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Chin. Phys. B  
  Chin. Phys. B--2020, Vol.29, No.1
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TOPICAL REVIEW—Strong-field atomic and molecular physics
SPECIAL TOPIC—Recent advances in thermoelectric materials and devices

Broadband visible light absorber based on ultrathin semiconductor nanostructures

Lin-Jin Huang, Jia-Qi Li, Man-Yi Lu, Yan-Quan Chen, Hong-Ji Zhu, Hai-Ying Liu
Chin. Phys. B 2020, 29 (1): 014201;  doi: 10.1088/1674-1056/ab5787
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It is desirable to have electromagnetic wave absorbers with ultrathin structural thickness and broader spectral absorption bandwidth with numerous applications in optoelectronics. In this paper, we theoretically propose and numerically demonstrate a novel ultrathin nanostructure absorber composed of semiconductor nanoring array and a uniform gold substrate. The results show that the absorption covers the entire visible light region, achieving an average absorption rate more than 90% in a wavelength range from 300 nm to 740 nm and a nearly perfect absorption from 450 nm to 500 nm, and the polarization insensitivity performance is particularly great. The absorption performance is mainly caused by the electrical resonance and magnetic resonance of semiconductor nanoring array as well as the field coupling effects. Our designed broadband visible light absorber has wide application prospects in the fields of thermal photovoltaics and photodetectors.

Noise-like rectangular pulses in a mode-locked double-clad Er:Yb laser with a record pulse energy

Tianyi Wu, Zhiyuan Dou, Bin Zhang, Jing Hou
Chin. Phys. B 2020, 29 (1): 014202;  doi: 10.1088/1674-1056/ab578a
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Generation of noise-like rectangular pulse was investigated systematically in an Er-Yb co-doped fiber laser based on an intra-cavity coupler with different coupling ratios. When the coupling ratio was 5/95, stable mode-locked pulses could be obtained with the pulse packet duration tunable from 4.86 ns to 80 ns. The repetition frequency was 1.186 MHz with the output spectrum centered at 1.6 μ. The average output power and single pulse energy reached a record 1.43 W and 1.21 μJ, respectively. Pulse characteristics under different coupling ratios (5/95, 10/90, 20/80, 30/70, 40/60) were also presented and discussed.

Propagation characteristics of parallel dark solitons in silicon-on-insulator waveguide

Zhen Liu, Weiguo Jia, Yang Wang, Hongyu Wang, Neimule Men-Ke, Jun-Ping Zhang
Chin. Phys. B 2020, 29 (1): 014203;  doi: 10.1088/1674-1056/ab577d
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The propagation characteristic of two identical and parallel dark solitons in a silicon-on-insulator (SOI) waveguide is simulated numerically using the split-step Fourier method. The parallel dark solitons imposed by the initial chirp are investigated mainly by changing their power, their relative time delay. The simulation shows that the time delay deforms the parallel dark soliton pulse, forming a bright-like soliton in the transmission process and making the transmission quality down. By increasing the power of one dark soliton, the energy of the other dark soliton can be increased, and larger increase in a soliton's power leads to larger increase in the energy of the other. When the initial chirp is introduced into one of the dark solitons, higher energy consumption is observed. In particular, positive chirps resulting in pulse broadening width while negative chirps narrowing, with an obvious compression effect on the other dark soliton. Finally, large negative chirps are found to have a profound impact on parallel and nonparallel dark solitons.

Soliton evolution and control in a two-mode fiber with two-photon absorption

Qianying Li
Chin. Phys. B 2020, 29 (1): 014204;  doi: 10.1088/1674-1056/ab5784
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Soliton dynamics are numerically investigated in a two-mode fiber with the two-photon absorption, and the effects of the two-photon absorption on the soliton propagation and interaction are demonstrated in different dispersion regimes. Soliton dynamics depend strictly on the sign and magnitude of the group velocity dispersion (GVD) coefficient of each mode and the strength (coefficient) of the two-photon absorption. The two-photon absorption leads to the soliton collapse, enhances the neighboring soliton interaction in both modes, and increases the energy exchange between the two modes. Finally, an available control is proposed to suppress the effects by the use of the nonlinear gain with filter.

Acquisition performance analysis for intersatellite optical communications with vibration influence

Jing Ma, Gaoyuan Lu, Siyuan Yu, Liying Tan, Yulong Fu, Fajun Li
Chin. Phys. B 2020, 29 (1): 014205;  doi: 10.1088/1674-1056/ab577e
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A stable and accurate pointing, acquisition system is an important part of initially building intersatellite optical communication links. Satellite platform vibration can cause the system instability and reduce the system precision in building and maintenance of a satellite optical communication system. In this paper, vibration influence is consciously discussed by acquisition time for intersatellite optical communications. Analytical expression of acquisition possibility is derived, taking the scan parameters and platform vibration into account, and vibration influence on the multi-scan acquisition time is also presented. The theoretical result calculated by the proposed analytical expression is approximate to the result by the Monte Carlo simulation.

