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Chin. Phys. B  
  Chin. Phys. B--2020, Vol.29, No.5
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SPECIAL TOPIC—Recent advances in thermoelectric materials and devices
SPECIAL TOPIC—Terahertz physics

Single-shot measurement of THz pulses

Lei Yang, Lei Hou, Chengang Dong, Wei Shi
Chin. Phys. B 2020, 29 (5): 057803;  doi: 10.1088/1674-1056/ab821f
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Terahertz (THz) waves have shown a broad prospect in the analysis of some dielectric materials because of their special properties. For the ultrafast irreversible processes, the THz single-shot measurement is a good choice. In this paper, a single-shot system is investigated, where a pump beam is used to generate THz pulses with high electrical field by optical rectification in LiNbO3, the probe beam with wavefront tilted by a blazed grating is used for single-shot measurement. The time window is up to 90 ps, the signal to noise ratio is 2000:1, the spectrum covers from 0.1 THz to about 2.0 THz, and the spectral resolution is 0.011 THz. The single-shot measurement result agrees well with that of a traditional electrical-optic sampling method.

Noise temperature distribution of superconducting hot electron bolometer mixers

Kang-Min Zhou, Wei Miao, Yue Geng, Yan Delorme, Wen Zhang, Yuan Ren, Kun Zhang, Sheng-Cai Shi
Chin. Phys. B 2020, 29 (5): 058505;  doi: 10.1088/1674-1056/ab81f9
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We report on the investigation of optimal bias region of a wide-band superconducting hot electron bolometer (HEB) mixer in terms of noise temperature performance for multi-pixel heterodyne receiver application in the 5-meter Dome A Terahertz Explorer (DATE5) telescope. By evaluating the double sideband (DSB) receiver noise temperature (Trec) across a wide frequency range from 0.2 THz to 1.34 THz and with a large number of bias points, a broad optimal bias region has been observed, illustrating a good bias applicability for multipixel application since the performance of the HEB mixer is uniquely determined by each bias point. The noise temperature of the HEB mixer has been analyzed by calibrating the noise contribution of all RF components, whose transmissions have been measured by a time-domain spectroscopy. The corrected noise temperature distribution shows a frequency independence relation. The dependence of the optimal bias region on the bath temperature of the HEB mixer has also been investigated, the bath temperature has limited effect on the lowest receiver noise temperature until 7 K, however the optimal bias region deteriorates obviously with increasing bath temperature.

Single-order soft x-ray spectra with spectroscopic photon sieve

Yu-Lin Gao, Lai Wei, Qiang-Qiang Zhang, Zu-Zua Yang, Wei-Min Zhou, Lei-Feng Cao
Chin. Phys. B 2020, 29 (5): 054101;  doi: 10.1088/1674-1056/ab81f0
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A single-order diffraction transmission grating named spectroscopic photon sieve (SPS) for soft x-ray region is proposed and demonstrated in this paper. The SPS consists of many circular pinholes located randomly, and can realize both free-standing diffractions and the suppression of higher-order differations. In this paper, the basic concept, numerical simulations, and calibration results of a 1000-lines/mm SPS for soft x-ray synchrotron radiation are presented. As predicted by theoretical calculations, the calibration results of a 1000-lines/mm SPS verify that the higher-order diffractions can be significantly suppressed along the symmetry axis. With the current nanofabrication technique, the SPS can potentially have a higher line density, and can be widely used in synchrotron radiation, laser-induced plasma diagnostics, and astrophysics.

Trajectory engineering via a space-fractional Schrödinger equation with dynamic linear index potential

Yunji Meng, Youwen Liu, Haijiang Lv
Chin. Phys. B 2020, 29 (5): 054201;  doi: 10.1088/1674-1056/ab7b4c
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We theoretically and numerically study the propagation dynamics of a Gaussian beam modeled by the fractional Schrödinger equation with different dynamic linear potentials. For the limited case α=1 (α is the Lévy index) in the momentum space, the beam suffers a frequency shift which depends on the applied longitudinal modulation and the involved chirp. While in the real space, by precisely controlling the linear chirp, the beam will exhibit two different evolution characteristics: one is the zigzag trajectory propagation induced by multi-reflection occurring at the zeros of spatial spectrum, the other is diffraction-free propagation. Numerical simulations are in full accordance with the theoretical results. Increase of the Lévy index not only results in the drift of those turning points along the transverse direction, but also leads to the delocalization of the Gaussian beam.

Pulse shaping of bright-dark vector soliton pair

Yan Zhou, Yuefeng Li, Xia Li, Meisong Liao, Jingshan Hou, Yongzheng Fang
Chin. Phys. B 2020, 29 (5): 054202;  doi: 10.1088/1674-1056/ab7b56
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We simulate pulse shaping of bright-dark vector soliton pair in an optical fiber system. Through changing input pulse parameters (amplitude ratio, projection angle, time delay, and phase difference), different kinds of pulse shapes and spectra can be generated. For input bright-dark vector soliton pair with the same central wavelength, "2+1"- and "2+2"-type pseudo-high-order bright-dark vector soliton pairs are achieved. While for the case of different central wavelengths, bright-dark vector soliton pairs with multiple pulse peaks/dips are demonstrated with appropriate pulse parameter setting.

An image compressed sensing algorithm based on adaptive nonlinear network

Yuan Guo, Wei Chen, Shi-Wei Jing
Chin. Phys. B 2020, 29 (5): 054203;  doi: 10.1088/1674-1056/ab7b4e
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Traditional compressed sensing algorithm is used to reconstruct images by iteratively optimizing a small number of measured values. The computation is complex and the reconstruction time is long. The deep learning-based compressed sensing algorithm can greatly shorten the reconstruction time, but the algorithm emphasis is placed on reconstructing the network part mostly. The random measurement matrix cannot measure the image features well, which leads the reconstructed image quality to be improved limitedly. Two kinds of networks are proposed for solving this problem. The first one is ReconNet's improved network IReconNet, which replaces the traditional linear random measurement matrix with an adaptive nonlinear measurement network. The reconstruction quality and anti-noise performance are greatly improved. Because the measured values extracted by the measurement network also retain the characteristics of image spatial information, the image is reconstructed by bilinear interpolation algorithm (Bilinear) and dilate convolution. Therefore a second network USDCNN is proposed. On the BSD500 dataset, the sampling rates are 0.25, 0.10, 0.04, and 0.01, the average peak signal-noise ratio (PSNR) of USDCNN is 1.62 dB, 1.31 dB, 1.47 dB, and 1.95 dB higher than that of MSRNet. Experiments show the average reconstruction time of USDCNN is 0.2705 s, 0.3671 s, 0.3602 s, and 0.3929 s faster than that of ReconNet. Moreover, there is also a great advantage in anti-noise performance.

Light slowing and all-optical time division multiplexing of hybrid four-wave mixing signal in nitrogen-vacancy center

Ruimin Wang, Irfan Ahmed, Faizan Raza, Changbiao Li, Yanpeng Zhang
Chin. Phys. B 2020, 29 (5): 054204;  doi: 10.1088/1674-1056/ab7b50
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We report the experimental results of hybrid four-wave mixing and fluorescence signals from nitrogen-vacancy (NV) centers in diamond. The fluorescence signals are slowed owing to dark state. The observed delay time of light slowing due to interconversion between NV- and NV0 is about 6.4 μs. The relative intensities of read-out signals change with the wavelength and power of writing pulse. Based on light slowing, we present the model of all-optical time division multiplexing. The intensity ratio in different demultiplexed channels is modulated by the wavelength and power of control field. It has potential applications in quantum communication and all-optical network.

Cascaded optical frequency transfer over 500-km fiber link using regenerative amplifier

Xue Deng, Dong-Dong Jiao, Jie Liu, Qi Zang, Xiang Zhang, Dan Wang, Jing Gao, Rui-Fang Dong, Tao Liu, Shou-Gang Zhang
Chin. Phys. B 2020, 29 (5): 054205;  doi: 10.1088/1674-1056/ab7b4f
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We demonstrate a 300-km+200-km cascaded coherent phase transfer via fiber link. The transfer is divided into a 300-km span and a 200-km span with independent phase locking loops, aiming to extend the phase control bandwidth of the whole link. The phase noise and transfer instability of the cascaded transmission are investigated and compared with those in the case of a single-span 500-km transfer. We achieve the transfer instabilities of 1.8×10-14 at 1 s, 8.9×10-20 at 104 s for the 300-km +200-km cascaded transmission, and 2.7×10-14 at 1 s for the 500-km single-span transfer.

