Chinese Physics B
Co-Editors-in-Chief
Zhongcan Ouyang
2020
Volume: 29 Issue 8
82 Article(s)
Ke Zhi-Jin, Wang Yi-Tao, Yu Shang, Liu Wei, Meng Yu, Li Zhi-Peng, Wang Hang, Li Qiang, Xu Jin-Shi, Xiao Ya, Tang Jian-Shun, Li Chuan-Feng, and Guo Guang-Can

Entanglement is the key resource in quantum information processing, and an entanglement witness (EW) is designed to detect whether a quantum system has any entanglement. However, prior knowledge of the target states should be known first to design a suitable EW, which weakens this method. Nevertheless, a recent theory shows that it is possible to design a universal entanglement witness (UEW) to detect negative-partial-transpose (NPT) entanglement in unknown bipartite states with measurement-device-independent (MDI) characteristic. The outcome of a UEW can also be upgraded to be an entanglement measure. In this study, we experimentally design and realize an MDI UEW for two-qubit entangled states. All of the tested states are well-detected without any prior knowledge. We also show that it is able to quantify entanglement by comparing it with concurrence estimated through state tomography. The relation between them is also revealed. The entire experimental framework ensures that the UEW is MDI.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Zhao Wen-Lei, and Jie Quanlin

We investigate the quantum to classical transition induced by two-particle interaction via a system of periodically kicked particles. The classical dynamics of particle 1 is almost unaffected in condition that its mass is much larger than that of particle 2. Interestingly, such classically weak influence leads to the quantum to classical transition of the dynamical behavior of particle 1. Namely, the quantum diffusion of this particle undergoes the transition from dynamical localization to the classically chaotic diffusion with the decrease of the effective Planck constant ?eff. The behind physics is due to the growth of entanglement in the system. The classically very weak interaction leads to the exponential decay of purity in condition that the classical dynamics of external degrees freedom is strongly chaotic.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Jing Jian, Ma Yao-Yao, Zhang Qiu-Yue, Wang Qing, and Dong Shi-Hai

We show that it is possible to simulate an anyon by a trapped atom which possesses an induced electric dipole moment in the background of electric and magnetic fields in a specific configuration. The electric and magnetic fields we applied contain a magnetic and two electric fields. We find that when the atom is cooled down to the limit of the negligibly small kinetic energy, the atom behaves like an anyon because its angular momentum takes fractional values. The fractional part of the angular momentum is determined by both the magnetic and one of the electric fields. Roles electric and magnetic fields played are analyzed.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Wang Bing-Jie, Xu Li, Zeng Wei-You, and Wang Qing-Lan

Testing the extreme weak gravitational forces between torsion pendulum and surrounding objects will indicate new physics which attracts many interests. In these measurements, the fiber alignment plays a crucial role in fulfilling high precision placement measurement, especially in measuring the deviation between the fiber and source mass or other objects. The traditional way of the fiber alignment requires to measure the component of the pendulum body and then transfer to the torsion fiber by some complicated calculations. A new method is reported here by using a CCD camera to get the projection image of the torsion fiber, which is a direct and no-contact measurement. Furthermore, the relative position change of the torsion fiber can also be monitored during the experiment. In our experiment, the alignment between the fiber and the center of the turntable has been operated as an example. Our result reaches the accuracy of several micrometers which is higher than the previous method.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Song Jia, Weng Tong-Feng, Gu Chang-Gui, and Yang Hui-Jie

A complex system contains generally many elements that are networked by their couplings. The time series of output records of the system’s dynamical process is subsequently a cooperative result of the couplings. Discovering the coupling structure stored in the time series is an essential task in time series analysis. However, in the currently used methods for time series analysis the structural information is merged completely by the procedure of statistical average. We propose a concept called mode network to preserve the structural information. Firstly, a time series is decomposed into intrinsic mode functions and residue by means of the empirical mode decomposition solution. The mode functions are employed to represent the contributions from different elements of the system. Each mode function is regarded as a mono-variate time series. All the mode functions form a multivariate time series. Secondly, the co-occurrences between all the mode functions are then used to construct a threshold network (mode network) to display the coupling structure. This method is illustrated by investigating gait time series. It is found that a walk trial can be separated into three stages. In the beginning stage, the residue component dominates the series, which is replaced by the mode function numbered M14 with peaks covering ～680 strides (～12 min) in the second stage. In the final stage more and more mode functions join into the backbone. The changes of coupling structure are mainly induced by the co-occurrent strengths of the mode functions numbered as M11, M12, M13, and M14, with peaks covering 200–700 strides. Hence, the mode network can display the rich and dynamical patterns of the coupling structure. This approach can be extended to investigate other complex systems such as the oil price and the stock market price series.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Lou Sen-Yue

The celebrated (1+1)-dimensional Korteweg de–Vries (KdV) equation and its (2+1)-dimensional extension, the Kadomtsev–Petviashvili (KP) equation, are two of the most important models in physical science. The KP hierarchy is explicitly written out by means of the linearized operator of the KP equation. A novel (2+1)-dimensional KdV extension, the cKP3–4 equation, is obtained by combining the third member (KP3, the usual KP equation) and the fourth member (KP4) of the KP hierarchy. The integrability of the cKP3–4 equation is guaranteed by the existence of the Lax pair and dual Lax pair. The cKP3–4 system can be bilinearized by using Hirota’s bilinear operators after introducing an additional auxiliary variable. Exact solutions of the cKP3–4 equation possess some peculiar and interesting properties which are not valid for the KP3 and KP4 equations. For instance, the soliton molecules and the missing D–Alembert type solutions (the arbitrary travelling waves moving in one direction with a fixed model dependent velocity) including periodic kink molecules, periodic kink-antikink molecules, few-cycle solitons, and envelope solitons exist for the cKP3–4 equation but not for the separated KP3 equation and the KP4 equation.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Tan Zhi-Zhong, and Tan Zhen

We consider the problem of electrical properties of an m × n cylindrical network with two arbitrary boundaries, which contains multiple topological network models such as the regular cylindrical network, cobweb network, globe network, and so on. We deduce three new and concise analytical formulae of potential and equivalent resistance for the complex network of cylinders by using the RT-V method (a recursion-transform method based on node potentials). To illustrate the multiplicity of the results we give a series of special cases. Interestingly, the results obtained from the resistance formulas of cobweb network and globe network obtained are different from the results of previous studies, which indicates that our research work creates new research ideas and techniques. As a byproduct of the study, a new mathematical identity is discovered in the comparative study.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Fei Xin-Xing, Wang Ying, Luo Xin, and Yu Cheng-Hao

In this paper, we propose a new enhanced GaN MISFET with embedded pn junction, i.e., EJ-MISFET, to enhance the breakdown voltage. The embedded pn junction is used to improve the simulated device electric field distribution between gate and drain, thus achieving an enhanced breakdown voltage (BV). The proposed simulated device with LGD = 15μm presents an excellent breakdown voltage of 2050 V, which is attributed to the improvement of the device electric field distribution between gate and drain. In addition, the ON-resistance (RON) of 15.37 Ω ?mm and Baliga’s figure of merit of 2.734 GW?cm-2 are achieved in the optimized EJ-MISFET. Compared with the field plate conventional GaN MISFET (FPC-MISFET) without embedded pn junction structure, the proposed simulated device increases the BV by 32.54% and the Baliga’s figure of merit is enhanced by 71.3%.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Al-Baradi Ateyyah M, Altowairqi Fatimah A, Atta A A, Badawi Ali, Algarni Saud A, Almalki Abdulraheem S A, Hassanien A M, Alodhayb A, Kamal A M, and El-Nahass M M

In this study, CdS/ZnO (2:3 mol%) thin films are successfully deposited on quartz substrates by using the sputtering technique. Good images on the structural and optical characteristic features of CdS/ZnO thin films before and after annealing are obtained. The CdS/ZnO thin films are annealed respectively at temperatures of 373 K, 473 K, and 573 K and the structural features are examined by XRD, ATR-FTIR, and FESEM. The optical properties of CdS/ZnO thin films such as refractive indices, absorption coefficients, optical band gap energy values, Urbach energy values, lattice dielectric constants, and high frequency dielectric constants are determined from spectrophotometer data recorded over the spectral range of 300 nm–2500 nm. Dispersion parameters are investigated by using a single-oscillator model. Photoluminescence spectra of CdS/ZnO thin films show an overall decrease in their intensity peaks after annealing. The third-order nonlinear optical parameter, and nonlinear refractive index are also estimated.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Wang Endong, Zhu Beien, and Gao Yi