Unitary transformation of general nonoverlapping-image multimode interference couplers with any input and output ports

Ze-Zheng Li, Wei-Hua Han, Zhi-Yong Li
Chin. Phys. B 2020, 29 (1): 014206;  doi: 10.1088/1674-1056/ab5783
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An explanation of optical unitary transformation is presented for general nonoverlapping-image multimode interference (MMI) couplers with any number of input and output ports. The light transformation in the MMI coupler can be considered as an optical field matrix acting on an input light column vector. We investigate the general phase principle of output light image. The complete proof of nonoverlapping-image MMI coupler's optical unitarity along with the phase analysis of matrix element is provided. Based on a two-dimensional finite-difference time-domain (2D-FDTD) simulation, the unitary transformation is obtained for a 4×4 nonoverlapping-image MMI coupler within a deviation of 4×10-2 for orthogonal invariance and 8×10-2 for transvection invariance in the C-band spectral range. A compact 1×4 splitter based on cascaded MMI coupler is proposed, showing a phase deviation less than 5.4° while maintaining a low-loss performance in C-band spectra.

A novel particle tracking velocimetry method for complex granular flow field Hot!

Bi-De Wang, Jian Song, Ran Li, Ren Han, Gang Zheng, Hui Yang
Chin. Phys. B 2020, 29 (1): 014207;  doi: 10.1088/1674-1056/ab5936
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Particle tracking velocimetry (PTV) is one of the most commonly applied granular flow velocity measurement methods. However, traditional PTV methods may have issues such as high mismatching rates and a narrow measurement range when measuring granular flows with large bulk density and high-speed contrast. In this study, a novel PTV method is introduced to solve these problems using an optical flow matching algorithm with two further processing steps. The first step involves displacement correction, which is used to solve the mismatching problem in the case of high stacking density. The other step is trajectory splicing, which is used to solve the problem of a measurement range reduction in the case of high-speed contrast The hopper flow experimental results demonstrate superior performance of this proposed method in controlling the number of mismatched particles and better measuring efficiency in comparison with the traditional PTV method.

Linear optical approach to supersymmetric dynamics

Yong-Tao Zhan, Xiao-Ye Xu, Qin-Qin Wang, Wei-Wei Pan, Munsif Jan, Fu-Ming Chang, Kai Sun, Jin-Shi Xu, Yong-Jian Han, Chuan-Feng Li, Guang-Can Guo
Chin. Phys. B 2020, 29 (1): 014209;  doi: 10.1088/1674-1056/ab5efe
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The concept of supersymmetry developed in particle physics has been applied to various fields of modern physics. In quantum mechanics, the supersymmetric systems refer to the systems involving two supersymmetric partner Hamiltonians, whose energy levels are degeneracy except one of the systems has an extra ground state possibly, and the eigenstates of the partner systems can be mapped onto each other. Recently, an interferometric scheme has been proposed to show this relationship in ultracold atoms[Phys. Rev. A 96 043624 (2017)]. Here this approach is generalized to linear optics for observing the supersymmetric dynamics with photons. The time evolution operator is simulated approximately via Suzuki-Trotter expansion with considering the realization of the kinetic and potential terms separately. The former is realized through the diffraction nature of light and the later is implemented using a phase plate. Additionally, we propose an interferometric approach which can be implemented perfectly using an amplitude alternator to realize the non-unitary operator. The numerical results show that our scheme is universal and can be realized with current technologies.

Displacement damage in optocouplers induced by high energy neutrons at back-n in China Spallation Neutron Source

Rui Xu, Zu-Jun Wang, Yuan-Yuan Xue, Hao Ning, Min-Bo Liu, Xiao-Qiang Guo, Zhi-Bin Yao, Jiang-Kun Sheng, Wu-Ying Ma, Guan-Tao Dong
Chin. Phys. B 2020, 29 (1): 014210;  doi: 10.1088/1674-1056/ab5efb
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Neutron radiation experiments of optocouplers at back-streaming white neutrons (back-n) in China Spallation Neutron Source (CSNS) are presented. The displacement damages induced by neutron radiation are analyzed. The performance degradations of two types of optocouplers are compared. The degradations of current transfer ratio (CTR) are analyzed, and the mechanisms induced by radiation are also demonstrated. With the increase of the accumulated fluence, the CTR is degrading linearly with neutron fluence. The radiation hardening of optocouplers can be improved when the forward current is increased. Other parameters related to CTR degradation of optocouplers are also analyzed.

Dielectric or plasmonic Mie object at air-liquid interface: The transferred and the traveling momenta of photon

M R C Mahdy, Hamim Mahmud Rivy, Ziaur Rahman Jony, Nabila Binte Alam, Nabila Masud, Golam Dastegir Al Quaderi, Ibraheem Muhammad Moosa, Chowdhury Mofizur Rahman, M Sohel Rahman
Chin. Phys. B 2020, 29 (1): 014211;  doi: 10.1088/1674-1056/ab5efa
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Considering the inhomogeneous or heterogeneous background, we have demonstrated that if the background and the half-immersed object are both non-absorbing, the transferred photon momentum to the pulled object can be considered as the one of Minkowski exactly at the interface. In contrast, the presence of loss inside matter, either in the half-immersed object or in the background, causes optical pushing of the object. Our analysis suggests that for half-immersed plasmonic or lossy dielectric, the transferred momentum of photon can mathematically be modeled as the type of Minkowski and also of Abraham. However, according to a final critical analysis, the idea of Abraham momentum transfer has been rejected. Hence, an obvious question arises:whence the Abraham momentum? It is demonstrated that though the transferred momentum to a half-immersed Mie object (lossy or lossless) can better be considered as the Minkowski momentum, Lorentz force analysis suggests that the momentum of a photon traveling through the continuous background, however, can be modeled as the type of Abraham. Finally, as an interesting sidewalk, a machine learning based system has been developed to predict the time-averaged force within a very short time avoiding time-consuming full wave simulation.

