrates are demonstrated to basically have the same good electric characteristics as the chips on Si substrates. Furthermore, the better heat dissipation of HEMTs on Cu substrates compared to HEMTs on Si substrates is clearly observed by thermoreflectance imaging, showing the promising potential for very high-power and high-temperature operation. This work shows the outstanding ability of HEMT chips on Cu substrates for solving the self-heating effect with the advantages of process simplicity, high yield, and low production requirement.

Investigation of active-region doping on InAs/GaSb long wave infrared detectors

Su-Ning Cui, Dong-Wei Jiang, Ju Sun, Qing-Xuan Jia, Nong Li, Xuan Zhang, Yong Li, Fa-Ran Chang, Guo-Wei Wang, Ying-Qiang Xu, Zhi-Chuan Niu
Chin. Phys. B 2020, 29 (4): 048502;  doi: 10.1088/1674-1056/ab773c
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The eight-band k·p model is used to establish the energy band structure model of the type-II InAs/GaSb superlattice detectors with a cut-off wavelength of 10.5 μm, and the best composition of M-structure in this type of device is calculated theoretically. In addition, we have also experimented on the devices designed with the best performance to investigate the effect of the active region p-type doping temperature on the quantum efficiency of the device. The results show that the modest active region doping temperature (Be: 760℃) can improve the quantum efficiency of the device with the best performance, while excessive doping (Be: >760℃) is not conducive to improving the photo response. With the best designed structure and an appropriate doping concentration, a maximum quantum efficiency of 45% is achieved with a resistance-area product of 688 Ω·cm2, corresponding to a maximum detectivity of 7.35×1011 cm·Hz3/W.

Dark count in single-photon avalanche diodes: A novel statistical behavioral model

Wen-Juan Yu, Yu Zhang, Ming-Zhu Xu, Xin-Miao Lu
Chin. Phys. B 2020, 29 (4): 048503;  doi: 10.1088/1674-1056/ab7808
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Dark count is one of the inherent noise types in single-photon diodes, which may restrict the performances of detectors based on these diodes. To formulate better designs for peripheral circuits of such diodes, an accurate statistical behavioral model of dark current must be established. Research has shown that there are four main mechanisms that contribute to the dark count in single-photon avalanche diodes. However, in the existing dark count models only three models have been considered, thus leading to inaccuracies in these models. To resolve these shortcomings, the dark current caused by carrier diffusion in the neutral region is deduced by multiplying the carrier detection probability with the carrier particle current at the boundary of the depletion layer. Thus, a comprehensive dark current model is constructed by adding the dark current caused by carrier diffusion to the dark current caused by the other three mechanisms. To the best of our knowledge, this is the first dark count simulation model into which incorporated simultaneously are the thermal generation, trap-assisted tunneling, band-to-band tunneling mechanisms, and carrier diffusion in neutral regions to evaluate dark count behavior. The comparison between the measured data and the simulation results from the models shows that the proposed model is more accurate than other existing models, and the maximum of accuracy increases up to 31.48% when excess bias voltage equals 3.5 V and temperature is 50℃.

Stackable luminescent device integrating blue light emitting diode with red organic light emitting diode

Kang Su, Jing Li, Chang Ge, Xing-Dong Lu, Zhi-Cong Li, Guo-Hong Wang, Jin-Min Li
Chin. Phys. B 2020, 29 (4): 048504;  doi: 10.1088/1674-1056/ab77ff
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We present a novel stackable luminescent device integrating a blue light emitting diode (LED) with a red organic LED (OLED) in series. The anode of the OLED is connected with the cathode of the LED through a via in the insulation layer on the LED. The LED-OLED hybrid device is electroluminescent and two electroluminescence (EL) peaks (the blue peak around 454 nm and the red peak around 610 nm) are observed clearly. The effect of the indium tin oxide (ITO) layer on the device performance is analyzed. Compared with the individual LED and OLED, their combination shows great potential applications in the field of white lighting, plant lighting, and display.