High common mode rejection ratio InP 90° optical hybrid in ultra-broadband at 60 nm with deep-rigded waveguide based on ×4 MMI coupler

Zi-Qing Lu, Qin Han, Han Ye, Shuai Wang, Feng Xiao, Fan Xiao
Chin. Phys. B 2020, 29 (5): 054206;  doi: 10.1088/1674-1056/ab7b52
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An InP optical 90° hybrid based on a×4 MMI coupler with a deep ridged waveguide is designed and fabricated. The working principle of the 90° hybrid is systematically introduced. Three-dimensional beam ropagation method (3D BPM) is used to optimize the structure parameters of the 90° hybrid. The designed compact structure is demonatrated to have a low excess loss less than -0.15 dB, a high common mode rejection ratio better than 40 dB, and a low relative phase deviation less than ±2.5°. The designed hybrid is manufactured on a sandwitched structure deposited on an InP substrate. The measured results show that the common mode rejection ratios are larger than 20 dB in a range from 1520 nm to 1580 nm. The phase deviations are less than ±5° in a range from 1545 nm to 1560 nm and less than ±7° across the C band. The designed 90° optical hybrid is suitable well for realizing miniaturization, high-properties, and high bandwidth of coherent receiver.

High gain fiber-solid hybrid double-passing end-pumped Nd: YVO4 picosecond amplifier with high beam quality

Xueyan Dong, Pingxue Li, Shun Li, Dongsheng Wang
Chin. Phys. B 2020, 29 (5): 054207;  doi: 10.1088/1674-1056/ab8218
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We propose a fiber-solid hybrid system which consists of a semiconductor saturable absorber mirror (SESAM) mode-locked fiber seed with a pulse width of 10.2 ps and a repetition rate of 18.9 MHz, a two-level fiber pre-amplifier and a double-passing end-pumped Nd:YVO4 amplifier. In the solid-state amplifier, to enhance the gain and the extraction efficiency, a specially designed structure in which the seed light passes through the gain medium four times and makes full use of population inversion is used as the double-passing amplifier. Besides, the beam filling factor (the ratio of the seed light diameter to the pump light diameter) and the thermal lens effect of the double-passing amplifier are considered and its optical-to-optical conversion efficiency is further improved. To preserve the beam quality of the double-passing amplifier, a new method of spherical-aberration self-compensation based on the principles of geometrical optics is used and discussed. Our system achieves a maximum average power of 9.5 W at the pump power of 28 W, corresponding to an optical-to-optical efficiency of 27%. And the beam quality factor M2 reaches 1.3 at the maximum output power.

Influences of annealing temperature on properties of Fe2+: ZnSe thin films deposited by electron beam evaporation and their applications to Q-switched fiber laser

Du-Xin Qing, Shu-Tong Wang, Shou-Gui Ning, Wei Zhang, Xiao-Xu Chen, Hong Zhang, Guo-Ying Feng, Shou-Huan Zhou
Chin. Phys. B 2020, 29 (5): 054208;  doi: 10.1088/1674-1056/ab81f2
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Fe2+:ZnSe thin films are prepared on sapphire substrate at room temperature by electron beam evaporation and then annealed in vacuum (about 1×10-4 Pa) at different temperatures. The influences of thermal annealing on the structural and optical properties of these films such as grain size and optical transmittance are investigated. The x-ray diffraction patterns show that the Fe2+:ZnSe thin film is preferred to be oriented along the (111) plane at different annealing temperatures. After the film is annealed, the full-width-at-half-maximum (FWHM ) of the x-ray diffraction peak profile (111) of the film decreases and its crystal quality is improved. Scanning electron microscope images show that the films are more dense after being annealed. Finally, the sample is used as a saturable absorber in ZBLAN fiber laser. The annealed Fe2+:ZnSe thin films can be used to realize stable Q-switching modulation on ZBLAN fiber laser. The results demonstrate that the Fe2+:ZnSe thin film is a promising material for generating the high-power pulses of mid-infrared Q-switched fiber lasers.

Optical modulation of repaired damage site on fused silica produced by CO2 laser rapid ablation mitigation

Chao Tan, Lin-Jie Zhao, Ming-Jun Chen, Jian Cheng, Zhao-Yang Yin, Qi Liu, Hao Yang, Wei Liao
Chin. Phys. B 2020, 29 (5): 054209;  doi: 10.1088/1674-1056/ab8206
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CO2 laser rapid ablation mitigation (RAM) of fused silica has been used in high-power laser systems owing to its advantages of high efficiency, and ease of implementing batch and automated repairing. In order to study the effect of repaired morphology of RAM on laser modulation and to improve laser damage threshold of optics, an finite element method (FEM) mathematical model of 351 nm laser irradiating fused silica optics is developed based on Maxwell electromagnetic field equations, to explore the 3D near-field light intensity distribution inside optics with repaired site on its surface. The influences of the cone angle and the size of the repaired site on incident laser modulation are studied as well. The results have shown that for the repaired site with a cone angle of 73.3°, the light intensity distribution has obvious three-dimensional characteristics. The relative light intensity on z-section has a circularly distribution, and the radius of the annular intensification zone increases with the decrease of z. While the distribution of maximum relative light intensity on y-section is parabolical with the increase of y. As the cone angle of the repaired site decreases, the effect of the repaired surface on light modulation becomes stronger, leading to a weak resistance to laser damage. Moreover, the large size repaired site would also reduce the laser damage threshold. Therefore, a repaired site with a larger cone angle and smaller size is preferred in practical CO2 laser repairing of surface damage. This work will provide theoretical guidance for the design of repaired surface topography, as well as the improvement of RAM process.

Narrowband perfect terahertz absorber based on polar-dielectrics metasurface

Meng-Meng Zhao, Shu-Fang Fu, Sheng Zhou, Yu-Ling Song, Qiang Zhang, Yong-Qi Yin, Yu-Tian Zhao, Hong Liang, Xuan-Zhang Wang
Chin. Phys. B 2020, 29 (5): 054210;  doi: 10.1088/1674-1056/ab81fa
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We theoretically propose a narrowband perfect absorber metasurface (PAMS) based on surface phonon polaritons in the terahertz range. The PAMS has unit cell consisting of a silver biarc on the top, a thin polar-dielectric in the middle and a silver layer at the bottom. The phonon polaritons are excited at the interface between the silver biarc and the polar dielectric, and enhance the absorption of the PAMS. The absorption peak is at 36.813 μm and the full width half maximum (FWHM) is nearly 36 nm, independent of the polarization and incidence angle. The electric fields are located at the split of the biarc silver layer and the quality factor Q is 1150. The FWHM decreases with the decreasing split width. When the thickness of the bottom layer is larger than 50 nm, the narrow band and high absorption are insensitive to the thickness of those layers. The designed absorber may have useful applications in terahertz spectra such as energy harvesting, thermal emitter, and sensing.

Electromagnetically induced transparency and electromagnetically induced absorption in Y-type system

Kalan Mal, Khairul Islam, Suman Mondal, Dipankar Bhattacharyya, Amitava Bandyopadhyay
Chin. Phys. B 2020, 29 (5): 054211;  doi: 10.1088/1674-1056/ab7ea0
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The propagation of a probe field through a four-level Y-type atomic system is described in the presence of two additional coherent radiation fields, namely, the control field and the coupling field. An expression for the probe response is derived analytically from the optical Bloch equations under steady state condition to study the absorptive properties of the system under probe field propagation through an ensemble of stationary atoms as well as in a Doppler broadened atomic vapor medium. The most striking result is the conversion of electromagnetically induced transparency (EIT) into electromagnetically induced absorption (EIA) as we start switching from weak probe regime to strong probe regime. The dependence of this conversion on residual Doppler averaging due to wavelength mismatch is also shown by choosing the coupling transition as a Rydberg transition.