Orientation-dependent depolarization of supercontinuum in BaF2 crystal

Zi-Xi Li, Cheng Gong, Tian-Jiao Shao, Lin-Qiang Hua, Xue-Bin Bian, Xiao-Jun Liu
Chin. Phys. B 2020, 29 (1): 014212;  doi: 10.1088/1674-1056/ab5fc7
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We present a systematic investigation of the depolarization properties of a supercontinuum accompanied with femtosecond laser filamentation in barium fluoride (BaF2) crystal. It is found that the depolarization of the supercontinuum depends strongly on the crystal orientations with respect to the incident laser polarization. At most crystal orientations, the depolarization of the supercontinuum rises with the increase of the input laser energies and finally saturates. While at 45°, the depolarization of the supercontinuum is not changed and keeps nearly negligible with the increase of the input laser energies. These peculiar depolarization properties of the supercontinuum can be ascribed to the orientation dependence of the cross-polarized wave (XPW) generation and ionization-induced plasma scattering in the BaF2 crystal.

Noise properties of multi-combination information in x-ray grating-based phase-contrast imaging

Wali Faiz, Ji Li, Kun Gao, Zhao Wu, Yao-Hu Lei, Jian-Heng Huang, Pei-Ping Zhu
Chin. Phys. B 2020, 29 (1): 014301;  doi: 10.1088/1674-1056/ab5780
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Grating-based x-ray phase contrast imaging has attracted increasing interest in recent decades as multimodal and laboratory source usable method. Specific efforts have been focused on establishing a new extraction method to perform practical applications. In this work, noise properties of multi-combination information of newly established information extraction method, so-called angular signal radiography method, are investigated to provide guidelines for targeted and specific applications. The results show that how multi-combination of images can be used in targeted practical applications to obtain a high-quality image in terms of signal-to-noise ratio. Our conclusions can also hold true for upcoming targeted practical applications such as biomedical imaging, non-destructive imaging, and materials science.

Micro-crack detection of nonlinear Lamb wave propagation in three-dimensional plates with mixed-frequency excitation

Wei-Guang Zhu, Yi-Feng Li, Li-Qiang Guan, Xi-Li Wan, Hui-Yang Yu, Xiao-Zhou Liu
Chin. Phys. B 2020, 29 (1): 014302;  doi: 10.1088/1674-1056/ab5931
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We propose a nonlinear ultrasonic technique by using the mixed-frequency signals excited Lamb waves to conduct micro-crack detection in thin plate structures. Simulation models of three-dimensional (3D) aluminum plates and composite laminates are established by ABAQUS software, where the aluminum plate contains buried crack and composite laminates comprises cohesive element whose thickness is zero to simulate delamination damage. The interactions between the S0 mode Lamb wave and the buried micro-cracks of various dimensions are simulated by using the finite element method. Fourier frequency spectrum analysis is applied to the received time domain signal and fundamental frequency amplitudes, and sum and difference frequencies are extracted and simulated. Simulation results indicate that nonlinear Lamb waves have different sensitivities to various crack sizes. There is a positive correlation among crack length, height, and sum and difference frequency amplitudes for an aluminum plate, with both amplitudes decreasing as crack thickness increased, i.e., nonlinear effect weakens as the micro-crack becomes thicker. The amplitudes of sum and difference frequency are positively correlated with the length and width of the zero-thickness cohesive element in the composite laminates. Furthermore, amplitude ratio change is investigated and it can be used as an effective tool to detect inner defects in thin 3D plates.

Sound propagation in inhomogeneous waveguides with sound-speed profiles using the multimodal admittance method

Qi Li, Juan Liu, Wei Guo
Chin. Phys. B 2020, 29 (1): 014303;  doi: 10.1088/1674-1056/ab5940
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The multimodal admittance method and its improvement are presented to deal with various aspects in underwater acoustics, mostly for the sound propagation in inhomogeneous waveguides with sound-speed profiles, arbitrary-shaped liquid-like scatterers, and range-dependent environments. In all cases, the propagation problem governed by the Helmholtz equation is transformed into initial value problems of two coupled first-order evolution equations with respect to the modal components of field quantities (sound pressure and its derivative), by projecting the Helmholtz equation on a constructed orthogonal and complete local basis. The admittance matrix, which is the modal representation of Direchlet-to-Neumann operator, is introduced to compute the first-order evolution equations with no numerical instability caused by evanescent modes. The fourth-order Magnus scheme is used for the numerical integration of differential equations in the numerical implementation. The numerical experiments of sound field in underwater inhomogeneous waveguides generated by point sources are performed. Besides, the numerical results computed by simulation software COMSOL Multiphysics are given to validate the correction of the multimodal admittance method. It is shown that the multimodal admittance method is an efficient and stable numerical method to solve the wave propagation problem in inhomogeneous underwater waveguides with sound-speed profiles, liquid-like scatterers, and range-dependent environments. The extension of the method to more complicated waveguides such as horizontally stratified waveguides is available.