A method of generating random bits by using electronic bipolar memristor

Bin-Bin Yang, Nuo Xu, Er-Rui Zhou, Zhi-Wei Li, Cheng Li, Pin-Yun Yi, Liang Fang
Chin. Phys. B 2020, 29 (4): 048505;  doi: 10.1088/1674-1056/ab77fd
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The intrinsic stochasticity of resistance switching process is one of the holdblocks for using memristor as a fundamental element in the next-generation nonvolatile memory. However, such a weakness can be used as an asset for generating the random bits, which is valuable in a hardware security system. In this work, a forming-free electronic bipolar Pt/Ti/Ta2O5/Pt memristor is successfully fabricated to investigate the merits of generating random bits in such a device. The resistance switching mechanism of the fabricated device is ascribed to the electric field conducted electrons trapping/de-trapping in the deep-energy-level traps produced by the “oxygen grabbing” process. The stochasticity of the electrons trapping/de-trapping governs the random distribution of the set/reset switching voltages of the device, making a single memristor act as a random bit in which the resistance of the device represents information and the applied voltage pulse serves as the triggering signal. The physical implementation of such a random process provides a method of generating the random bits based on memristors in hardware security applications.

Structural and thermal stabilities of Au@Ag core-shell nanoparticles and their arrays: A molecular dynamics simulation

Hai-Hong Jia, De-Liang Bao, Yu-Yang Zhang, Shi-Xuan Du
Chin. Phys. B 2020, 29 (4): 048701;  doi: 10.1088/1674-1056/ab7da9
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Thermal stability of core-shell nanoparticles (CSNPs) is crucial to their fabrication processes, chemical and physical properties, and applications. Here we systematically investigate the structural and thermal stabilities of single Au@Ag CSNPs with different sizes and their arrays by means of all-atom molecular dynamics simulations. The formation energies of all Au@Ag CSNPs we reported are all negative, indicating that Au@Ag CSNPs are energetically favorable to be formed. For Au@Ag CSNPs with the same core size, their melting points increase with increasing shell thickness. If we keep the shell thickness unchanged, the melting points increase as the core sizes increase except for the CSNP with the smallest core size and a bilayer Ag shell. The melting points of Au@Ag CSNPs show a feature of non-monotonicity with increasing core size at a fixed NP size. Further simulations on the Au@Ag CSNP arrays with 923 atoms reveal that their melting points decrease dramatically compared with single Au@Ag CSNPs. We find that the premelting processes start from the surface region for both the single NPs and their arrays.

Effect of C60 nanoparticles on elasticity of small unilamellar vesicles composed of DPPC bilayers

Tanlin Wei, Lei Zhang, Yong Zhang
Chin. Phys. B 2020, 29 (4): 048702;  doi: 10.1088/1674-1056/ab7d9f
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The interaction between C60 nanoparticles and biomembranes has been of great interest in researches over the past decades due to their novel applications as well as potential cytotoxicity. In this work, we study the deformation of the small unilamellar vesicles composed of dipalmitoylphosphatidylcholine (DPPC) lipid bilayers infiltrated with C60 nanoparticles of different molecular concentrations through coarse-grained molecular dynamics simulations. By employing the Helfrich spontaneous curvature model, the bending modulus and the spontaneous curvature of the vesicles with C60 nanoparticles of different concentrations are obtained according to the simulation data. The results show that the bending modulus and the spontaneous curvature of pure DPPC vesicle membranes are approximately 1.6×10-19 J and 1.4 nm-1, respectively. Both of them increase linearly approximately as the C60 concentration increases from 0 to 16.3%. The density profiles of vesicles, the order of lipid packing and the diffusion characteristics of DPPC and C60 are also investigated.

Electron beam irradiation on novel coronavirus (COVID-19): A Monte-Carlo simulation

Guobao Feng, Lu Liu, Wanzhao Cui, Fang Wang
Chin. Phys. B 2020, 29 (4): 048703;  doi: 10.1088/1674-1056/ab7dac
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The novel coronavirus pneumonia triggered by COVID-19 is now raging the whole world. As a rapid and reliable killing COVID-19 method in industry, electron beam irradiation can interact with virus molecules and destroy their activity. With the unexpected appearance and quickly spreading of the virus, it is urgently necessary to figure out the mechanism of electron beam irradiation on COVID-19. In this study, we establish a virus structure and molecule model based on the detected gene sequence of Wuhan patient, and calculate irradiated electron interaction with virus atoms via a Monte Carlo simulation that track each elastic and inelastic collision of all electrons. The characteristics of irradiation damage on COVID-19, atoms' ionizations and electron energy losses are calculated and analyzed with regions. We simulate the different situations of incident electron energy for evaluating the influence of incident energy on virus damage. It is found that under the major protecting of an envelope protein layer, the inner RNA suffers the minimal damage. The damage for a ~100-nm-diameter virus molecule is not always enhanced by irradiation energy monotonicity, for COVID-19, the irradiation electron energy of the strongest energy loss damage is 2 keV.