Location of micro-cracks in plates using time reversed nonlinear Lamb waves

Yaoxin Liu, Aijun He, Jiehui Liu, Yiwei Mao, Xiaozhou Liu
Chin. Phys. B 2020, 29 (5): 054301;  doi: 10.1088/1674-1056/ab81f7
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A promising tool to detect micro-cracks in plate-like structures is used for generating higher harmonic Lamb waves. In this paper, a method combining nonlinear S0 mode Lamb waves with time reversal to locate micro-cracks is presented and verified by numerical simulations. Two different models, the contact acoustic nonlinearity (CAN) model and the Preisach-Mayergoyz (PM) model, are used to simulate a localized damage in a thin plate. Pulse inversion method is employed to extract the second and fourth harmonics from the received signal. Time reversal is performed to compensate the dispersion of S0 mode Lamb waves. Consequently, the higher harmonics generated from the damaged area can be refocused on their source. By investigating the spatial distribution of harmonic wave packets, the location of micro-cracks will be revealed. The numerical simulations indicate that this method gives accurate locations of the damaged area in a plate. Furthermore, the PM model is proved to be a suitable model to simulate the micro-cracks in plates for generation of higher harmonics.

Pulling force of acoustic-vortex beams on centered elastic spheres based on the annular transducer model

Yuzhi Li, Qingdong Wang, Gepu Guo, Hongyan Chu, Qingyu Ma, Juan Tu, Dong Zhang
Chin. Phys. B 2020, 29 (5): 054302;  doi: 10.1088/1674-1056/ab8210
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To solve the difficulty of generating an ideal Bessel beam, an simplified annular transducer model is proposed to study the axial acoustic radiation force (ARF) and the corresponding negative ARF (pulling force) exerted on centered elastic spheres for acoustic-vortex (AV) beams of arbitrary orders. Based on the theory of acoustic scattering, the axial distributions of the velocity potential and the ARF for AV beams of different orders generated by the annular transducers with different physical sizes are simulated. It is proved that the pulling force can be generated by AV beams of arbitrary orders with multiple axial regions. The pulling force is more likely to exert on the sphere with a smaller k0a (product of the wave number and the radius) for the AV beam with a bigger topological charge due to the strengthened off-axis acoustic scattering. The pulling force decreases with the increase of the axial distance for the sphere with a bigger k0a. More pulling force areas with wider axial regions can be formed by AV beams using a bigger-sized annular transducer. The theoretical results demonstrate the feasibility of generating the pulling force along the axes of AV beams using the experimentally applicable circular array of planar transducers, and suggest application potentials for multi-position stable object manipulations in biomedical engineering.

Single SH guided wave mode generation method for PPM EMATs

Guo-Fu Zhai, Yong-Qian Li
Chin. Phys. B 2020, 29 (5): 054303;  doi: 10.1088/1674-1056/ab81f8
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Using periodic permanent magnet (PPM) electromagnetic acoustic transducers (EMATs), different shear horizontal (SH) guided wave modes can form simultaneously in some situations, which can interfere with the inspection. The main cause of this phenomenon (typically named multiple modes) is related to the frequency bandwidth of excitation signals and the transducer spatial bandwidth. Simply narrowing the frequency bandwidth cannot effectively limit the number of different SH modes. Previous researches showed that unnecessary SH wave modes can be eliminated by using dual EMATs. However, in practical applications, it is more convenient to change the excitation frequency than to use dual EMATs. In this paper, the stress boundary conditions of the PPM-EMAT are analyzed, the analytical expression of SH guided wave is established, and the magnitude of SH guided wave mode under continuous tone and tone-burst input is obtained. A method to generate a single SH mode by re-selecting an operating point is proposed. Furthermore, the influence of the frequency bandwidth of the tone-burst signal is analyzed. Finally, a single SH mode excitation is achieved with tone-burst input.

Efficient tensor decomposition method for noncircular source in colocated coprime MIMO radar

Qian-Peng Xie, Xiao-Yi Pan, Shun-Ping Xiao
Chin. Phys. B 2020, 29 (5): 054304;  doi: 10.1088/1674-1056/ab81f6
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An effective method via tensor decomposition is proposed to deal with the joint direction-of-departure (DOD) and direction-of-arrival (DOA) estimation of noncircular sources in colocated coprime MIMO radar. By decomposing the transmitter and receiver into two sparse subarrays, noncircular property of source can be used to construct new extended received signal model for two sparse subarrays. The new received model can double the virtual array aperture due to the elliptic covariance of imping sources is nonzero. To further exploit the multidimensional structure of the noncircular received model, we stack the subarray output and its conjugation according to mode-1 unfolding and mode-2 unfolding of a third-order tensor, respectively. Thus, the corresponding extended tensor model consisted of noncircular information for DOA and DOD can be obtained. Then, the higher-order singular value decomposition technique is utilized to estimate the accurate signal subspace and angular parameter can be automatically paired via the rotational invariance relationship. Specifically, the ambiguous angle can be eliminated and the true targets can be achieved with the aid of the coprime property. Furthermore, a closed-form expression for the deterministic CRB under the NC sources scenario is also derived. Simulation results verify the superiority of the proposed estimator.

Magnetoacoustic position imaging for liquid metal in animal interstitial structure

Xiao-He Zhao, Guo-Qiang Liu, Hui Xia, Yan-Hong Li
Chin. Phys. B 2020, 29 (5): 054305;  doi: 10.1088/1674-1056/ab8207
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Magnetoacoustic tomography with magnetic induction (MAT-MI), as a new kind of in-vivo imaging method, has potential application value in interstitial fluid research. In this paper, we propose the application of MAT-MI with liquid metal serving as a tracer of the interstitial structure to study its fluid behavior, and use it to implement the positional imaging of the spatial distribution of liquid metal. Owing to the particularity of liquid metal magnetoacoustic pressure (MAP) signals, we propose an envelope analysis method to extract the rising edge of the amplitude envelope of the detected waveform as effective position data. And for the first time, we propose the method of superpositing pixel matrix to achieve the position imaging of liquid metal. Finally, the positional imaging of the liquid metal sample embedded in the gel is achieved to have relatively accurate results. This study provides a method of effectively extracting data and implementing the position imaging for liquid metal in the interstitial structure in the frame of MAT-MI.

Growth induced buckling of morphoelastic rod in viscous medium

Yitong Zhang, Shuai Zhang, Peng Wang
Chin. Phys. B 2020, 29 (5): 054501;  doi: 10.1088/1674-1056/ab7b4d
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Biological growth is a common phenomenon in nature, and some organisms such as DNA molecules and bacterial filaments grow in viscous media. The growth induced instability of morphoelastic rod in a viscous medium is studied in this paper. Based on the Kirchhoff kinetic analogy method, the mechanical model for growing elastic thin rod in the viscous medium is established. A perturbation analysis is used to analyze the stability of the growing elastic rod in the viscous medium. We apply the results into planar growing ring and get its criterion of instability. Take the criterion into DNA ring to discuss the influence of viscous resistance on its instability.

Parametric study of the clustering transition in vibration driven granular gas system

Qi-Lin Wu, Mei-Ying Hou, Lei Yang, Wei Wang, Guang-Hui Yang, Ke-Wei Tao, Liang-Wen Chen, Sheng Zhang
Chin. Phys. B 2020, 29 (5): 054502;  doi: 10.1088/1674-1056/ab8217
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A parametric study of the clustering transition of a vibration-driven granular gas system is performed by simulation. The parameters studied include the global volume fraction of the system, the size of the system, the friction coefficient, and the restitution coefficient among particles and among particle-walls. The periodic boundary and fixed boundary of sidewalls are also checked in the simulation. The simulation results provide us the necessary "heating" time for the system to reach steady state, and the friction term needed to be included in the "cooling" time. A gas-cluster phase diagram obtained through Kolmogorov-Smirnov (K-S) test analysis using similar experimental parameters is given. The influence of the parameters to the transition is then investigated in simulations. This simulation investigation helps us gain understanding which otherwise cannot be obtained by experiment alone, and makes suggestions on the determination of parameters to be chosen in experiments.