The hydrated-proton structure is critical for understanding the proton transport in water. However, whether the hydrated proton adopts Zundel or Eigen structure in solution has been highly debated in the past several decades. Current experimental techniques cannot directly visualize the dynamic structures in situ, while the available theoretical results on the infrared (IR) spectrum derived from current configurational models cannot fully reproduce the experimental results and thus are unable to provide their precise structures. In this work, using H5O2+ as a model, we performed first-principles calculations to demonstrate that both the structural feature and the IR frequency of proton stretching, characteristics to discern the Zundel or Eigen structures, evolve discontinuously with the change of the O–O distance. A simple formula was introduced to discriminate the Zundel, Zundel-like, and Eigen-like structures. This work arouses new perspectives to understand the proton hydration in water.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Shen Xu-Xu, Wang Jun, Guo Fu-Ming, Chen Ji-Gen, and Yang Yun-Jun

We investigate high-order harmonic generation from atoms irradiated by bichromatic counter-rotating circularly polarized laser pulses by numerically solving the time-dependent Schr?dinger equation. It is found that the minimum energy position of the harmonic spectrum and the non-integer order optical radiation are greatly discrepant for different atomic potentials. By analyzing the quantum trajectory of the harmonic emission, discrepancies among the harmonic spectra from different potentials can be attributed to the action of the potential on the ionized electrons. In addition, based on the influence of the driving light intensity on the overall intensity and ellipticity of higher order harmonics, the physical conditions for generating a high-intensity circularly polarized harmonic can be obtained.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Hai Kuo, Zhu Wenhua, Chen Qiong, and Hai Wenhua

By investigating a harmonically confined and periodically driven particle system with spin–orbit coupling (SOC) and a specific controlled parameter, we demonstrate an exactly solvable two-level model with a complete set of spin-motion entangled Schr?dinger kitten (or cat) states. In the undriven case, application of a modulation resonance results in the exact stationary states. We show a decoherence-averse effect of SOC and implement a transparent coherent control by exchanging positions of the probability-density wavepackets to create transitions between the different degenerate ground states. The expected energy consisting of quantum and continuous parts is derived, and the energy deviations caused by the exchange operations are much less than the quantum gap. The results could be directly extended to a weakly coupled single-particle chain for transparently encoding spin–orbit qubits via the robust spin-motion entangled degenerate ground states.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Li Liang, Zheng Yu-Lu, Hu Yu-Xin, Li Fang-Fei, Zhou Qiang, and Cui Tian

As is well known, the basic intrinsic properties of materials can be significant for their practical applications. In this work, the room-temperature absorption, transmittance, reflectance spectra, and relative photoelectricities parameters of the Mg4Ta2O9 crystals are demonstrated. Meanwhile, the polarized Raman spectra of Mg4Ta2O9 crystals are also described. The room-temperature photoluminescence (PL) and the temperature-dependent PL for Mg4Ta2O9 crystals are obtained. Significantly, we observe a phonon-participated PL process in Mg4Ta2O9.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Yang Yang, Zhang Qiang, Mi Wenbo, and Zhang Xixiang

The lattice structures of epitaxial Fe3O4 films deposited on MgO were studied systematically using polarized Raman spectroscopy as a function of film thickness, where interesting phenomena were observed. Firstly, the spectral conflict to the Raman selection rules (RSRs) was observed under cross-sectional configuration, which can be attributed to the tetragonal deformation in the growth direction due to the lattice mismatch between Fe3O4 and MgO. Secondly, the blue shift and broadening of Raman peaks evidenced the decrease of the tensile strain in Fe3O4 films with decreasing thickness. Thirdly, distinct from the other Raman modes, the lowest T2g mode exhibited asymmetric lineshape, which can be interpreted using the spatial correlation model. The increased correlation length introduced in the model can well explain the enhanced peak asymmetry feature with decreasing thickness. These results provide useful information for understanding the lattice structure of epitaxial Fe3O4 film.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Wu Ji-Zhou, Li Yu-Qing, Liu Wen-Liang, Li Peng, Wang Xiao-Feng, Chen Peng, Ma Jie, Xiao Lian-Tuan, and Jia Suo-Tang

We report an effective method for enhancing the photoassociation of ultracold atoms using a non-resonant magnetic field, which enables the manipulation of the coupling between the wavefunctions of the colliding atomic pairs and the excited molecules. A series of photoassociation spectra are measured for different magnetic fields. We show that the photoassociation rate is significantly dependent on the non-resonant magnetic field. A qualitatively theoretical explanation is provided, and shows a good agreement with the experimental result.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Zhang Han, Shao Zhe, and Zhao Ke

The properties of one-photon absorption (OPA), emission and two-photon absorption (TPA) of a bipyridine-based zinc ion probe are investigated employing the density functional theory in combination with response functions. The responsive mechanism and coordination mode effect are explored. The structural fluctuation is illustrated by molecular dynamics simulation. The calculated OPA and emission wavelengths of the probe are consistent with the experimental data. It is found that the red-shift of OPA wavelength and the enhancement of TPA intensity are induced by the increased intra-molecular charge transfer mechanism upon metal binding. The structural fluctuation could result in the blue-shift of TPA wavelength and the decrease of the TPA cross section. The TPA properties are quite different among the zinc complexes with different coordination modes. The TPA wavelength of the complexes with two ligands is close to that of the probe, which is in agreement with the experimental observation.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Wang Chao, Xu Wen, Mei Hong-Ying, Qin Hua, Zhao Xin-Nian, Wen Hua, Zhang Chao, Ding Lan, Xu Yong, Li Peng, Wu Dai, and Li Ming

Electron energy relaxation time τ is one of the key physical parameters for electronic materials. In this study, we develop a new technique to measure τ in a semiconductor via monochrome picosecond (ps) terahertz (THz) pump and probe experiment. The special THz pulse structure of Chinese THz free-electron laser (CTFEL) is utilized to realize such a technique, which can be applied to the investigation into THz dynamics of electronic and optoelectronic materials and devices. We measure the THz dynamical electronic properties of high-mobility n-GaSb wafer at 1.2 THz, 1.6 THz, and 2.4 THz at room temperature and in free space. The obtained electron energy relaxation time for n-GaSb is in line with that measured via, e.g., four-wave mixing techniques. The major advantages of monochrome ps THz pump–probe in the study of electronic and optoelectronic materials are discussed in comparison with other ultrafast optoelectronic techniques. This work is relevant to the application of pulsed THz free-electron lasers and also to the development of advanced ultrafast measurement technique for the investigation of dynamical properties of electronic and optoelectronic materials.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Lei Changyong, and Dong Guangjiong

Electron vortex beams (EVBs) have potential applications in nanoscale magnetic probes of condensed matter and nanoparticle manipulation as well as radiation physics. Recently, a relativistic electron vortex beam (REVB) has been proposed [Phys. Rev. Lett.107 174802 (2011)]. Compared with EVBs, except for orbital angular momentum, an REVB has intrinsic relativistic effect, i.e., spin angular momentum and spin–orbit coupling. We study the electromagnetic field of an REVB analytically. We show that the electromagnetic field can be separated into two parts, one is only related to orbital quantum number, and the other is related to spin–orbit coupling effect. Exploiting this separation property, the difference between the electromagnetic fields of the REVB in spin-up and spin-down states can be used as a demonstration of the relativistic quantum effect. The linear momentum and angular momentum of the generated electromagnetic field have been further studied and it is shown that the linear momentum is weakly dependent on the spin state; while the angular momentum is evidently dependent on the spin state and linearly increases with the topological charge of electron vortex beam. The electromagnetic and mechanical properties of the REVB could be useful for studying the interaction between REVBs and materials.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Hu Yong-Nan, Cheng Li-Hong, Yao Zheng-Wei, Zhang Xiao-Bo, Zhang Ai-Xia, and Xue Ju-Kui