Theoretical estimation of sonochemical yield in bubble cluster in acoustic field

Zhuang-Zhi Shen
Chin. Phys. B 2020, 29 (1): 014304;  doi: 10.1088/1674-1056/ab6108
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In order to learn more about the physical phenomena occurring in cloud cavitation, the nonlinear dynamics of a spherical cluster of cavitation bubbles and cavitation bubbles in cluster in an acoustic field excited by a square pressure wave are numerically investigated by considering viscosity, surface tension, and the weak compressibility of the liquid. The theoretical prediction of the yield of oxidants produced inside bubbles during the strong collapse stage of cavitation bubbles is also investigated. The effects of acoustic frequency, acoustic pressure amplitude, and the number of bubbles in cluster on bubble temperature and the quantity of oxidants produced inside bubbles are analyzed. The results show that the change of acoustic frequency, acoustic pressure amplitude, and the number of bubbles in cluster have an effect not only on temperature and the quantity of oxidants inside the bubble, but also on the degradation types of pollutants, which provides a guidance in improving the sonochemical degradation of organic pollutants.

Evaporation of saline colloidal droplet and deposition pattern

Hong-Hui Sun, Wei-Bin Li, Wen-Jie Ji, Guo-Liang Dai, Yong Huan, Yu-Ren Wang, Ding Lan
Chin. Phys. B 2020, 29 (1): 014701;  doi: 10.1088/1674-1056/ab5782
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The dynamic process of the evaporation and the desiccation of sessile saline colloidal droplets, and their final deposition are investigated. During the evaporation, the movement of the colloidal particles shows a strong dependence on the salt concentration and the droplet shape. The final deposition pattern indicates a weakened coffee-ring effect in this mixed droplet system. The microscopic observation reveals that as evaporation proceeds, the particle motion trail is affected by the salt concentration of the droplet boundary. The Marangoni flow, which is induced by surface tension gradient originating from the local evaporative peripheral salt enrichment, suppresses the compensation flow towards the contact line of the droplet. The inhomogeneous density and concentration field induced by evaporation or crystallization can be the major reason for various micro-flows. At last stage, the distribution and crystallization of NaCl are affected by the colloidal particles during the drying of the residual liquid film.

Mechanism from particle compaction to fluidization of liquid-solid two-phase flow

Yue Zhang, Jinchun Song, Lianxi Ma, Liancun Zheng, Minghe Liu
Chin. Phys. B 2020, 29 (1): 014702;  doi: 10.1088/1674-1056/ab5789
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A new model of particle yield stress including cohesive strength is proposed, which considers the friction and cohesive strength between particles. A calculation method for the fluidization process of liquid-solid two-phase flow in compact packing state is given, and the simulation and experimental studies of fluidization process are carried out by taking the sand-water two-phase flow in the jet dredging system as an example, and the calculation method is verified.

Acoustic characteristics of pulse detonation engine sound propagating in enclosed space

Yang Kang, Ning Li, Chun-Sheng Weng, Xiao-Long Huang
Chin. Phys. B 2020, 29 (1): 014703;  doi: 10.1088/1674-1056/ab5b88
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Acoustic characteristics of pulse detonation engine (PDE) sound propagating in enclosed space are numerically and experimentally investigated. The finite element software LS-DYNA is utilized to numerically simulate the PDE sound propagating in enclosed space. Acoustic measurement systems are established for testing the PDE sound in enclosed space, and the time-frequency characteristics of PDE sound in enclosed space are reported in detail. The experimental results show that the sound waveform of PDE sound in enclosed space are quite different from those in open space, and the reflection and superposition of PDE sound on the walls of enclosed space results in the sound pressure oscillating obviously. It is found that the peak sound pressure level (PSPL) and overall sound pressure level (OASPL) of PDE sound in enclosed space are higher than those in open space and their difference increases with the rise of propagation distance. The results of the duration of PDE sound indicate that the A duration of PDE sound in enclosed space is higher than that in open space except at measuring points located at 2-m and 5-m while the B duration is higher at each of all measuring points. Results show that the enclosed space has a great influence on the acoustic characteristic of PDE sound. This research is helpful in performing PDE experiments in enclosed laboratories to prevent the PDE sound from affecting the safety of laboratory environment, equipment, and staffs.