Numerical study on permeability characteristics of fractal porous media

Yongping Huang, Feng Yao, Bo Zhou, Chengbin Zhang
Chin. Phys. B 2020, 29 (5): 054701;  doi: 10.1088/1674-1056/ab7b53
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The fractal Brownian motion is utilized to describe pore structures in porous media. A numerical model of laminar flow in porous media is developed, and the flow characteristics are comprehensively analyzed and compared with those of homogeneous porous media. Moreover, the roles of the fractal dimension and porosity in permeability are quantitatively described. The results indicate that the pore structures of porous media significantly affect their seepage behaviors. The distributions of pressure and velocity in fractal porous media are both non-uniform; the streamline is no longer straight but tortuous. When Reynolds number Re < 1, the dimensionless permeability is independent of Reynolds number, but its further increase will lead to a smaller permeability. Moreover, due to the higher connectivity and enlarged equivalent aperture of internal channel network, the augment in porosity leads to the permeability enhancement, while it is small and insensitive to porosity variation when ε < 0.6. Fractal dimension also plays a significant role in the permeability of porous media. The increase in fractal dimension leads to the enhancement in pore connectivity and a decrease in channel tortuosity, which reduces the flow resistance and improves the transport capacity of porous media.

Droplets breakup via a splitting microchannel

Wei Gao, Cheng Yu, Feng Yao
Chin. Phys. B 2020, 29 (5): 054702;  doi: 10.1088/1674-1056/ab7b4b
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On the basis of a volume of fluid (VOF) liquid/liquid interface tracking method, we apply a two-dimensional model to investigate the dynamic behaviors of droplet breakup through a splitting microchannel. The feasibility and applicability of the theoretical model are experimentally validated. Four flow regimes are observed in the splitting microchannel, that is, breakup with permanent obstruction, breakup with temporary obstruction, breakup with tunnels, and non-breakup. The results indicate that the increase of the capillary number Ca provides considerable upstream pressure to accelerate the droplet deformation, which is favorable for the droplet breakup. The decrease of the droplet size contributes to its shape changing from the plug to the sphere, which results in weakening droplet deformation ability and generating the non-breakup flow regime.

Magnetic properties of La2CuMnO6 double perovskite ceramic investigated by Monte Carlo simulations

S Mtougui, I EL Housni, N EL Mekkaoui, S Ziti, S Idrissi, H Labrim, R Khalladi, L Bahmad
Chin. Phys. B 2020, 29 (5): 056101;  doi: 10.1088/1674-1056/ab7d95
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We present a theoretical study of the magnetic properties of the lanthanum copper manganate double perovskite La2CuMnO6 ceramic, using Monte Carlo simulations. We analyze and discuss the ground state phase diagrams in different planes to show the effect of every physical parameter. Based on the Monte Carlo simulations, which combine Metropolis algorithm and Ising model, we explore the thermal behavior of the total magnetization and susceptibility. We also present and discuss the influence of physical parameters such as the external magnetic field, the exchange coupling interactions between magnetic atoms, and the exchange magnetic field on the magnetization of the system. Moreover, the critical temperature of the system is about Tc=70 K, in agreement with the experimental value. Finally, the hysteresis loops of La2CuMnO6 are discussed.

Modification of the Peierls-Nabarro model for misfit dislocation

Shujun Zhang, Shaofeng Wang
Chin. Phys. B 2020, 29 (5): 056102;  doi: 10.1088/1674-1056/ab8459
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For a misfit dislocation, the balance equations satisfied by the displacement fields are modified, and an extra term proportional to the second-order derivative appears in the resulting misfit equation compared with the equation derived by Yao et al. This second-order derivative describes the lattice discreteness effect that arises from the surface effect. The core structure of a misfit dislocation and the change in interfacial spacing that it induces are investigated theoretically in the framework of an improved Peierls-Nabarro equation in which the effect of discreteness is fully taken into account. As an application, the structure of the misfit dislocation for a honeycomb structure in a two-dimensional heterostructure is presented.

Bose-Einstein condensates in an eightfold symmetric optical lattice

Zhen-Xia Niu, Yong-Hang Tai, Jun-Sheng Shi, Wei Zhang
Chin. Phys. B 2020, 29 (5): 056103;  doi: 10.1088/1674-1056/ab8211
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We investigate the properties of Bose-Einstein condensates (BECs) in a two-dimensional quasi-periodic optical lattice (OL) with eightfold rotational symmetry by numerically solving the Gross-Pitaevskii equation. In a stationary external harmonic trapping potential, we first analyze the evolution of matter-wave interference pattern from periodic to quasi-periodic as the OL is changed continuously from four-fold periodic to eight-fold quasi-periodic. We also investigate the transport properties during this evolution for different interatomic interaction and lattice depth, and find that the BEC crosses over from ballistic diffusion to localization. Finally, we focus on the case of eightfold symmetric lattice and consider a global rotation imposed by the external trapping potential. The BEC shows vortex pattern with eightfold symmetry for slow rotation, becomes unstable for intermediate rotation, and exhibits annular solitons with approximate axial symmetry for fast rotation. These results can be readily demonstrated in experiments using the same configuration as in Phys. Rev. Lett. 122 110404 (2019).

Electronic structure and phase transition engineering in NbS2: Crucial role of van der Waals interactions

Wei Wang, Wen Lei, Xiaojun Zheng, Huan Li, Xin Tang, Xing Ming
Chin. Phys. B 2020, 29 (5): 056201;  doi: 10.1088/1674-1056/ab8214
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Based on first-principles simulations, we revisit the crystal structures, electronic structures, and structural stability of the layered transition metal dichalcogenides (TMDCs) NbS2, and shed more light on the crucial roles of the van der Waals (vdW) interactions. Theoretically calculated results imply that the vdW corrections are important to reproduce the layered crystal structure, which is significant to correctly describe the electronic structure of NbS2. More interestingly, under hydrostatic pressure or tensile strain in ab plane, an isostructural phase transition from two-dimensional layered structure to three-dimensional bulk in the I4/mmm phase has been uncovered. The abnormal structural transition is closely related to the electronic structure instability and interlayer bonding effects. The interlayer Nb-S distances collapse and the interlayer vdW interactions disappear, concomitant with new covalent bond emerging and increasing coordination number. Present work highlights the significance of the vdW interactions, and provides new insights on the unconventional structural transitions in NbS2, which will attract wide audience working in the hectic field of TMDCs.

Effect of chemical ordering annealing on superelasticity of Ni-Mn-Ga-Fe ferromagnetic shape memory alloy microwires

Yanfen Liu, Xuexi Zhang, Hongxian Shen, Jianfei Sun, Qinan Li, Xiaohua Liu, Jianjun Li, Weidong Cheng
Chin. Phys. B 2020, 29 (5): 056202;  doi: 10.1088/1674-1056/ab8213
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Ni50Mn25Ga20Fe5 ferromagnetic shape memory alloy microwires with diameters of ~30-50μm and grain sizes of ~2-5μm were prepared by melt-extraction technique. A step-wise chemical ordering annealing was carried out to improve the superelasticity strain and recovery ratio which were hampered by the internal stress, compositional inhomogeneity, and high-density defects in the as-extracted Ni50Mn25Ga20Fe5 microwires. The annealed microwires exhibited enhanced atomic ordering degree, narrow thermal hysteresis, and high saturation magnetization under a low magnetic field. As a result, the annealed microwire showed decreased superelastic critical stress, improved reversibility, and a high superelastic strain (1.9%) with a large recovery ratio (>96%). This kind of filamentous material with superior superelastic effects may be promising materials for minor-devices.

Raman scattering study of two-dimensional magnetic van der Waals compound VI3

Yi-Meng Wang, Shang-Jie Tian, Cheng-He Li, Feng Jin, Jian-Ting Ji, He-Chang Lei, Qing-Ming Zhang
Chin. Phys. B 2020, 29 (5): 056301;  doi: 10.1088/1674-1056/ab8215
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The layered magnetic van der Waals materials have generated tremendous interest due to their potential applications and importance in fundamental research. Previous x-ray diffraction (XRD) studies on the magnetic van der Waals compound VI3, revealed a structural transition above the magnetic transition but output controversial analysis on symmetry. In this paper we carried out polarized Raman scattering measurements on VI3 from 10 K to 300 K, with focus on the two Ag phonon modes at ~ 71.1 cm-1 and 128.4 cm-1. Our careful symmetry analysis based on the angle-dependent spectra demonstrates that the crystal symmetry can be well described by C2h rather than D3d both above and below structural phase transition. We further performed temperature-dependent Raman experiments to study the magnetism in VI3. Fano asymmetry and anomalous linewidth drop of two Ag phonon modes at low temperatures, point to a significant spin-phonon coupling. This is also supported by the softening of 71.1-cm-1 mode above the magnetic transition. The study provides the fundamental information on lattice dynamics and clarifies the symmetry in VI3. And spin-phonon coupling existing in a wide temperature range revealed here may be meaningful in applications.