We study the dynamics of single electron in an inhomogeneous cylindrical plasma channel during the direct acceleration by linearly polarized chirped laser pulse. By adjusting the parameters of the chirped laser pulse and the plasma channel, we obtain the energy gain, trajectory, dephasing rate and unstable threshold of electron oscillation in the channel. The influences of the chirped factor and inhomogeneous plasma density distribution on the electron dynamics are discussed in depth. We find that the nonlinearly chirped laser pulse and the inhomogeneous plasma channel have strong coupled influence on the electron dynamics. The electron energy gain can be enhanced, the instability threshold of the electron oscillation can be lowered, and the acceleration length can be shortened by chirped laser, while the inhomogeneity of the plasma channel can reduce the amplitude of the chirped laser.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Li Genyan, Li Xiao, Zhang Lei, and Chen Jun

Inducing a significant optical torque remains a challenging task, since the law of angular momentum conservation implies that one has to harvest a lot of light. Such a problem was partially resolved by using optical twist via strong internal multiple scattering to recycle the photons, and one can induce a large torque per unit of radiation cross section. By using the Maxwell stress tensor and the generalized Lorentz–Mie scattering theory for multi-spheres, we investigate the influence of gain materials in further amplifying optical torque in the optical twist settings. It is found that, when combined with a gain layer, the optical torque of lossy (both in PT - and non-PT-symmetric structures) or lossless (low dielectric materials) clusters at resonance could be one order of magnitude larger than those of a single layer and previous studied plasmonic double layer structures. Moreover, the gain-enhanced large opposite rotations (i.e., optical twist) of the two layers arise at resonances in these structures. In contrast, in the gain–gain double-layer cluster, optical torques on both layers have no significant increase and the two layers rotate in the same direction at resonances. This work provides an elaborate investigation on the gain media-induced optical twist, which offers more choices for optical micromanipulation.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Huang Wei-Qi, Liu Shi-Rong, Peng Hong-Yan, Li Xin, and Huang Zhong-Mei

Silicene, silicon analogue to graphene which possesses a two-dimensional (2D) hexagonal lattice, has attracted increasing attention in the last few years due to predicted unique properties. However, silicon naturally possesses a three-dimensional (3D) diamond structure, so there seems to be not any natural solid phase of silicon similar to graphite. Here we report the synthesis of new silicene structure with a unique rectangular lattice by using a coherent electron beam to irradiate amorphous silicon nanofilm produced by pulsed laser deposition (PLD). Under the irradiation of coherent electron beam with proper kinetic energy, the surface layer of silicon nanofilm can be crystallized into silicene. The dynamic stability and the energy band properties of this new silicene structure are investigated by using first-principle calculations and density function theory (DFT) with the help of the observed crystalline structure and lattice constant. The new silicene structure has a real direct bandgap of 0.78 eV. Interestingly, the simulating calculation shows that the convex bond angle is 118° in the new silicene structure with rectangular lattices. The DFT simulations reveal that this new silicene structure has a Dirac-cone-like energy band. The experimental realization of silicene and the theoretically predicted properties shed light on the silicon material with potential applications in new devices.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Liu Menghan, Chen Peng, Xie Zili, Xiu Xiangqian, Chen Dunjun, Liu Bin, Han Ping, Shi Yi, Zhang Rong, Zheng Youdou, Cheng Kai, and Zhang Liyang

Resonance effects caused by the photon–electron interaction are a focus of attention in semiconductor optoelectronics, as they are able to increase the efficiency of emission. GaN-on-silicon microdisks can provide a perfect cavity structure for such resonance to occur. Here we report GaN-based microdisks with different diameters, based on a standard blue LED wafer on a Si substrate. A confocal photoluminescence spectroscopy is performed to analyze the properties of all microdisks. Then, we systematically study the effects of radial modes and axial modes of these microdisks on photon–electron coupling efficiency by using three-dimensional finite-difference time-domain simulations. For thick microdisks, photon–electron coupling efficiency is found to greatly depend on the distributions of both the radial modes and the axial modes, and the inclined sidewalls make significant influences on the axial mode distributions. These results are important for realization of high-efficiency resonant emission in GaN-based microcavity devices.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Huang Shuai, Wang Qing, Zhang Meng, Chen Chaoyong, Wang Kaixin, Gao Mingwei, and Gao Chunqing

A novel Er:YAG laser system operating at 1645 nm with high pulse-repetition-frequency (PRF) of kHz level is demonstrated. A ring cavity with double gain medium end-pumped by two fiber lasers is utilized to obtain high pulse energy. A novel ‘triple-reflection’ configuration on a piezoelectric actuator (PZT) is adopted to achieve high-repetition-rate at 3-kHz operation with the ramp-fire locking method. Single frequency pulses with maximum average power of 18.3 W at 3 kHz are obtained, and the pulse duration time is 318 ns. The full line width at half maximum (FWHM) of the pulses measured by the heterodyne technique is 1.71 MHz at 3 kHz. To the best of our knowledge, this is the highest PRF single-frequency laser pulses achieved based Er:YAG gain medium.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Zhao Xin, Song Zheng, Li Yuan-Ji, Feng Jin-Xia, and Zhang Kuan-Shou

Based on a theoretical model of Q-switched laser with the influences of the driving signal sent to the Pockels cell and the doping concentration of the gain medium taken into account, a method of achieving high energy sub-nanosecond Q-switched lasers is proposed and verified in experiment. When a Nd:YVO4 crystal with a doping concentration of 0.7 at.% is used as a gain medium and a driving signal with the optimal high-level voltage is applied to the Pockels cell, a stable single-transverse-mode electro–optical Q-switched laser with a pulse width of 0.77 ns and a pulse energy of 1.04 mJ operating at the pulse repetition frequency of 1 kHz is achieved. The precise tuning of the pulse width is also demonstrated.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Wei Li, Meng Wei-Dong, Sun Li-Cun, Cao Xin-Fei, and Pu Xiao-Yun

Ray tracing method is used to study the propagation of collimated beams in a liquid–core cylindrical lens (LCL), which has dual functions of diffusion cell and image formation. The diffusion images on the focal plane of the used LCL are simulated by establishing and solving both linear and nonlinear ray equations, the calculated results indicate that the complex imaging results of LCL in inhomogeneous media can be treated by the law of ray propagation in homogeneous media under the condition of small refractive index gradient of diffusion solution. Guided by the calculation conditions, the diffusion process of triethylene glycol aqueous solution is experimentally studied at room temperature by using the LCL in this paper. The spatial and temporal concentration profile Ce(z, t) of diffusion solution is obtained by analyzing diffusion image appearing on the focal plane of the LCL; Then, the concentration-dependent diffusion coefficient is assumed to be a polynomial D(C) = D0 × (1 + α1C + α2C2 + α3C3 + ?). The finite difference method is used to solve the Fick diffusion equation for calculating numerically the concentration profiles Cn(z, t). The D(C)of triethylene glycol aqueous solution is obtained by comparing the Cn(z, t) with Ce(z,t). Finally, the obtained polynomial D(C) is used to calculate the refractive index profiles nn(z,t)s of diffusion solution in the used LCL. Based on the ray propagation law in inhomogeneous media and the calculated n(z,t), the ray tracing method is used again to simulate the dynamic images of the whole experimental diffusion process to varify the correctness of the calculated D(C). The method presented in this work opens up a new way for both measuring and verifying the concentration-dependent liquid diffusion coefficients.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Ji Lanting, Chen Wei, Gao Yang, Xu Yan, Wu Chi, Wang Xibin, Yi Yunji, Li Baohua, Sun Xiaoqiang, and Zhang Daming

Electro–optic modulator is a key component for on-chip optical signal processing. An electro–optic phase modulator based on multilayer graphene embedded in silicon nitride waveguide is demonstrated to fulfill low-power operation. Finite element method is adopted to investigate the interaction enhancement between the graphene flake and the optical mode. The impact of multilayer graphene on the performance of phase modulator is studied comprehensively. Simulation results show that the modulation efficiency improves with the increment of graphene layer number, as well as the modulation length. The 3-dB bandwidth of around 48 GHz is independent of graphene layer number and length. Compared to modulator with two- or four-layer graphene, the six-layer graphene/silicon nitride waveguide modulator can realize π phase shift at a low-power consumption of 14 fJ/bit when the modulation length is 240 μm.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Wang Xiao-Long, Zou Yong-Gang, He Zhi-Fang, Liu Guo-Jun, and Ma Xiao-Hui

We design an 850 nm tunable vertical-cavity surface-emitting laser (VCSEL) structure using an internal-cavity sub-wavelength grating. The use of such a tuning structure allows for wider wavelength tuning range and more stable single-polarization as compared to conventional tunable VCSELs. The features of the internal-cavity grating effect on the wavelength tuning and polarization characteristics of the tunable VCSEL are analyzed. The simulation results show that the largest wavelength tuning range achieves 44.2 nm, and the maximum orthogonal polarization suppression ratio (OPSR) is 33.4 dB (TE-type) and 38.7 dB (TM-type).