Beryllium carbide as diffusion barrier against Cu: First-principles study

Hua-Liang Cao, Xin-Lu Cheng, Hong Zhang
Chin. Phys. B 2020, 29 (1): 016601;  doi: 10.1088/1674-1056/ab593e
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Beryllium carbide is used in inertial confinement fusion (ICF) capsule ablation material due to its low atomic number, low opacity, and high melting point properties. We used the method of climbing image nudged elastic band (CINEB) to calculate the diffusion barrier of copper atom in the crystal of beryllium and beryllium carbide. The diffusion barrier of copper atom in crystal beryllium is only 0.79 eV, and the barrier in beryllium carbide is larger than 2.85 eV. The three structures of beryllium carbide:anti-fluorite Be2C, Be2C-I, and Be2C-III have a good blocking effect to the diffusion of copper atom. Among them, the Be2C-III structure has the highest diffusion barrier of 6.09 eV. Our research can provide useful help for studying Cu diffusion barrier materials.

Sodium decorated net-Y nanosheet for hydrogen storage and adsorption mechanism: A first-principles study

Yunlei Wang, Yuhong Chen, Yunhui Wang
Chin. Phys. B 2020, 29 (1): 016801;  doi: 10.1088/1674-1056/ab5785
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Using first-principles calculations based on density functional theory (DFT), we investigate the potential hydrogen storage capacity of the Na-decorated net-Y single layer nanosheet. For double-side Na decoration, the average binding energy is 1.54 eV, which is much larger than the cohesive energy of 1.13 eV for bulk Na. A maximum of four H2 molecules can be adsorbed around each Na with average adsorption energies of 0.25-0.32 eV/H2. Also, H2 storage gravimetric of 8.85 wt% is obtained, and this meets the U.S. Department of Energy (DOE) ultimate target. These results are instrumental in seeking a promising hydrogen energy carrier.

First-principles study of high performance lithium/sodium storage of Ti3C2T2 nanosheets as electrode materials

Li-Na Bai, Ling-Ying Kong, Jing Wen, Ning Ma, Hong Gao, Xi-Tian Zhang
Chin. Phys. B 2020, 29 (1): 016802;  doi: 10.1088/1674-1056/ab592e
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Ti3C2Tx nanosheet, the first synthesized MXene with high capacity performance and charge/discharge rate, has attracted increasingly attention in renewable energy storage applications. By performing systematic density functional theory calculations, the theoretical capacity of the intrinsic structure of single- and multi-layered Ti3C2T2 (T=F or O) corresponding to M (M=Li and Na) atoms are investigated. Theoretical volumetric capacity and gravimetric capacity are obtained, which are related to the stacking degree. The optimal ratios of capacity to structure are determined under different stacking degrees for understanding the influence of surface functional groups on energy storage performance. Its performance can be tuned by performing surface modification and increasing the interlayer distance. In addition, the reason for theoretical capacity differences of M atoms is analyzed, which is attributed to difference in interaction between the M-ions and substrate and the difference in electrostatic exclusion between adsorbed M-ions. These results provide an insight into the understanding of the method of efficiently increasing the energy storage performance, which will be useful for designing and using high performance electrode materials.

Giant topological Hall effect of ferromagnetic kagome metal Fe3Sn2 Hot!

Qi Wang, Qiangwei Yin, Hechang Lei
Chin. Phys. B 2020, 29 (1): 017101;  doi: 10.1088/1674-1056/ab5fbc
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We present the experiment observation of a giant topological Hall effect (THE) in a frustrated kagome bilayer magnet Fe3Sn2. The negative topologically Hall resistivity appears when the field is below 1.3 T and it increases with increasing temperature up to 300 K. Its maximum absolute value reaches ~2.01 μΩ·cm at 300 K and 0.76 T. The origins of the observed giant THE can be attributed to the coexistence of the field-induced skyrmion state and the non-collinear spin configuration, possibly related to the magnetic frustration interaction in Fe3Sn2.

Improvement of radiative recombination rate in deep ultraviolet laser diodes with step-like quantum barrier and aluminum-content graded electron blocking layer

Yi-Fu Wang, Mussaab I Niass, Fang Wang, Yu-Huai Liu
Chin. Phys. B 2020, 29 (1): 017301;  doi: 10.1088/1674-1056/ab592c
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The design of the active region structures, including the modifications of structures of the quantum barrier (QB) and electron blocking layer (EBL), in the deep ultraviolet (DUV) AlGaN laser diode (LD) is investigated numerically with the Crosslight software. The analyses focus on electron and hole injection efficiency, electron leakage, hole diffusion, and radiative recombination rate. Compared with the reference QB structure, the step-like QB structure provides high radiative recombination and maximum output power. Subsequently, a comparative study is conducted on the performance characteristics with four different EBLs. For the EBL with different Al mole fraction layers, the higher Al-content AlGaN EBL layer is located closely to the active region, leading the electron current leakage to lower, the carrier injection efficiency to increase, and the radiative recombination rate to improve.