Effect of Ni substitution on the formability and magnetic properties of Gd50Co50 amorphous alloy

Ben-Zheng Tang, Xiao-Ping Liu, Dong-Mei Li, Peng Yu, Lei Xia
Chin. Phys. B 2020, 29 (5): 056401;  doi: 10.1088/1674-1056/ab7da8
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A small amount of Ni was added into the binary Gd50Co50 amorphous alloy to replace Gd in order to obtain ternary Co50Gd50-xNx (x= 1, 2, and 3) amorphous alloys. Compared to the binary Gd50Co50 amorphous alloy, the Co50Gd50-xNx amorphous alloys show an enhanced Curie temperature (TC) with a weakened formability. The maximum magnetic entropy change (-ΔSmpeak) of the Co50Gd50-xNx amorphous alloys is found to decrease with the increasing TC. The adiabatic temperature rise (ΔTad) of the Co50Gd47Ni3 amorphous alloy is superior to that of the Fe-based metallic glasses at room temperature. The variation of the TC and -ΔSmpeak of the Gd50Co50 amorphous alloy with Ni addition, and the mechanism involved, were discussed.

Growth and structural characteristics of metastable β-In2Se3 thin films on H-terminated Si(111) substrates by molecular beam epitaxy

Yi-Fan Shen, Xi-Bo Yin, Chao-Fan Xu, Jing He, Jun-Ye Li, Han-Dong Li, Xiao-Hong Zhu, Xiao-Bin Niu
Chin. Phys. B 2020, 29 (5): 056402;  doi: 10.1088/1674-1056/ab820f
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Epitaxial growth and structural characteristics of metastable β-In2Se3 thin films on H-terminated Si(111) substrates are studied. The In2Se3 thin films grown below the β-to-α phase transition temperature (453 K) are characterized to be strained β-In2Se3 mixed with significant γ-In2Se3 phases. The pure-phased single-crystalline β-In2Se3 can be reproducibly achieved by in situ annealing the as-deposited poly-crystalline In2Se3 within the phase equilibrium temperature window of β-In2Se3. It is suggeted that the observed γ-to-β phase transition triggered by quite a low annealing temperature should be a rather lowered phase transition barrier of the epitaxy-stabilized In2Se3 thin-film system at a state far from thermodynamic equilibrium.

STM study of selenium adsorption on Au(111) surface

Bin Liu, Yuan Zhuang, Yande Que, Chaoqiang Xu, Xudong Xiao
Chin. Phys. B 2020, 29 (5): 056801;  doi: 10.1088/1674-1056/ab821d
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Adsorption of chalcogen atoms on metal surfaces has attracted increasing interest for both the fundamental research and industrial applications. Here, we report a systematic study of selenium (Se) adsorption on Au(111) at varies substrate temperatures by scanning tunneling microscopy. At room temperature, small Se clusters are randomly dispersed on the surface. Increasing the temperature up to 200℃, a well-ordered lattice of Se molecules consisting of 8 Se atoms in ring-like structure is formed. Further increasing the temperature to 250℃ gives rise to the formation of Se monolayer with Au(111)-√3×√3 lattices superimposed with a quasi-hexagonal lattice. Desorption of Se atoms rather than the reaction between the Se atoms and the Au substrate occurs if further increasing the temperature. The ordered structures of selenium monolayers could serve as templates for self-assemblies and our findings in this work might provide insightful guild for the epitaxial growth of the two-dimensional transition metal dichalcogenides.

Influence of spherical inclusions on effective thermoelectric properties of thermoelectric composite materials

Wen-Kai Yan, Ai-Bing Zhang, Li-Jun Yi, Bao-Lin Wang, Ji Wang
Chin. Phys. B 2020, 29 (5): 057301;  doi: 10.1088/1674-1056/ab7d96
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A homogenization theory is developed to predict the influence of spherical inclusions on the effective thermoelectric properties of thermoelectric composite materials based on the general principles of thermodynamics and Mori-Tanaka method. The closed-form solutions of effective Seebeck coefficient, electric conductivity, heat conductivity, and figure of merit for such thermoelectric materials are obtained by solving the nonlinear coupled transport equations of electricity and heat. It is found that the effective figure of merit of thermoelectric material containing spherical inclusions can be higher than that of each constituent in the absence of size effect and interface effect. Some interesting examples of actual thermoelectric composites with spherical inclusions, such as insulated cavities, inclusions subjected to conductive electric and heat exchange and thermoelectric inclusions, are considered, and the numerical results lead to the conclusion that considerable enhancement of the effective figure of merit is achievable by introducing inclusions. In this paper, we provide a theoretical foundation for analytically and computationally treating the thermoelectric composites with more complicated inclusion structures, and thus pointing out a new route to their design and optimization.

Selective excitation of multipolar surface plasmon in a graphene-coated dielectric particle by Laguerre Gaussian beam

Yang Yang, Guanghua Zhang, Xiaoyu Dai
Chin. Phys. B 2020, 29 (5): 057302;  doi: 10.1088/1674-1056/ab7e97
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Localized surface plasmonic resonance has attracted extensive attention since it allows for great enhancement of local field intensity on the nanoparticle surface. In this paper, we make a systematic study on the excitation of localized surface plasmons of a graphene coated dielectric particle. Theoretical results show that both the intensity and frequency of the plasmonic resonant peak can be tuned effectively through modifying the graphene layer. Furthermore, high order localized surface plasmons could be excited and tuned selectively by the Laguerre Gaussian beam, which is induced by the optical angular orbital momentum transfer through the mutual interaction between the particle and the helical wavefront. Moreover, the profiles of the multipolar localized surface plasmons are illustrated in detail. The study provides rich potential applications in the plasmonic devices and the wavefront engineering nano-optics.

Seeing Dirac electrons and heavy fermions in new boron nitride monolayers

Yu-Jiao Kang, Yuan-Ping Chen, Jia-Ren Yuan, Xiao-Hong Yan, Yue-E Xie
Chin. Phys. B 2020, 29 (5): 057303;  doi: 10.1088/1674-1056/ab7e9e
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Most three-dimensional (3D) and two-dimensional (2D) boron nitride (BN) structures are wide-band-gap insulators. Here, we propose two BN monolayers having Dirac points and flat bands, respectively. One monolayer is named as 5-7 BN that consists of five- and seven-membered rings. The other is a Kagome BN made of triangular boron rings and nitrogen dimers. The two structures show not only good dynamic and thermodynamic stabilities but also novel electronic properties. The 5-7 BN has Dirac points on the Fermi level, indicating that the structure is a typical Dirac material. The Kagome BN has double flat bands just below the Fermi level, and thus there are heavy fermions in the structure. The flat-band-induced ferromagnetism is also revealed. We analyze the origination of the band structures by partial density of states and projection of orbitals. In addition, a possible route to experimentally grow the two structures on some suitable substrates such as the PbO2 (111) surface and the CdO (111) surface is also discussed, respectively. Our research not only extends understanding on the electronic properties of BN structures, but also may expand the applications of BN materials in 2D electronic devices.

Dynamics of entropic uncertainty for three types of three-level atomic systems under the random telegraph noise

Xiong Xu, Mao-Fa Fang
Chin. Phys. B 2020, 29 (5): 057305;  doi: 10.1088/1674-1056/a67e9c
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We study the dynamics of the entropic uncertainty for three types of three-level atomic systems coupled to an environment modeled by random matrices. The results show that the entropic uncertainty in the Ξ-type atomic system is lower than that in the V-type atomic system which is exactly the same as that in the Λ-type atomic system. In addition, the effect of relative coupling strength on entropic uncertainty is opposite in Markov region and non-Markov region, and the influence of a common environment and independent environments in Markov region and non-Markov region is also opposite. One can reduce the entropic uncertainty by decreasing relative coupling strength or placing the system in two separate environments in the Markov case. In the non-Markov case, the entropic uncertainty can be reduced by increasing the relative coupling strength or by placing the system in a common environment.