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Chen Lin, Li Huihui, Hao Weiming, Yin Xiang, and Wang Jian

Metasurfaces have exhibited considerable capability for generating Airy beams. However, the available plasmonic/dielectric metasurfaces Airy-beam generators have low transmission efficiency and/or poor quality of generated beam because they lack the amplitude modulation. Hyperbolic metamaterials (HMMs) have recently provided an alternative strategy for building high-performance meta-devices that are capable of flexibly modulating the phase, amplitude and polarization state of light. Here we reveal that both the propagation phase and the Pancharatnam–Berry phase can contribute to the local transmission phase of circularly polarized electromagnetic waves by using HMMs. This thus provides us with great freedom to design HMM units with different cross-sections to independently control the transmission phase and amplitude. Here, we design circularly polarized Airy-beam generators in the microwave and near-infrared domains, which require binary phase and polynary amplitude, and validate the good performance in the microwave experiment. Our work can facilate the generation of a complicated light field that highly requires independent and complete control of the transmission phase and amplitude under circularly polarized incidence.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Wang Bo-Yun, Zhu Yue-Hong, Zhang Jing, Zeng Qing-Dong, Du Jun, Wang Tao, and Yu Hua-Qing

An ultrafast and low-power slow light tuning mechanism based on plasmon-induced transparency (PIT) for two disk cavities aperture-coupled to a metal-dielectric-metal plasmonic waveguide system is investigated numerically and analytically. The optical Kerr effect is enhanced by the local electromagnetic field of surface plasmon polaritons, slow light, and graphene–Ag composite material structures with a large effective Kerr nonlinear coefficient. Through the dynamic adjustment of the frequency of the disk nanocavity, the group velocity is controlled between c/53.2 and c/15.1 with the pump light intensity increased from 0.41 MW/cm2 to 2.05 MW/cm2. Alternatively, through the dynamic adjustment of the propagation phase of the plasmonic waveguide, the group velocity is controlled between c/2.8 and c/14.8 with the pump light intensity increased from 5.88 MW/cm2 to 11.76 MW/cm2. The phase shift multiplication of the PIT effect is observed. Calculation results indicate that the entire structure is ultracompact and has a footprint of less than 0.8 μm2. An ultrafast responsive time in the order of 1 ps is reached due to the ultrafast carrier relaxation dynamics of graphene. All findings are comprehensively analyzed through finite-difference time-domain simulations and with a coupling-mode equation system. The results can serve as a reference for the design and fabrication of nanoscale integration photonic devices with low power consumption and ultrafast nonlinear responses.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Shi Jie, Liu Yulin, Shi Shengguo, Deng Anding, and Li Hongdao

Measurements of bubble size distribution require the understanding of the acoustic characteristics of the medium. The bubbles show highly nonlinear properties under finite amplitude acoustic excitation, so the acoustic fields from bubble population are easily observed at the second harmonics as well as at the fundamental frequency, which shows that the nonlinear coefficient increases obviously. The inversion method of bubble size distribution based on nonlinear acoustic effects can peel off the influence of complex environment and obtain the size distribution coefficient information of bubbles more accurately. The previous nonlinear inversion methods of bubble size distribution are mostly based on the nonlinear scattering cross-section characteristics of bubbles. However, the stability of inversion is not high enough. In this paper, we introduce a new acoustic inversion method for bubble size distribution, which is based on the nonlinear coefficients of bubble medium. Compared with other inversion methods based on linear or nonlinear scattering cross section, the inversion method based on nonlinear coefficients of bubble medium proposed in this paper shows good robustness in both simulation and experiment.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Li Yi-yi, and Zhang Hao-chun

The advanced heat flux manipulating structures inspired by TO-based spatial mapping have aroused wide interests owing to huge potential in high-efficient thermal energy utilization. However, most researches are limited to the realization of single function in one specific structure and appropriate evaluation of the energy transfer process is relatively lacking. In this work, based on time-dependent two-dimensional heat conduction equation, a co-effect structure capable of accomplishing concentration and rotation functions simultaneously is established and validated by finite element simulations compared with the conventional single concentrator and singe rotator. In addition, from the perspective of thermodynamics, the transformed local entropy production rate and total entropy production are theoretically derived and applied to evaluate the quality of energy transfer processes. The proposed co-effect structure can help to explore other potential mass/flux manipulating devices and the evaluation method is valuable for the further manufacturing as well as optimization of these devices in engineering applications.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Wang Qilang, Chen Yunyu, Aiyiti Adili, Zheng Minrui, Li Nianbei, and Xu Xiangfan

Unveiling the thermal transport properties of various one-dimensional (1D) or quasi-1D materials like nanowires, nanotubes, and nanorods is of great importance both theoretically and experimentally. The dimension or size dependence of thermal conductivity is crucial in understanding the phonon–phonon interaction in the low-dimensional systems. In this paper, we experimentally investigate the size-dependent thermal conductivity of individual single crystalline α-Fe2O3 nanowires collaborating the suspended thermal bridge method and the focused electron-beam self-heating technique, with the sample diameter (d) ranging from 180 nm to 661 nm and length (L) changing from 4.84 μm to 20.73 μm. An empirical relationship for diameter-/length-dependent thermal conductivity is obtained, which shows an approximately linear dependence on the aspect ratio (L/(1 + Cd)) at T = 300 K, where C is a fitting parameter. This is related to the boundary scattering and diameter effect of α-Fe2O3 nanowires although rigorous calculations are needed to confirm the result.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Peng Guang-Han

Traffic interruption phenomena frequently occur with the number of vehicles increasing. To investigate the effect of the traffic interruption probability on traffic flow, a new optimal velocity model is constructed by considering the driver anticipation term in the interruption case for car-following theory. Furthermore, the effect of driver anticipation in the interruption case is investigated via linear stability analysis. Also, the MKdV equation is obtained concerning the effect of driver anticipation in the interruption case. Moreover, numerical simulation states that the driver anticipation term in the interruption case contributes to the stability of traffic flow.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Wang Wei-Li, Wan Fang-Fang, and Lo Siu-Ming

Exit choice is one of the most important pedestrian behaviors during evacuation. Distance to the exit is a generally recognized factor influencing expected moving time to the exit. Visual range determines how much information a pedestrian can perceive, thus the number of pedestrians within the visual field can be used to estimate waiting time at the exit. Besides, the choice firmness that reflects the degree to which a pedestrian would persist in his/her previous choice of exit is proposed. By integrating game theory into a cellular automata simulation framework, the pedestrian exit choice mechanism is investigated and explicitly modeled in this paper. A systematic analysis of the key factors influencing pedestrian evacuation is conducted, including visual radius and choice firmness of a pedestrian, initial crowd distribution of the room, exit layout as well as exit width. It is found that low choice firmness level can lead to unnatural pedestrian behavior such as wandering, which is adverse to evacuation. The longer the pedestrian’s visual radius, the earlier the pedestrian can determine his/her final selection of the exit. Compared with the scenario where the pedestrians are randomly distributed, pedestrians clustered together in a corner of the room lead to high crowd density and imbalanced use of exits. Furthermore, the exit layout and exit width also have a certain influence on pedestrian evacuation process. The results of this paper may be of benefit to the formulation of behavioral rules in other pedestrian simulation models.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Raeisi Amin, Bidokhti Abbasali, Jafar Nazemosadat Seyed Mohammad, and Lari Kamran