Two-step processed efficient perovskite solar cells via improving perovskite/PTAA interface using solvent engineering in PbI2 precursor

Cao-Yu Long, Ning Wang, Ke-Qing Huang, Heng-Yue Li, Biao Liu, Jun-Liang Yang
Chin. Phys. B 2020, 29 (4): 048801;  doi: 10.1088/1674-1056/ab7744
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The morphology and interface of perovskite film are very important for the performance of perovskite solar cells (PSCs). The quality of perovskite film, fabricated via two-step spin-coating process, is significantly influenced by the morphology and crystallinity of PbI2 film. With the addition of additive dimethyl sulfoxide (DMSO) into the PbI2 precursor, the roughness and trap-state density of perovskite film have been significantly reduced, leading to the excellent contact between perovskite layer and subsequent deposited carrier transport layer. Accordingly, the planar heterojunction PSCs with an architecture of ITO/SnO2/perovskite/PTAA/Ag show an efficiency up to 19.02%. Furthermore, PSCs exhibit promising stability in air with a humidity of ~ 45%, and retain 80% of initial efficiency after being exposed to air for 400 h without any encapsulation.

Network correlation between investor's herding behavior and overconfidence behavior

Mao Zhang, Yi-Ming Wang
Chin. Phys. B 2020, 29 (4): 048901;  doi: 10.1088/1674-1056/ab7740
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It is generally accepted that herding behavior and overconfidence behavior are unrelated or even mutually exclusive. However, these behaviors can both lead to some similar market anomalies, such as excessive trading volume and volatility in the stock market. Due to the limitation of traditional time series analysis, we try to study whether there exists network relevance between the investor's herding behavior and overconfidence behavior based on the complex network method. Since the investor's herding behavior is based on market trends and overconfidence behavior is based on past performance, we convert the time series data of market trends into a market network and the time series data of the investor's past judgments into an investor network. Then, we update these networks as new information arrives at the market and show the weighted in-degrees of the nodes in the market network and the investor network can represent the herding degree and the confidence degree of the investor, respectively. Using stock transaction data of Microsoft, US S&P 500 stock index, and China Hushen 300 stock index, we update the two networks and find that there exists a high similarity of network topological properties and a significant correlation of node parameter sequences between the market network and the investor network. Finally, we theoretically derive and conclude that the investor's herding degree and confidence degree are highly related to each other when there is a clear market trend.

Identifying influential spreaders in complex networks based on entropy weight method and gravity law

Xiao-Li Yan, Ya-Peng Cui, Shun-Jiang Ni
Chin. Phys. B 2020, 29 (4): 048902;  doi: 10.1088/1674-1056/ab77fe
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In complex networks, identifying influential spreader is of great significance for improving the reliability of networks and ensuring the safe and effective operation of networks. Nowadays, it is widely used in power networks, aviation networks, computer networks, and social networks, and so on. Traditional centrality methods mainly include degree centrality, closeness centrality, betweenness centrality, eigenvector centrality, k-shell, etc. However, single centrality method is one-sided and inaccurate, and sometimes many nodes have the same centrality value, namely the same ranking result, which makes it difficult to distinguish between nodes. According to several classical methods of identifying influential nodes, in this paper we propose a novel method that is more full-scaled and universally applicable. Taken into account in this method are several aspects of node's properties, including local topological characteristics, central location of nodes, propagation characteristics, and properties of neighbor nodes. In view of the idea of the multi-attribute decision-making, we regard the basic centrality method as node's attribute and use the entropy weight method to weigh different attributes, and obtain node's combined centrality. Then, the combined centrality is applied to the gravity law to comprehensively identify influential nodes in networks. Finally, the classical susceptible-infected-recovered (SIR) model is used to simulate the epidemic spreading in six real-society networks. Our proposed method not only considers the four topological properties of nodes, but also emphasizes the influence of neighbor nodes from the aspect of gravity. It is proved that the new method can effectively overcome the disadvantages of single centrality method and increase the accuracy of identifying influential nodes, which is of great significance for monitoring and controlling the complex networks.
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