Improvements in reverse breakdown characteristics of THz GaAs Schottky barrier varactor based on metal-brim structure

Lu-Wei Qi, Xiao-Yu Liu, Jin Meng, De-Hai Zhang, Jing-Tao Zhou
Chin. Phys. B 2020, 29 (5): 057306;  doi: 10.1088/1674-1056/ab81fc
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The excellent reverse breakdown characteristics of Schottky barrier varactor (SBV) are crucially required for the application of high power and high efficiency multipliers. The SBV with a novel Schottky structure named metal-brim is fabricated and systemically evaluated. Compared with normal structure, the reverse breakdown voltage of the new type SBV improves from -7.31 V to -8.75 V. The simulation of the Schottky metal-brim SBV is also proposed. Three factors, namely distribution of leakage current, the electric field, and the area of space charge region are mostly concerned to explain the physical mechanism. Schottky metal-brim structure is a promising approach to improve the reverse breakdown voltage and reduce leakage current by eliminating the accumulation of charge at Schottky electrode edge.

High performance InAlN/GaN high electron mobility transistors for low voltage applications

Minhan Mi, Meng Zhang, Sheng Wu, Ling Yang, Bin Hou, Yuwei Zhou, Lixin Guo, Xiaohua Ma, Yue Hao
Chin. Phys. B 2020, 29 (5): 057307;  doi: 10.1088/1674-1056/ab821e
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A high performance InAlN/GaN high electron mobility transistor (HEMT) at low voltage operation (6-10 V drain voltage) has been fabricated. An 8 nm InAlN barrier layer is adopted to generate large 2DEG density thus to reduce sheet resistance. Highly scaled lateral dimension (1.2 μm source-drain spacing) is to reduce access resistance. Both low sheet resistance of the InAlN/GaN structure and scaled lateral dimension contribute to an high extrinsic transconductance of 550 mS/mm and a large drain current of 2.3 A/mm with low on-resistance (Ron) of 0.9 Ω·mm. Small signal measurement shows an fT/fmax of 131 GHz/196 GHz. Large signal measurement shows that the InAlN/GaN HEMT can yield 64.7%-52.7% (Vds=6-10 V) power added efficiency (PAE) associated with 1.6-2.4 W/mm output power density at 8 GHz. These results demonstrate that GaN-based HEMTs not only have advantages in the existing high voltage power and high frequency rf field, but also are attractive for low voltage mobile compatible rf applications.

Application of graphene vertical field effect to regulation of organic light-emitting transistors

Hang Song, Hao Wu, Hai-Yang Lu, Zhi-Hao Yang, Long Ba
Chin. Phys. B 2020, 29 (5): 057401;  doi: 10.1088/1674-1056/ab81f3
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The luminescence intensity regulation of organic light-emitting transistor (OLED) device can be achieved effectively by the combination of graphene vertical field effect transistor (GVFET) and OLED. In this paper, we fabricate and characterize the graphene vertical field-effect transistor with gate dielectric of ion-gel film, confirming that its current switching ratio reaches up to 102. Because of the property of high light transmittance in ion-gel film, the OLED device prepared with graphene/PEDOT:PSS as composite anode exhibits good optical properties. We also prepare the graphene vertical organic light-emitting field effect transistor (GVOLEFET) by the combination of GVFET and graphene OLED, analyzing its electrical and optical properties, and confirming that the luminescence intensity can be significantly changed by regulating the gate voltage.

Effect of annealing temperature on coercivity of Nd-Fe-B magnets with TbFeAl doping by process of hot-pressing

Ze-Teng Shu, Bo Zheng, Guang-Fei Ding, Shi-Cong Liao, Jing-Hui Di, Shuai Guo, Ren-Jie Chen, A-Ru Yan, Lei Shi
Chin. Phys. B 2020, 29 (5): 057501;  doi: 10.1088/1674-1056/ab7b57
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The Nd-Fe-B magnets are pre-sintered and then processed with hot-pressing, and the resulting magnets are called the hot-pressed pretreated (HPP) magnets. The coercivity of the HPP magnets increases as the annealed temperature increases. When the annealing temperature is 900℃, the coercivity of the magnet is only 17.6 kOe (1 Oe=79.5775 A·m-1), but when the annealing temperature rises up to 1060℃, the coercivity of the magnet reaches 23.53 kOe, which is remarkably increased by 33.7%. The microstructure analysis indicates that the grain surface of the HPP magnet becomes smoother as the annealed temperature increases. The microstructure factor α is changed according to the intrinsic coercivity model formula. The α of the magnet at 900℃ is only 0.578, but it is 0.825 at 1060℃. Microstructural optimization is due mainly to the increase of coercivity of the HPP magnet.

Microstructure and ferromagnetism ofheavily Mn doped SiGe thin flims Hot!

Huanming Wang, Sen Sun, Jiayin Xu, Xiaowei Lv, Yuan Wang, Yong Peng, Xi Zhang, Gang Xiang
Chin. Phys. B 2020, 29 (5): 057504;  doi: 10.1088/1674-1056/ab8219
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Heavily Mn-doped SiGe thin films were grown by radio frequency magnetron sputtering and then treated by post-growth thermal annealing. Structural characterizations reveal the coexistence of Mn-diluted SiGe crystals and Mn-rich nanoclusters in the annealed films. Magnetic measurements indicate the ferromagnetic ordering of the annealed samples above room temperature . The data suggest that the ferromagnetism is probably mainly contributed by the Ge-rich nanoclusters and partially contributed by the tensile-strained Mn-diluted SiGe crystals. The results may be useful for room temperature spintronic applications based on group IV semiconductors.

Role of the spin anisotropy of the interchain interaction in weakly coupled antiferromagnetic Heisenberg chains Hot!

Yuchen Fan, Rong Yu
Chin. Phys. B 2020, 29 (5): 057505;  doi: 10.1088/1674-1056/ab820b
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In quasi-one-dimensional (q1D) quantum antiferromagnets, the complicated interplay of intrachain and interchain exchange couplings may give rise to rich phenomena. Motivated by recent progress on field-induced phase transitions in the q1D antiferromagnetic (AFM) compound YbAlO3, we study the phase diagram of spin-1/2 Heisenberg chains with Ising anisotropic interchain couplings under a longitudinal magnetic field via large-scale quantum Monte Carlo simulations, and investigate the role of the spin anisotropy of the interchain coupling on the ground state of the system. We find that the Ising anisotropy of the interchain coupling can significantly enhance the longitudinal spin correlations and drive the system to an incommensurate AFM phase at intermediate magnetic fields, which is understood as a longitudinal spin density wave (LSDW). With increasing field, the ground state changes to a canted AFM order with transverse spin correlations. We further provide a global phase diagram showing how the competition between the LSDW and the canted AFM states is tuned by the Ising anisotropy of the interchain coupling.

A novel diluted magnetic semiconductor (Ca,Na)(Zn,Mn)2Sb2 with decoupled charge and spin dopings

Yilun Gu, Haojie Zhang, Rufei Zhang, Licheng Fu, Kai Wang, Guoxiang Zhi, Shengli Guo, Fanlong Ning
Chin. Phys. B 2020, 29 (5): 057507;  doi: 10.1088/1674-1056/ab892e
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We report the successful synthesis of a new diluted magnetic semiconductor (Ca,Na)(Zn,Mn)2Sb2. Na and Mn are doped into the parent compound CaZn2Sb2, which has the same crystal structure as that of "122" type iron-based superconductor CaFe2As2. Na substitution for Ca and Mn substitution for Zn introduce carriers and spins, respectively. Doping Mn atoms alone up to 5% does not induce any type of magnetic ordering. When both Na and Mn are co-doped, a ferromagnetic ordering with maximum TC~10 K has been observed. Iso-thermal magnetization shows that the coercive field is up to ~245 Oe at 2 K. Below TC, a negative magneto-resistance with MR ~12% has also been achieved.