As the mesoscale eddies in oceans and semi-enclosed seas are significant in horizontal dispersion of pollutants, we investigate the seasonal variations of these eddies in the Persian Gulf (PG) that are usually generated due to seasonal winds and baroclinic instability. The sea surface height (SSH) data from 2010 to 2014 of AVISO are used to identify and track eddies, using the SSH-based method. Then seasonal horizontal dispersion coefficients are estimated for the PG, using the properties of eddies. The results show an annual mean of 78 eddies with a minimum lifetime of one week. Most of the eddies are predominantly cyclonic (59.1%) and have longer lifetimes and higher diffusion coefficients than the anti-cyclonic eddies. The eddy activity is higher in warm seasons, compared to that of cold seasons. As locations with high eddy diffusion coefficients are high-risk areas by using maps of horizontal eddy diffusion coefficients, perilous times and locations of the release of pollutants are specified to be within the longitude from 51.38°E to 55.28°E. The mentioned areas are located from the Strait of Hormuz towards the northeast of the PG, closer to Iranian coast. Moreover, July can be considered as the most dangerous time of pollution release.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Song Xianzhao, Li Bin, and Xie Lifeng

We experimentally observed properties of liquid film breakup for shock-wave-initiated disturbances in air at normal temperature and pressure. The tested liquids include water and various glycerol mixtures. High speed camera and multiple-spark high speed camera were utilized to record the process of liquid film breakup. A phase Doppler particle analyzer was also used to record droplet size and velocity. The experimental results show that liquid viscosity plays a vital role in the deformation, breakup and atomization of liquid films. After the interaction of shock waves, the droplet size of various glycerol mixtures is significantly smaller than either water or glycerol. Richtmyer–Meshkov instability is an important factor in the breakup and atomization of liquid films induced by shock waves. Furthermore, a dispersal model is established to study breakup mechanisms of liquid films. The correlation between droplet size and velocity is revealed quantitatively. The research results may provide improved understanding of breakup mechanisms of liquid films, and have important implications for many fields, especially for heterogeneous detonations of gas/liquid mixtures.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Xiong Lun, Li Qiang, Yang Cheng-Fu, Xie Qing-Shuang, and Zhang Jun-Ran

The equation of state (EOS) of Cr3C2 at high pressure is studied by the synchrotron radiation x-ray diffraction (XRD) in a diamond anvil cell (DAC) at ambient temperature, and density functional theory (DFT). The XRD analysis shows that the orthorhombic structure is maintained to a maximum pressure of 44.5 GPa. The XRD data show that the bulk modulus is K0 = 292 (18) GPa with K0′=3.25(0.85). In addition, the high-pressure compression behavior of Cr3C2 is studied by first principles calculations. The obtained bulk modulus of Cr3C2 is 323 (1) GPa.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Hao Qi, Qiao Ji-Chao, Goncharova E V, Afonin G V, Liu Min-Na, Cheng Yi-Ting, and Khonik V A

A relationship between thermal effects and relaxation of the high-frequency shear modulus upon heat treatment of bulk Zr48(Cu5/6Ag1/6)44Al8 metallic glass is found. This relationship is attributed to the relaxation of a interstitial-type defect system frozen-in from the melt upon glass production. Calorimetric data show that thermal effects occurring on heating include heat release below the glass transition temperature, heat absorption above it and heat release caused by crystallization. The equation derived within the Interstitialcy theory can be used to calculate the shear modulus relaxation using the calorimetric data. The obtained results are used to trace the defect concentration as functions of temperature and thermal prehistory.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Dong Jiansheng, and Ouyang Gang

Understanding the physical mechanism of structural stability and transition in various polytypes of layered transition metal dichalcogenides under the external stimulus is of crucial importance for their new applications. Here, we investigate the thickness-dependent structural properties of MoS2 under the condition of hydrostatic pressure in terms of bond relaxation and thermodynamics considerations. For both types of MoS2 structures, we find that the transition and metallization are significantly modulated by hydrostatic pressure and the number of layers. We establish a pressure-size phase diagram to address the transition mechanism. Our study not only provides insights into the thickness-dependent structural properties of MoS2, but also shows a theoretical guidance for the design and fabrication of MoS2-based devices.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Xu Sen, Hu Yangming, Liang Yuan, Shi Chenfei, Su Yuling, Guo Juan, Gao Qilong, Chao Mingju, and Liang Erjun

Oxygen vacancies have a profound effect on the magnetic, electronic, and transport properties of transition metal oxides but little is known about their effect on thermal expansion. Herein we report the effect of oxygen defects on the structure formation and thermal expansion properties of the layered perovskite Ca2RuO4 (CRO). It is shown that the CRO containing excess oxygen crystallizes in a metallic L-CRO phase without structure transition from 100 K to 500 K and displays a normal thermal expansion behavior, whereas those with oxygen vacancies adopt at room temperature an insulating S-CRO phase and exhibit an enormous negative thermal expansion (NTE) from 100 K to about 360 K, from where they undergo a structure transition to a high temperature metallic L-CRO phase. Compared to the L-CRO containing excess oxygen, the S-CRO structure has increasingly large orthorhombic strain and distinctive in-plane distortion upon cooling. The in-plane distortion of the RuO6 octahedra reaches a maximum across 260 K and then relaxes monotonically, providing a structure evidence for the appearance of an antiferromagnetic orbital ordering in the paramagnetic phase and the Ag phonon mode suppression and phase flip across the same temperature found recently. Both the L- and S-CRO display an antiferromagnetic ordering at about 150–110 K, with ferromagnetic ordering components at lower temperature. The NTE in S-CRO is a result of a complex interplay among the spin, orbital, and lattice.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Yang Heng-Yu, Chen Ya-Li, Zhou Wu-Xing, Xie Guo-Feng, and Xu Ning

The ultra-low thermal conductivity of roughened silicon nanowires (SiNWs) can not be explained by the classical phonon–surface scattering mechanism. Although there have been several efforts at developing theories of phonon–surface scattering to interpret it, but the underlying reason is still debatable. We consider that the bond order loss and correlative bond hardening on the surface of roughened SiNWs will deeply influence the thermal transport because of their ultra-high surface-to-volume ratio. By combining this mechanism with the phonon Boltzmann transport equation, we explicate that the suppression of high-frequency phonons results in the obvious reduction of thermal conductivity of roughened SiNWs. Moreover, we verify that the roughness amplitude has more remarkable influence on thermal conductivity of SiNWs than the roughness correlation length, and the surface-to-volume ratio is a nearly universal gauge for thermal conductivity of roughened SiNWs.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Pan Yuhao, Lei Bao, Qiao Jingsi, Hu Zhixin, Zhou Wu, and Ji Wei

Point and line defects are of vital importance to the physical and chemical properties of certain two-dimensional (2D) materials. Although electron beams have been demonstrated to be capable of creating single- and multi-atom defects in 2D materials, the products are often random and difficult to predict without theoretical inputs. In this study, the thermal motion of atoms and electron incident angle were additionally considered to study the vacancy evolution in a black phosphorus (BP) monolayer by using an improved first-principles molecular dynamics method. The P atoms in monolayer BP tend to be struck away one by one under an electron beam within the displacement threshold energy range of 8.55–8.79 eV, which ultimately induces the formation of a zigzag-like chain vacancy. The chain vacancy is a thermodynamically metastable state and is difficult to obtain by conventional synthesis methods because the vacancy formation energy of 0.79 eV/edge atom is higher than the typical energy in monolayer BP. Covalent-like quasi-bonds and a charge density wave are formed along the chain vacancy, exhibiting rich electronic properties. This work proposes a theoretical protocol for simulating a complete elastic collision process of electron beams with 2D layers and will facilitate the establishment of detailed theoretical guidelines for experiments on 2D material etching using focused high-energy electron beams.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Li Yong, Xu Peng, Zhang Xiaoming, Liu Guodong, Liu Enke, and Li Lingwei