Tetraalkyl-substituted zinc phthalocyanines used as anode buffer layers for organic light-emitting diodes

Qian Chen, Songhe Yang, Lei Dong, Siyuan Cai, Jiaju Xu, Zongxiang Xu
Chin. Phys. B 2020, 29 (1): 017302;  doi: 10.1088/1674-1056/ab5933
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Two soluble tetraalkyl-substituted zinc phthalocyanines (ZnPcs) for use as anode buffer layer materials in tris(8-hydroxyquinoline)aluminum (Alq3)-based organic light-emitting diodes (OLEDs) are presented in this work. The hole-blocking properties of these ZnPc layers slowed the hole injection process into the Alq3 emissive layer greatly and thus reduced the production of unstable cationic Alq3 (Alq3+) species. This led to the enhanced brightness and efficiency when compared with the corresponding properties of OLEDs based on the popular poly-(3, 4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) buffer layer. Furthermore, because of the high thermal and chemical stabilities of these ZnPcs, a nonaqueous film fabrication process was realized together with improved charge balance in the OLEDs and enhanced OLED lifetimes.

Tunneling magnetoresistance in ferromagnet/organic-ferromagnet/metal junctions

Yan-Qi Li, Hong-Jun Kan, Yuan-Yuan Miao, Lei Yang, Shuai Qiu, Guang-Ping Zhang, Jun-Feng Ren, Chuan-Kui Wang, Gui-Chao Hu
Chin. Phys. B 2020, 29 (1): 017303;  doi: 10.1088/1674-1056/ab5d02
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Spin-dependent transport in ferromagnet/organic-ferromagnet/metal junctions is investigated theoretically. The results reveal a large tunneling magnetoresistance up to 3230% by controlling the relative magnetization orientation between the ferromagnet and the central organic ferromagnet. The mechanism is explained by distinct efficient spin-resolved tunneling states in the ferromagnet between the parallel and antiparallel spin configurations. The key role of the organic ferromagnet in generating the large magnetoresistance is explored, where the spin selection effect is found to enlarge the difference of the tunneling states between the parallel and antiparallel configurations by comparing with the conventional organic spin valves. The effects of intrinsic interactions in the organic ferromagnet including electron-lattice interaction and spin coupling with radicals on the magnetoresistance are discussed. This work demonstrates a promising potential of organic ferromagnets in the design of high-performance organic spin valves.

Surface Majorana flat bands in j=3/2 superconductors with singlet-quintet mixing Hot!

Jiabin Yu, Chao-Xing Liu
Chin. Phys. B 2020, 29 (1): 017402;  doi: 10.1088/1674-1056/ab5b89
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Recent experiments[Science Advances 4 eaao4513 (2018)] have revealed the evidence of nodal-line superconductivity in half-Heusler superconductors, e.g., YPtBi. Theories have suggested the topological nature of such nodal-line superconductivity and proposed the existence of surface Majorana flat bands on the (111) surface of half-Heusler superconductors. Due to the divergent density of states of the surface Majorana flat bands, the surface order parameter and the surface impurity play essential roles in determining the surface properties. We study the effect of the surface order parameter and the surface impurity on the surface Majorana flat bands of half-Heusler superconductors based on the Luttinger model. To be specific, we consider the topological nodal-line superconducting phase induced by the singlet-quintet pairing mixing, classify all the possible translationally invariant order parameters for the surface states according to irreducible representations of C3v point group, and demonstrate that any energetically favorable order parameter needs to break the time-reversal symmetry. We further discuss the energy splitting in the energy spectrum of surface Majorana flat bands induced by different order parameters and non-magnetic or magnetic impurities. We propose that the splitting in the energy spectrum can serve as the fingerprint of the pairing symmetry and mean-field order parameters. Our theoretical prediction can be examined in the future scanning tunneling microscopy experiments.

Non-parabolic effect for femtosecond laser-induced ultrafast electro-absorption in solids

Li-Bo Liu, Hong-Xiang Deng, Xiao-Tao Zu, Xiao-Dong Yuan, Wan-Guo Zheng
Chin. Phys. B 2020, 29 (1): 017801;  doi: 10.1088/1674-1056/ab577c
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A theoretical study for femtosecond laser-induced ultrafast electro-absorption of bulk solids is presented. Our numerical results show that, in the case of low intensity of the pump laser where the interaction between the pump laser and solids is in the multi-photon regime, the energy band of solids can be approximately taken as a parabolic band and electro-absorption spectrums from the parabolic band and real band are nearly the same. While, in the case of high intensity where the interaction is in the tunneling regime, spectrums from the parabolic band and real band are quite different. The physical mechanism for the difference in the tunneling regime is found. We find that the non-parabolic parts of the real energy band and Bragger scattering of electrons near the first Brillouin zone boundaries, which are neglected in previous studies, strongly influence the electro-absorption spectrum in the tunneling regime. These two physical processes cause the difference of spectrums. Our theoretical results are in accordance with the experiment result.