Variable-K double trenches SOI LDMOS with high-concentration P-pillar

Lijuan Wu, Lin Zhu, Xing Chen
Chin. Phys. B 2020, 29 (5): 057701;  doi: 10.1088/1674-1056/ab7e94
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A variable-K trenches silicon-on-insulator (SOI) lateral diffused metal-oxide-semiconductor field-effect transistor (MOSFET) with a double conductive channel is proposed based on the enhancement of low dielectric constant media to electric fields. The device features variable-K dielectric double trenches and a P-pillar between the trenches (VK DT-P LDMOS). The low-K dielectric layer on the surface increases electric field of it. Adding a variable-K material introduces a new electric field peak to the drift region, so as to optimize electric field inside the device. Introduction of the high-concentration vertical P-pillar between the two trenches effectively increases doping concentration of the drift region and maintains charge balance inside it. Thereby, breakdown voltage (BV) of the device is increased. The double conductive channels provide two current paths that significantly reduce specific on-resistance (Ron,sp). Simulation results demonstrate that a 17-μm-length device can achieve a BV of 554 V and a low on-resistance of 13.12 mΩ·cm2. The Ron,sp of VK DT-P LDMOS is reduced by 78.9% compared with the conventional structure.

Effect of heating time on structural, morphology, optical, and photocatalytic properties of g-C3N4 nanosheets

Nguyen Manh Hung, Le Thi Mai Oanh, Lam Thi Hang, Pham Do Chung, Pham Thi Duyen, Dao Viet Thang, Nguyen Van Minh
Chin. Phys. B 2020, 29 (5): 057801;  doi: 10.1088/1674-1056/ab7e95
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Effect of heating time on the structural, morphology, optical, and photocatalytic properties of graphitic carbon nitride (g-C3N4) nanosheets prepared at 550℃ in Ar atmosphere is studied. The investigations are carried out by using x-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), UV-vis absorption, and photoluminescence (PL). At a heating temperature of 550℃, g-C3N4 nanocrystals are formed after 0.5 h and become more orderly as the heating time increases. The surface area of the g-C3N4 nanosheets significantly increases as the preparation time increases. The g-C3N4 prepared in 2.5 h shows the highest photocatalytic performance, decomposing completely 10 ppm RhB solution under xenon lamp irradiation for 2.0 h.

Insulator-metal transition in CaTiO3 quantum dots induced by ultrafast laser pulses

Tong Liu, Hong Zhang, Xin-Lu Cheng
Chin. Phys. B 2020, 29 (5): 058101;  doi: 10.1088/1674-1056/ab7d9e
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According to time-dependent density functional theory (TDDFT), we study the interactions between ultra-fast laser pulses and two kinds of calcium titanate quantum dots (PCTO-QDs and MCTO-QDs). Under the action of localized field effect, ultrafast laser can induce quantum dots to make the transition from insulator to metal. The PCTO-QDs are ultimately metallic, while the MCTO-QDs are still insulator after experiencing metal state. This is bacause the stability of the unsaturated atoms in the outermost layer of PCTO-QDs is weak and the geometric configuration of MCTO-QDs as a potential well will also reduce the damage of laser. Moreover, laser waveforms approaching to the intrinsic frequency of quantum dots tend to cause the highest electron levels to cross the Fermi surface. In this paper, it is reported that the insulating quantum dots can be transformed into metal by adjusting the intensity and frequency of laser. The importance of local morphology is emphasized by comparing two kinds of CTO-QDs. More importantly, it is an important step to identify the potential properties of perovskite materials.

Influence of N+ implantation on structure, morphology, and corrosion behavior of Al in NaCl solution

Hadi Savaloni, Rezvan Karami, Helma Sadat Bahari, Fateme Abdi
Chin. Phys. B 2020, 29 (5): 058102;  doi: 10.1088/1674-1056/ab7d97
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Structural and morphological changes as well as corrosion behavior of N+ implanted Al in 0.6 M NaCl solution as function of N+ fluence are investigated. The x-ray diffraction results confirmed AlN formation. The atomic force microscope (AFM) images showed larger grains on the surface of Al with increasing N+ fluence. This can be due to the increased number of impacts of N+ with Al atoms and energy conversion to heat, which increases the diffusion rate of the incident ions in the target. Hence, the number of the grain boundaries is reduced, resulting in corrosion resistance enhancement. Electrochemical impedance spectroscopy (EIS) and polarization results showed the increase of corrosion resistance of Al with increasing N+ fluence. EIS data was used to simulate equivalent electric circuits (EC) for the samples. Strong dependence of the surface morphology on the EC elements was observed. The scanning electron microscope (SEM) analysis of the samples after corrosion test also showed that the surfaces of the implanted Al samples remain more intact relative to the untreated Al sample, consistent with the EIS and polarization results.

Mg acceptor activation mechanism and hole transport characteristics in highly Mg-doped AlGaN alloys

Qing-Jun Xu, Shi-Ying Zhang, Bin Liu, Zhen-Hua Li, Tao Tao, Zi-Li Xie, Xiang-Qian Xiu, Dun-Jun Chen, Peng Chen, Ping Han, Ke Wang, Rong Zhang, You-Liao Zheng
Chin. Phys. B 2020, 29 (5): 058103;  doi: 10.1088/1674-1056/ab7e93
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The Mg acceptor activation mechanism and hole transport characteristics in AlGaN alloy with Mg doping concentration (~ 1020 cm-3) grown by metal-organic chemical vapor deposition (MOCVD) are systematically studied through optical and electrical properties. Emission lines of shallow oxygen donors and (VIII complex)1- as well as VN3+ and neutral Mg acceptors are observed, which indicate that self-compensation is occurred in Mg-doped AlGaN at highly doping levels. The fitting of the temperature-dependent Hall effect data shows that the acceptor activation energy values in Mg-doped AlxGa1-xN (x=0.23, 0.35) are 172 meV and 242 meV, and the hole concentrations at room temperature are 1.2×1018 cm-3 and 3.3×1017 cm-3, respectively. Therefore, it is believed that there exists the combined effect of the Coulomb potentials of the dopants and screening of the Coulomb potentials by a high hole concentration. Moreover, due to the high ionized acceptors' concentration and compensation ratio, the impurity conduction becomes more prominent and the valence band mobility drops sharply at low temperature.

Prediction of structured void-containing 1T-PtTe2 monolayer with potential catalytic activity for hydrogen evolution reaction

Bao Lei, Yu-Yang Zhang, Shi-Xuan Du
Chin. Phys. B 2020, 29 (5): 058104;  doi: 10.1088/1674-1056/ab8203
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Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted considerable attention because of their unique properties and great potential in nano-technology applications. Great efforts have been devoted to fabrication of novel structured TMD monolayers by modifying their pristine structures at the atomic level. Here we propose an intriguing structured 1T-PtTe2 monolayer as hydrogen evolution reaction (HER) catalyst, namely, Pt4Te7, using first-principles calculations. It is found that Pt4Te7 is a stable monolayer material verified by the calculation of formation energy, phonon dispersion, and ab initio molecular dynamics simulations. Remarkably, the novel structured void-containing monolayer exhibits superior catalytic activity toward HER compared with the pristine one, with a Gibbs free energy very close to zero (less than 0.07 eV). These features indicate that Pt4Te7 monolayer is a high-performance HER catalyst with a high platinum utilization. These findings open new perspectives for the functionalization of 2D TMD materials at an atomic level and its application in HER catalysis.

Relationship between ESIPT properties and antioxidant activities of 5-hydroxyflavone derivates

Chaofan Sun, Bifa Cao, Hang Yin, Ying Shi
Chin. Phys. B 2020, 29 (5): 058202;  doi: 10.1088/1674-1056/ab7ea1
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It is of great significance to study the relationship between the excited state intramolecular proton transfer (ESIPT) properties and antioxidant activities of compounds in the field of life sciences. In this work, two novel compounds 5HF-OMe and 5HF-NH2 are designed through introducing a methoxy- and amino-group into the structure of 5-hydroxyflavone (5HF) respectively. The relationship between the ESIPT reaction and antioxidant activities of the three compounds is studied via the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The calculated potential energy curves suggest that the rate of ESIPT reaction will gradually slow down from 5HF to 5HF-OMe and 5HF-NH2. In addition, the antioxidant activities of the three compounds gradually enhance from 5HF to 5HF-OMe and 5HF-NH2, which can be seen from the calculated energy gaps and ionization potential values. Interestingly, the above results imply that the rate of ESIPT reaction has a negative relationship with the antioxidant activities of the compounds, i.e., the slower rate of ESIPT reaction will reflect the higher antioxidant activity of the compound, which will provide valuable reference for detecting the antioxidant activity of compound via the photophysical method.