The electronic structures, magnetic properties, and martensitic transformation in all-d-metal Heusler-like alloys Cd2MnTM (TM = Fe, Ni, Cu) were investigated by the first-principles calculations based on density-functional theory. The results indicate that all three alloys are stabilized in the ferromagnetic L21-type structure. The total magnetic moments mainly come from Mn and Fe atoms for Cd2MnFe, whereas, only from Mn atoms for Cd2MnNi and Cd2MnCu. The magnetic moment at equilibrium lattice constant of Cd2MnFe (6.36 μB) is obviously larger than that of Cd2MnNi (3.95 μB) and Cd2MnCu (3.82 μB). The large negative energy differences (ΔE) between martensite and austenite in Cd2MnFe and Cd2MnNi under tetragonal distortion and different uniform strains indicate the possible occurrence of ferromagnetic martensitic transformation (FMMT). The minimum total energies in martensitic phase are located with the c/a ratios of 1.41 and 1.33 for Cd2MnFe and Cd2MnNi, respectively. The total moments in martensitic state still maintain large values compared with those in cubic state. The study is useful to find the new all-d-metal Heusler alloys with FMMT.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Li Shu-Fa, and Zhu Tao

The current-induced spin-orbit torque (SOT) plays a dominant role to manipulate the magnetization in a heavy metal/ferromagnetic metal bilayer. We separate the contributions of interfacial and bulk spin-orbit coupling (SOC) to the effective field of field-like SOT in a typical NiFe/Pt bilayer by planar Hall effect (PHE). The effective field from interfacial SOC is directly measured at the transverse PHE configuration. Then, at the longitudinal configuration, the effective field from bulk SOC is determined, which is much smaller than that from interfacial SOC. The giant interface SOT in NiFe/Pt bilayers suggests that further analysis of interfacial effects on the current-induced manipulation of magnetization is necessary.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Guan Yurou, Song Lingling, Zhao Hui, Du Renjun, Liu Liming, Yan Cuixia, and Cai Jinming

The fascinating Dirac cone in honeycomb graphene, which underlies many unique electronic properties, has inspired the vast endeavors on pursuing new two-dimensional (2D) Dirac materials. Based on the density functional theory method, a 2D material Zn3Si2 of honeycomb transition-metal silicide with intrinsic Dirac cones has been predicted. The Zn3Si2 monolayer is dynamically and thermodynamically stable under ambient conditions. Importantly, the Zn3Si2 monolayer is a room-temperature 2D Dirac material with a spin–orbit coupling energy gap of 1.2 meV, which has an intrinsic Dirac cone arising from the special hexagonal lattice structure. Hole doping leads to the spin polarization of the electron, which results in a Dirac half-metal feature with single-spin Dirac fermion. This novel stable 2D transition-metal-silicon-framework material holds promises for electronic device applications in spintronics.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Zhang Hao, Tang Hui-Li, He Nuo-Tian, Zhu Zhi-Chao, Chen Jia-Wen, Liu Bo, and Xu Jun

High quality 0.02 mol%, 0.05 mol%, and 0.08 mol% Fe: β-Ga2O3 single crystals were grown by the floating zone method. The crystal structure, optical, electrical, and thermal properties were measured and discussed. Fe: β-Ga2O3 single crystals showed transmittance of higher than 80% in the near infrared region. With the increase of the Fe doping concentration, the optical bandgaps reduced and room temperature resistivity increased. The resistivity of 0.08 mol% Fe: β-Ga2O3 crystal reached to 3.63 × 1011 Ω ?cm. The high resistivity Fe: β-Ga2O3 single crystals could be applied as the substrate for the high-power field effect transistors (FETs).

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Zhu Guodong, Guo Yangzhe, Dong Bin, and Fang Yurui

Quantum theory of surface plasmons is very important for studying the interactions between light and different metal nanostructures in nanoplasmonics. In this work, using the canonical quantization method, the SPPs on nanowires and their orbital and spin angular momentums are investigated. The results show that the SPPs on nanowire carry both orbital and spin momentums during propagation. Later, the result is applied to the plasmonic nanowire waveguide to show the agreement of the theory. The study is helpful for the nano wire based plasmonic interactions and the quantum information based optical circuit in the future.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Li Xue-Chao, Ye Chun-Bao, Gao Juan, and Wang Bing

We theoretically investigate the optical absorption coefficient (OAC) in asymmetrical Gaussian potential quantum dots subject to an applied electric field. Confined wave functions together with energies of electron energies in an effective mass approximation framework are obtained. The OAC is expressed according to the iterative method and the compact-density-matrix approach. Based on our results, OAC is sensitively dependent on external electric field together with the incident optical intensity. Additionally, peak shifts into greater energy as the quantum dot radius decrease. Moreover, the parameters of Gaussian potential have a significant influence on the OAC.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Zan Ying, Li Yong-Liang, Cheng Xiao-Hong, Zhao Zhi-Qian, Liu Hao-Yan, Hu Zhen-Hua, Du An-Yan, and Wang Wen-Wu

A high crystalline quality of SiGe fin with an Si-rich composition area using the replacement fin processing is systematically demonstrated in this paper. The fin replacement process based on a standard FinFET process is developed. A width of less than 20-nm SiGe fin without obvious defect impact both in the direction across the fin and in the direction along the fin is verified by using the high angle annular dark field scanning transmission electron microscopy and the scanning moiré fringe imaging technique. Moreover, the SiGe composition is inhomogenous in the width of the fin. This is induced by the formation of {111} facets. Due to the atomic density of the {111} facets being higher, the epitaxial growth in the direction perpendicular to these facets is slower than in the direction perpendicular to {001}. The Ge incorporation is then higher on the {111} facets than on the {001} facets. So, an Si-rich area is observed in the central area and on the bottom of SiGe fin.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Lv Quan-Jiang, Zhang Yi-Hong, Zheng Chang-Da, Gao Jiang-Dong, Zhang Jian-Li, and Liu Jun-Lin

Inhomogeneous electroluminescence (EL) of InGaN green LEDs grown on mesh-patterned Si (111) substrate had been investigated. Sample with n-AlGaN inserted between the pre-strained layers and the first quantum well showed the inhomogeneous EL in the low current density range. Near-field EL emission intensity distribution images depicted that inhomogeneity in the form of premature turn-on at the periphery of the LED chip, results in stronger emission intensity at the edges. This premature turn-on effect significantly reduces the luminous efficacy and higher ideality factor value due to locally current crowding effect. Raman measurement and fluorescence microscopy results indicated that the partially relaxed in-plane stress at the edge of the window region acts as a parasitic diode with a smaller energy band gap, which is a source of edge emission. Numerical simulations showd that the tilted triangular n-AlGaN functions like a forward-biased Schottky diode, which not only impedes carrier transport, but also contributes a certain ideality factor.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Liu Qiaoli, Zhang Haiyan, Hao Lingxiang, Hu Anqi, Wu Guang, and Guo Xia

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Yang Yuan-Zhi, Hu Min, and Huang Tai-Yu

Identifying influential nodes in complex networks is essential for network robust and stability, such as viral marketing and information control. Various methods have been proposed to define the influence of nodes. In this paper, we comprehensively consider the global position and local structure to identify influential nodes. The number of iterations in the process of k-shell decomposition is taken into consideration, and the improved k-shell decomposition is then put forward. The improved k-shell decomposition and degree of target node are taken as the benchmark centrality, in addition, as is well known, the effect between node pairs is inversely proportional to the shortest path length between two nodes, and then we also consider the effect of neighbors on target node. To evaluate the performance of the proposed method, susceptible-infected (SI) model is adopted to simulate the spreading process in four real networks, and the experimental results show that the proposed method has obvious advantages over classical centrality measures in identifying influential nodes.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Pan Qi, and Chu Bao-Jin

The piezoelectric, ferromagnetism, and magnetoelectric response of BiFeO3–BaTiO3 ceramics with the compositions around the morphotropic phase boundary (MPB) of the solid solution are systematically investigated after the ceramics have been quenched from a high temperature. We find that the ferromagnetism of the quenched ceramics is greatly enhanced. An enhanced piezoelectric response d33 larger than 200 pC/N, which could be sustained up to 350 °C, is measured. As a result of enhanced ferromagnetism and piezoelectric response, a large magnetoelectric response ～ 1.3 V/cm·Oe (1 Oe = 79.5775 A·m-1) is obtained near the mechanical resonance frequency of the quenched ceramic samples. Our research also shows that in addition to the ferromagnetism and piezoelectric response, the mechanical quality factor is another important parameter to achieve high magnetoelectric response because the physical effects are coupled through mechanical interaction in BiFeO3-based materials. Our work suggests that quenching is an effective approach to enhancing the magnetoelectric response of BiFeO3-based materials and the materials belong to single-phase multiferroic materials with high magnetoelectric response.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Dong Ke-Xiu, Chen Dun-Jun, Cai Qing, liu Yan-Li, and Wang Yu-Jie