Thickness-dependent excitonic properties of atomically thin 2H-MoTe2

Jin-Huan Li, Dan Bing, Zhang-Ting Wu, Guo-Qing Wu, Jing Bai, Ru-Xia Du, Zheng-Qing Qi
Chin. Phys. B 2020, 29 (1): 017802;  doi: 10.1088/1674-1056/ab5a3a
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Two-dimensional (2D) 2H-MoTe2 is a promising semiconductor because of its small bandgap, strong absorption, and low thermal conductivity. In this paper, we systematically study the optical and excitonic properties of atomically thin 2H-MoTe2 (1-5 layers). Due to the fact that the optical contrast and Raman spectra of 2H-MoTe2 with different thicknesses exhibit distinctly different behaviors, we establish a quantitative method by using optical images and Raman spectra to directly identify the layers of 2H-MoTe2 thin films. Besides, excitonic states and binding energy in monolayer/bilayer 2H-MoTe2 are measured by temperature-dependent photoluminescence (PL) spectroscopy. At temperature T=3.3 K, we can observe an exciton emission at ~1.19 eV and trion emission at ~1.16 eV for monolayer 2H-MoTe2. While at room temperature, the exciton emission and trion emission both disappear for their small binding energy. We determine the exciton binding energy to be 185 meV (179 meV), trion binding energy to be 20 meV (18 meV) for the monolayer (bilayer) 2H-MoTe2. The thoroughly studies of the excitonic states in atomically thin 2H-MoTe2 will provide guidance for future practical applications.

Growth characteristics of type IIa large single crystal diamond with Ti/Cu as nitrogen getter in FeNi-C system

Ming-Ming Guo, Shang-Sheng Li, Mei-Hua Hu, Tai-Chao Su, Jun-Zuo Wang, Guang-Jin Gao, Yue You, Yuan Nie
Chin. Phys. B 2020, 29 (1): 018101;  doi: 10.1088/1674-1056/ab592f
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High-quality type I!Ia large diamond crystals are synthesized with Ti/Cu as nitrogen getter doped in an FeNi-C system at temperature ranging from 1230 ℃ to 1380 ℃ and at pressure 5.3-5.9 GPa by temperature gradient method. Different ratios of Ti/Cu are added to the FeNi-C system to investigate the best ratio for high-quality type I!Ia diamond. Then, the different content of nitrogen getter Ti/Cu (Ti:Cu=4:3) is added to this synthesis system to explore the effect on diamond growth. The macro and micro morphologies of synthesized diamonds with Ti/Cu added, whose nitrogen concentration is determined by Fourier transform infrared (FTIR), are analyzed by optical microscopy (OM) and scanning electron microscopy (SEM), respectively. It is found that the inclusions in the obtained crystals are minimal when the Ti/Cu ratio is 4:3. Furthermore, the temperature interval for diamond growth becomes narrower when using Ti as the nitrogen getter. Moreover, the lower edge of the synthesis temperature of type IIa diamond is 25 ℃ higher than that of type Ib diamond. With the increase of the content of Ti/Cu (Ti:Cu=4:3), the color of the synthesized crystals changes from yellow and light yellow to colorless. When the Ti/Cu content is 1.7 wt%, the nitrogen concentration of the crystal is less than 1 ppm. The SEM results show that the synthesized crystals are mainly composed by (111) and (100) surfaces, including (311) surface, when the nitrogen getter is added into the synthesis system. At the same time, there are triangular pits and dendritic growth stripes on the crystal surface. This work will contribute to the further research and development of high-quality type I!Ia diamond.

Room temperature non-balanced electric bridge ethanol gas sensor based on a single ZnO microwire

Yun-Zheng Li, Qiu-Ju Feng, Bo Shi, Chong Gao, De-Yu Wang, Hong-Wei Liang
Chin. Phys. B 2020, 29 (1): 018102;  doi: 10.1088/1674-1056/ab593f
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In this paper, ultra-long and large-scaled ZnO microwire arrays are grown by the chemical vapor deposition method, and a single ZnO microwire-based non-balanced electric bridge ethanol gas sensor is fabricated. The experimental results show that the gas sensor has good repeatability, high response rate, short response, and recovery time at room temperature (25 ℃). The response rate of the gas sensor exposed to 90-ppm ethanol is about 93%, with a response time and recovery time are 0.3 s and 0.7 s respectively. As a contrast, the traditional resistive gas sensor of a single ZnO microwire shows very small gas response rate. Therefore, ethanol gas sensor based on non-balanced electric bridge can obviously enhance gas sensing characteristics, which provides a feasible method of developing the high performance ZnO-based gas sensor.

Influences of grain size and microstructure on optical properties of microcrystalline diamond films

Jia-Le Wang, Cheng-Ke Chen, Xiao Li, Mei-Yan Jiang, Xiao-Jun Hu
Chin. Phys. B 2020, 29 (1): 018103;  doi: 10.1088/1674-1056/ab593d
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Microcrystalline diamond (MCD) films with different grain sizes ranging from 160 nm to 2200 nm are prepared by using a hot filament chemical vapor deposition (HFCVD) system, and the influences of grain size and structural features on optical properties are investigated. The results show that the film with grain size in a range of 160 nm-310 nm exhibits a higher refractive index in a range of (2.77-2.92). With grain size increasing to 620±300 nm, the refractive index shows a value between 2.39 and 2.47, approaching to that of natural diamond (2.37-2.55), and a lower extinction coefficient value between 0.08 and 0.77. When the grain size increases to 2200 nm, the value of refractive index increases to a value between 2.66 and 2.81, and the extinction coefficient increases to a value in a range of 0.22-1.28. Visible Raman spectroscopy measurements show that all samples have distinct diamond peaks located in a range of 1331 cm-1-1333 cm-1, the content of diamond phase increases gradually as grain size increases, and the amount of trans-polyacetylene (TPA) content decreases. Meanwhile, the sp2 carbon clusters content and its full-width-at-half-maximum (FWHM) value are significantly reduced in MCD film with a grain size of 620 nm, which is beneficial to the improvement of the optical properties of the films.