Characteristic enhancement in tunnel field-effect transistors via introduction of vertical graded source

Zhijun Lyu, Hongliang Lu, Yuming Zhang, Yimen Zhang, Bin Lu, Yi Zhu, Fankang Meng, Jiale Sun
Chin. Phys. B 2020, 29 (5): 058501;  doi: 10.1088/1674-1056/ab7d99
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A novel vertical graded source tunnel field-effect transistor (VGS-TFET) is proposed to improve device performance. By introducing a source with linearly graded component, the on-state current increases by more than two decades higher than that of the conventional GaAs TFETs without sacrificing the subthreshold swing (SS) due to the improved band-to-band tunneling efficiency. Compared with the conventional TFETs, much larger drive current range can be achieved by the proposed VGS-TFET with SS below the thermionic limitation of 60 mV/dec. Furthermore, the minimum SS about 20 mV/dec indicates its promising potential for further ultralow power applications.

Multistability and coexisting transient chaos in a simple memcapacitive system

Fu-Ping Wang, Fa-Qiang Wang
Chin. Phys. B 2020, 29 (5): 058502;  doi: 10.1088/1674-1056/ab7e98
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The self-excited attractors and hidden attractors in a memcapacitive system which has three elements are studied in this paper. The critical parameter of stable and unstable states is calculated by identifying the eigenvalues of Jacobian matrix. Besides, complex dynamical behaviors are investigated in the system, such as coexisting attractors, hidden attractors, coexisting bifurcation modes, intermittent chaos, and multistability. From the theoretical analyses and numerical simulations, it is found that there are four different kinds of transient transition behaviors in the memcapacitive system. Finally, field programmable gate array (FPGA) is used to implement the proposed chaotic system.

High-performance inverters based on ambipolar organic-inorganic heterojunction thin-film transistors

Sheng Sun, Yuzhi Li, Shengdong Zhang
Chin. Phys. B 2020, 29 (5): 058503;  doi: 10.1088/1674-1056/ab7e96
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This work reports on the integration of organic and inorganic semiconductors as heterojunction active layers for high-performance ambipolar transistors and complementary metal-oxide-semiconductor (CMOS)-like inverters. Pentacene is employed as a p-type organic semiconductor for its stable electrical performance, while the solution-processed scandium (Sc) substituted indium oxide (ScInO) is employed as an n-type inorganic semiconductor. It is observed that by regulating the doping concentration of Sc, the electrical performance of the n-type semiconductor could be well controlled to obtain a balance with the electrical performance of the p-type semiconductor, which is vital for achieving high-performance inverters. When the doping concentration of Sc is 10 at.%, the CMOS-like logic inverters exhibit a voltage gain larger than 80 and a wide noise margin (53% of the theoretical value). The inverters also respond well to the input signal with frequency up to 500 Hz.

Ultraviolet irradiation dosimeter based on persistent photoconductivity effect of ZnO

Chao-Jun Wang, Xun Yang, Jin-Hao Zang, Yan-Cheng Chen, Chao-Nan Lin, Zhong-Xia Liu, Chong-Xin Shan
Chin. Phys. B 2020, 29 (5): 058504;  doi: 10.1088/1674-1056/ab8891
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It is essential to determine the accumulative ultraviolet (UV) irradiation over a period of time in some cases, such as monitoring UV irradiation to the skin, solar disinfection of water, photoresist exposure, etc. UV colorimetric dosimeters, which use dyes' color change to monitor the amount of UV exposure, have been widely studied. However, the exposure data of these UV colorimetric dosimeters can hardly be converted to digital signals, limiting their applications. In this paper, a UV dosimeter has been proposed and demonstrated based on the persistent photoconductivity (PPC) in zinc oxide microwires (ZnO MWs). The PPC effect usually results in high photoconductivity gain but low response speed, which has been regarded as a disadvantage for photodetectors. However, in this work, the unique characteristics of the PPC effect have been utilized to monitoring the accumulative exposure. We demonstrate that the photocurrent in the ZnO MWs depends on the accumulative UV exposure due to the PPC effect, thus the photocurrent can be utilized to determine the UV accumulation. The dosimeter is immune to visible light and exhibits a photoconductive gain of 2654, and the relative error of the dosimeter is about 10%. This UV dosimeter with electrical output is reusable and convenient to integrate with other electronic devices and may also open a new application area for the PPC effect.

Generating mechanism of pathological beta oscillations in STN-GPe circuit model: A bifurcation study

Jing-Jing Wang, Yang Yao, Zhi-Wei Gao, Xiao-Li Li, Jun-Song Wang
Chin. Phys. B 2020, 29 (5): 058701;  doi: 10.1088/1674-1056/ab7e9b
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Parkinson's disease (PD) is characterized by pathological spontaneous beta oscillations (13 Hz-35 Hz) often observed in basal ganglia (BG) composed of subthalamic nucleus (STN) and globus pallidus (GPe) populations. From the viewpoint of dynamics, the spontaneous oscillations are related to limit cycle oscillations in a nonlinear system; here we employ the bifurcation analysis method to elucidate the generating mechanism of the pathological spontaneous beta oscillations underlined by coupling strengths and intrinsic properties of the STN-GPe circuit model. The results reveal that the increase of inter-coupling strength between STN and GPe populations induces the beta oscillations to be generated spontaneously, and causes the oscillation frequency to decrease. However, the increase of intra-coupling (self-feedback) strength of GPe can prevent the model from generating the oscillations, and dramatically increase the oscillation frequency. We further provide a theoretical explanation for the role played by the inter-coupling strength of GPe population in the generation and regulation of the oscillations. Furthermore, our study reveals that the intra-coupling strength of the GPe population provides a switching mechanism on the generation of the abnormal beta oscillations: for small value of the intra-coupling strength, STN population plays a dominant role in inducing the beta oscillations; while for its large value, the GPe population mainly determines the generation of this oscillation.

Current improvement in substrate structured Sb2S3 solar cells with MoSe2 interlayer

Lu Liu, Sheng-Li Zhang, Jian-Yu Wu, Wei-Huang Wang, Wei Liu, Li Wu, Yi Zhang
Chin. Phys. B 2020, 29 (5): 058801;  doi: 10.1088/1674-1056/ab8220
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Sb2S3 solar cells with substrate structure usually suffer from pretty low short circuit current (JSC) due to the defects and poor carrier transport. The Sb2S3, as a one-dimensional material, exhibits orientation-dependent carrier transport property. In this work, a thin MoSe2 layer is directly synthesized on the Mo substrate followed by depositing the Sb2S3 thin film. The x-ray diffraction (XRD) patterns confirm that a thin MoSe2 layer can improve the crystallization of the Sb2S3 film and induce (hk1) orientations, which can provide more carrier transport channels. Kelvin probe force microscopy (KPFM) results suggest that this modified Sb2S3 film has a benign surface with less defects and dangling bonds. The variation of the surface potential of Sb2S3 indicates a much more efficient carrier separation. Consequently, the power conversion efficiency (PCE) of the substrate structured Sb2S3 thin film solar cell is improved from 1.36% to 1.86%, which is the best efficiency of the substrate structured Sb2S3 thin film solar cell, and JSC significantly increases to 13.6 mA/cm2. According to the external quantum efficiency (EQE) and C-V measurements, the modified crystallization and elevated built-in electric field are the main causes.

Asynchronism of the spreading dynamics underlying the bursty pattern

Tong Wang, Ming-Yang Zhou, Zhong-Qian Fu
Chin. Phys. B 2020, 29 (5): 058901;  doi: 10.1088/1674-1056/ab81fe
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The potential mechanisms of the spreading phenomena uncover the organizations and functions of various systems. However, due to the lack of valid data, most of early works are limited to the simulated process on model networks. In this paper, we track and analyze the propagation paths of real spreading events on two social networks: Twitter and Brightkite. The empirical analysis reveals that the spreading probability and the spreading velocity present the explosive growth within a short period, where the spreading probability measures the transferring likelihood between two neighboring nodes, and the spreading velocity is the growth rate of the information in the whole network. Besides, we observe the asynchronism between the spreading probability and the spreading velocity. To explain the interesting and abnormal issue, we introduce the time-varying spreading probability into the susceptible-infected (SI) and linear threshold (LT) models. Both the analytic and experimental results reproduce the spreading phenomenon in real networks, which deepens our understandings of spreading problems.
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