To suppress the electric field crowding at sidewall and improve the detection sensitivity of the AlGaN separate absorption and multiplication (SAM) avalanche photodiodes (APDs), we propose the new AlGaN APDs structure combining a large-area mesa with a field plate (FP). The simulated results show that the proposed AlGaN APDs exhibit a significant increase in avalanche gain, about two orders of magnitude, compared to their counterparts without FP structure, which is attributed to the suppression of electric field crowding at sidewall of multiplication layer and the reduction of the maximum electric field at the p-type GaN sidewall in p–n depletion region. Meanwhile, the APDs can produce an obviously enhanced photocurrent due to the increase in cross sectional area of multiplication region.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Yan Zhao, Pan Hongyi, Wang Junyang, Chen Rusong, Luo Fei, Yu Xiqian, and Li Hong

The dissolution of transition metal (TM) cations from oxide cathodes and the subsequent migration and deposition on the anode lead to the deconstruction of cathode materials and uncontrollable growth of solid electrode interphase (SEI). The above issues have been considered as main causes for the performance degradation of lithium-ion batteries (LIBs). In this work, we reported that the solid oxide electrolyte Li1.5Al0.5Ti1.5(PO4)3 (LATP) coating on polyethylene (PE) polymer separator can largely block the TM dissolution and deposition in LIBs. Scanning electron microscopy (SEM), second ion mass spectroscopy (SIMS), and Raman spectroscopy characterizations reveal that the granular surface of the LATP coating layer is converted to a dense morphology due to the reduction of LATP at discharge process. The as-formed dense surface layer can effectively hinder the TM deposition on the anode electrode and inhibit the TM dissolution from the cathode electrode. As a result, both the LiCoO2/SiO-graphite and LiMn2O4/SiO-graphite cells using LATP coated PE separator show substantially enhanced cycle performances compared with those cells with Al2O3 coated PE separator.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Luo Linling, Ye Xiaoqiu, Zhang Guanghui, Kou Huaqin, Xiong Renjin, Sang Ge, Yu Ronghai, and Zhao Dongliang

The vacuum arc melting method was used to prepare ZrCo1 - xCrx (x = 0, 0.025, 0.05, 0.075, 0.1) alloys. Afterward, the crystal structure, hydrogenation kinetics, thermodynamic properties, and disproportionation performance of ZrCo1 - xCrx (x = 0-0.1) alloys were investigated. The x-ray diffraction spectra demonstrated that ZrCo1 - xCrx (x = 0-0.1) alloys contained ZrCo and ZrCo2 phases, and their corresponding hydrides consisted of ZrCoH3 and ZrH phases. The activation behaviors of Cr-substituted samples were significantly promoted. The activation time of ZrCo was 7715 s while that of ZrCo0.9Cr0.1 was 195 s. The improvement of kinetics can be attributed to the catalytic hydrogenation of ZrCr2. The activation energy for the hydrogenation of ZrCo was 44.88-kJ?mol-1 H2 and decreased to 40.34-kJ?mol-1 H2 for ZrCo0.95Cr0.05. The plateau pressure and width of the pressure–composition–temperature curves decreased slightly as Cr content increased. The extent of disproportionation of ZrCo was 83.68% after being insulated at 798 K for 10 h and decreased slightly to 70.52% for ZrCo0.9Cr0.1. The improvement of anti-disproportionation performance can be attributed to increase in the activation energy of disproportionation from 167.46-kJ?mol-1 H2 for ZrCo to 168.28-kJ?mol-1 H2 for ZrCo0.95Cr0.05.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Li Xin-Qi, Li Zhi-Lin, Zhao Jia-Ji, and Wu Xiao-Song

We report experimental investigation of the resistivity and Nernst effect in two-dimensional (2D) NbSe2 crystals. A strongly enhanced Nernst effect, 100 times larger than that in bulk NbSe2, caused by moving vortices is observed in thin film. It is found that in the low temperature, high magnetic field regime, pinning effects show little dependence on the thickness and resistivity of the superconductor films. Strong Nernst signals persist above the superconducting transition, suggesting that the Nernst effect is a sensitive probe to superconducting fluctuations. A magnetic field induced superconductor--insulator transition (SIT) is evident, which is surprising in that such a SIT usually takes place in disordered dirty superconductors, while our samples are highly crystalline and close to the clean limit. Hence, our results expand the scope of SIT into 2D crystal clean superconductors.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Nazir Mudassar, Yang Xiaoyan, Tian Huanfang, Song Pengtao, Wang Zhan, Xiang Zhongcheng, Guo Xueyi, Jin Yirong, You Lixing, and Zheng Dongning

We study superconducting properties of NbN thin film samples with different thicknesses and an ultra-thin NbTiN meander nanowire sample. For the ultra-thin samples, we found that the temperature dependence of upper critical field (Hc2) in parallel to surface orientation shows bending curvature close to critical temperature Tc, suggesting a two-dimensional (2D) nature of the samples. The 2D behavior is further supported by the angular dependence measurements of Hc2 for the thinnest samples. The temperature dependence of parallel upper critical field for the thick films could be described by a model based on the anisotropic Ginzburg–Landau theory. Interestingly, the results measured in the field perpendicular to the film surface orientation show a similar bending curvature but in a much narrow temperature region close to Tc for the ultra-thin samples. We suggest that this feature could be due to suppression of pair-breaking caused by local in-homogeneity. We further propose the temperature dependence of perpendicular Hc2 as a measure of uniformity of superconducting ultra-thin films. For the thick samples, we find that Hc2 shows maxima for both parallel and perpendicular orientations. The Hc2 peak for the perpendicular orientation is believed to be due to the columnar structure formed during the growth of the thick films. The presence of columnar structure is confirmed by transmission electron microscopy (TEM). In addition, we have measured the angular dependence of magneto-resistance, and the results are consistent with the Hc2 data.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Hu Sheng, Yang Ling, Mi Min-Han, Hou Bin, Liu Sheng, Zhang Meng, Wu Mei, Zhu Qing, Wu Sheng, Lu Yang, Zhu Jie-Jie, Zhou Xiao-Wei, Lv Ling, Ma Xiao-Hua, and Hao Yue

The graded AlGaN:Si back barrier can form the majority of three-dimensional electron gases (3DEGs) at the GaN/graded AlGaN:Si heterostructure and create a composite two-dimensional (2D)–three-dimensional (3D) channel in AlGaN/GaN/graded-AlGaN:Si/GaN:C heterostructure (DH:Si/C). Frequency-dependent capacitances and conductance are measured to investigate the characteristics of the multi-temperature trap states of in DH:Si/C and AlGaN/GaN/GaN:C heterostructure (SH:C). There are fast, medium, and slow trap states in DH:Si/C, while only medium trap states exist in SH:C. The time constant/trap density for medium trap state in SH:C heterostructure are (11 μs–17.7 μs)/(1.1 × 1013 cm-2·eV-1–3.9× 1013 cm-2·eV-1) and (8.7 μs–14.1 μs)/(0.7× 1013 cm-2·eV-1–1.9× 1013 cm-2·eV-1) at 300 K and 500 K respectively. The time constant/trap density for fast, medium, and slow trap states in DH:Si/C heterostructure are (4.2 μs–7.7 μs)/(1.5× 1013 cm-2·eV-1–3.2× 1013 cm-2·eV-1), (6.8 μs–11.8 μs)/(0.8× 1013 cm-2 · eV-1–2.8× 1013 cm-2 · eV-1), (30.1 μs–151 μs)/(7.5× 1012 cm-2 · eV-1–7.8× 1012 cm-2 · eV-1) at 300 K and (3.5 μs–6.5 μs)/(0.9× 1013 cm-2 · eV-1–1.8× 1013 cm-2 · eV-1), (4.9 μs–9.4 μs)/(0.6× 1013 cm-2 · eV-1–1.7× 1013 cm-2 · eV-1), (20.6 μs–61.9 μs)/(3.2× 1012 cm-2 · eV-1–3.5× 1012 cm-2·eV-1) at 500 K, respectively. The DH:Si/C structure can effectively reduce the density of medium trap states compared with SH:C structure.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Xiang Xiao-Jun, Song Guo-Zhu, Zhou Xue-Feng, Liang Hao, Xu Yue, Qin Shi-Jun, Wang Jun-Pu, Hong Fang, Dai Jian-Hong, Zhou Bo-Wen, Liang Wen-Jia, Yin Yun-Yu, Zhao Yu-Sheng, Peng Fang, Yu Xiao-Hui, and Wang Shan-Min