Optical and electrical properties of InGaZnON thin films

Jian Ke Yao, Fan Ye, Ping Fan
Chin. Phys. B 2020, 29 (1): 018105;  doi: 10.1088/1674-1056/ab5fc1
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The substrate temperature (Ts) and N2 partial pressure (PN2) dependent optical and electrical properties of sputtered InGaZnON thin films are studied. With the increased Ts and PN2, the thin film becomes more crystallized and nitrified. The Hall mobility, free carrier concentration (Ne), and electrical conductivity increase with the lowered interfacial potential barrier during crystal growing. The photoluminescence (PL) intensity decreases with the increased Ne. The band gap (Eg) narrows and the linear refractive index (n1) increases with the increasing concentration of N in the thin films. The Stokes shift between the PL peak and absorption edge decreases with Eg. The n1, dispersion energy, average oscillator wavelength, and oscillator length strength all increase with n1. The single oscillator energy decreases with n1. The nonlinear refractive index and third order optical susceptibility increase with n1. The Seebeck coefficient, electron effective mass, mean free path, scattering time, and plasma energy are all Ne dependent.

Infrared light-emitting diodes based on colloidal PbSe/PbS core/shell nanocrystals

Byung-Ryool Hyun, Mikita Marus, Huaying Zhong, Depeng Li, Haochen Liu, Yue Xie, Weon-kyu Koh, Bing Xu, Yanjun Liu, Xiao Wei Sun
Chin. Phys. B 2020, 29 (1): 018503;  doi: 10.1088/1674-1056/ab5fb7
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Colloidal PbSe nanocrystals (NCs) have gained considerable attention due to their efficient carrier multiplication and emissions across near-infrared and short-wavelength infrared spectral ranges. However, the fast degradation of colloidal PbSe NCs in ambient conditions hampers their widespread applications in infrared optoelectronics. It is well-known that the inorganic thick-shell over core improves the stability of NCs. Here, we present the synthesis of PbSe/PbS core/shell NCs showing wide spectral tunability, in which the molar ratio of lead (Pb) and sulfur (S) precursors, and the concentration of sulfur and PbSe NCs in solvent have a significant effect on the efficient PbS shell growth. The infrared light-emitting diodes (IR-LEDs) fabricated with the PbSe/PbS core/shell NCs exhibit an external quantum efficiency (EQE) of 1.3% at 1280 nm. The ligand exchange to optimize the distance between NCs and chloride treatment are important processes for achieving high performance on PbSe/PbS NC-LEDs. Our results provide evidence for the promising potential of PbSe/PbS NCs over the wide range of infrared optoelectronic applications.

Phase transition of DNA compaction in confined space: Effects of macromolecular crowding are dominant

Erkun Chen, Yangtao Fan, Guangju Zhao, Zongliang Mao, Haiping Zhou, Yanhui Liu
Chin. Phys. B 2020, 29 (1): 018701;  doi: 10.1088/1674-1056/ab5941
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With a view of detecting the effects of macromolecular crowding on the phase transition of DNA compaction confined in spherical space, Monte Carlo simulations of DNA compaction in free space, in confined spherical space without crowders and in confined spherical space with crowders were performed separately. The simulation results indicate that macromolecular crowding effects on DNA compaction are dominant over the roles of multivalent counterions. In addition, effects of temperature on the phase transition of DNA compaction have been identified in confined spherical space with different radii. In confined spherical space without crowders, the temperature corresponding to phase transition depends on the radius of the confined spherical space linearly. In contrast, with the addition of crowders to the confined spherical space, effects of temperature on the phase transition of DNA compaction become insignificant, whereas the phase transition at different temperatures strongly depends on the size of crowder, and the critical volume fraction of crowders pertains to the diameter of crowder linearly.

Major impact of queue-rule choice on the performance of dynamic networks with limited buffer size

Xiang Ling, Xiao-Kun Wang, Jun-Jie Chen, Dong Liu, Kong-Jin Zhu, Ning Guo
Chin. Phys. B 2020, 29 (1): 018901;  doi: 10.1088/1674-1056/ab5935
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We investigate the similarities and differences among three queue rules, the first-in-first-out (FIFO) rule, last-in-first-out (LIFO) rule and random-in-random-out (RIRO) rule, on dynamical networks with limited buffer size. In our network model, nodes move at each time step. Packets are transmitted by an adaptive routing strategy, combining Euclidean distance and node load by a tunable parameter. Because of this routing strategy, at the initial stage of increasing buffer size, the network density will increase, and the packet loss rate will decrease. Packet loss and traffic congestion occur by these three rules, but nodes keep unblocked and lose no packet in a larger buffer size range on the RIRO rule networks. If packets are lost and traffic congestion occurs, different dynamic characteristics are shown by these three queue rules. Moreover, a phenomenon similar to Braess' paradox is also found by the LIFO rule and the RIRO rule.
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