As one of important members of refractory materials, tungsten phosphide (WP) holds great potential for fundamental study and industrial applications in many fields of science and technology, due to its excellent properties such as superconductivity and as-predicted topological band structure. However, synthesis of high-quality WP crystals is still a challenge by using tradition synthetic methods, because the synthesis temperature for growing its large crystals is very stringently required to be as high as 3000 °C, which is far beyond the temperature capability of most laboratory-based devices for crystal growth. In addition, high temperature often induces the decomposition of metal phosphides, leading to off-stoichiometric samples based on which the materials’ intrinsic properties cannot be explored. In this work, we report a high-pressure synthesis of single-crystal WP through a direct crystallization from cooling the congruent W–P melts at 5 GPa and ～ 3200 °C. In combination of x-ray diffraction, electron microscope, and thermal analysis, the crystal structure, morphology, and stability of recovered sample are well investigated. The final product is phase-pure and nearly stoichiometric WP in a single-crystal form with a large grain size, in excess of one millimeter, thus making it feasible to implement most experimental measurements, especially, for the case where a large crystal is required. Success in synthesis of high-quality WP crystals at high pressure can offer great opportunities for determining their intrinsic properties and also making more efforts to study the family of transition-metal phosphides.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Niu Lei, Chen Yimin, Shen Xiang, and Xu Tiefeng

Ge–Ga–S thin films were deposited by magnetron sputtering with mean coordination number (MCN) ranging from 2.46 to 2.94. The physical properties of the Ge–Ga–S films, including optical band gap, refractive index, and thickness, vary with the time of heat treatment. Based on the analysis of the topology model, it is concluded that the Ge–Ga–S thin films with components close to the stoichiometric ratio can form the most Ga–S bonds and Ga–S bonds, and the physical properties of the Ge27.3Ga6.3S66.3 (MCN = 2.62) film are the most stable. This is an important reference for thin film photonic devices.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Jin Cheng-Jie, Jiang Rui, and Li Da-Wei

In order to investigate the influence of bottleneck on single-file pedestrian flow, we conduct two different bottleneck experiments. The first one is on ring road, while the second one is on straight route. For the first one, the global density is always set to be 1.5 ped/m. The corresponding critical flow rate for the bottleneck activation is about 0.57 ped/s. The data of the detectors set at different locations, including the velocities and time-headways, show that the amplitude of the oscillation of the stop-and-go waves gradually increases during the upstream propagation. Besides, when the measured flow rates are the same, the different situations in the single-file experiments with and without bottleneck are compared and discussed. For the second one, lower flow rates are used and the bottleneck is always activated. In all the runs, the system can reach one stable state, and the time needed is nearly the same. Inside the stable area, the statistics of pedestrians’ velocities keeps nearly constant in both time and space. Outside this area, when the waiting time is not long (X = 10 s), the phenomenon observed is similar to that found on ring road, e.g., the statistics of pedestrians’ velocities also gradually increases during the upstream propagation. This phenomenon is similar to that found in vehicular traffic flow, which shows the universality of different traffic flows. But when the waiting time becomes longer (X = 20 s), this situation will be broken since the actions of many pedestrians become much slower. All these results can facilitate understanding more about the influence of bottleneck on single-file pedestrian flow.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Gao Shan, Zhang Chong-Yang, Ao Hong-Rui, and Jiang Hong-Yuan

We demonstrate a piezoelectric vibration energy harvester with the ZnO piezoelectric film and an improved synchronous electric charge extraction energy harvesting circuit on the basis of the beam-type mechanical structure, especially investigate its output performance in vibration harvesting and ability to generate charges. By establishing the theoretical model for each of vibration and circuit, the numerical results of voltage and power output are obtained. By fabricating the prototype of this harvester, the quality of the sputtered film is explored. Theoretical and experimental analyses are conducted in open-circuit and closed-circuit conditions, where the open-circuit mode refers to the voltage output in relation to the ZnO film and external excitation, and the power output of the closed-circuit mode is relevant to resistance. Experimental findings show good agreement with the theoretical ones, in the output tendency. It is observed that the properties of ZnO film achieve regularly direct proportion to output performance under different excitations. Furthermore, a maximum experimental power output of 4.5 mW in a resistance range of 3 kΩ–8 kΩ is achieved by using an improved synchronous electric charge extraction circuit. The result is not only more than three times the power output of classic circuit, but also can broaden the resistance to a large range of 5 kΩ under an identical maximum value of power output. In this study we demonstrate the fundamental mechanism of piezoelectric materials under multiple conditions and take an example to show the methods of fabricating and testing the ZnO film. Furthermore, it may contribute to a novel energy harvesting circuit with high output performance.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Sheng Ling-Xiao, Chen Cheng-Ke, Jiang Mei-Yan, Li Xiao, and Hu Xiao-Jun

The microstructure and Ge-V photoluminescent properties of diamond particles treated by microwave oxygen plasma are investigated. The results show that in the first 5 min of microwave plasma treatment, graphite and disordered carbon on the surface of the particles are etched away, so that diamond with regular crystal plane, smaller lattice stress, and better crystal quality is exposed, producing a Ge-V photoluminescence (PL) intensity 4 times stronger and PL peak FWHM (full width at half maximum) value of 6.6 nm smaller than the as-deposited sample. It is observed that the cycles of ‘diamond is converted into graphite and disordered carbon, then the graphite and disordered carbon are etched’ can occur with the treatment time further increasing. During these cycles, the particle surface alternately appears smooth and rough, corresponding to the strengthening and weakening of Ge-V PL intensity, respectively, while the PL intensity is always stronger than that of the as-deposited sample. The results suggest that not only graphite but also disordered carbon weakens the Ge-V PL intensity. Our study provides a feasible way of enhancing the Ge-V PL properties and effectively controlling the surface morphology of diamond particle.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Zeng Jing, Chen Ke-Qiu, and Zhou Yanhong

Very recently, experimental evidence showed that the hydrogen is retained in dithiol-terminated single-molecule junction under the widely adopted preparation conditions, which is in contrast to the accepted view [Nat. Chem.11 351 (2019)]. However, the hydrogen is generally assumed to be lost in the previous physical models of single-molecule junctions. Whether the retention of the hydrogen at the gold—sulfur interface exerts a significant effect on the theoretical prediction of spin transport properties is an open question. Therefore, here in this paper we carry out a comparative study of spin transport in M-tetraphenylporphyrin-based (M = V, Cr, Mn, Fe, and Co; M-TPP) single-molecule junction through Au–SR and Au–S(H)R bondings. The results show that the hydrogen at the gold–sulfur interface may dramatically affect the spin-filtering efficiency of M-TPP-based single-molecule junction, depending on the type of transition metal ions embedded into porphyrin ring. Moreover, we find that for the Co-TPP-based molecular junction, the hydrogen at the gold–sulfur interface has no obvious effect on transmission at the Fermi level, but it has a significant effect on the spin-dependent transmission dip induced by the quantum interference on the occupied side. Thus the fate of hydrogen should be concerned in the physical model according to the actual preparation condition, which is important for our fundamental understanding of spin transport in the single-molecule junctions. Our work also provides guidance in how to experimentally identify the nature of gold–sulfur interface in the single-molecule junction with spin-polarized transport.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Hu Ji-Wei, Gao Song, Yan Jun-Wei, Lou Ping, and Yin Yong

With the increasingly fierce market competition, manufacturing enterprises have to continuously improve their competitiveness through their collaboration and labor division with each other, i.e. forming manufacturing enterprise collaborative network (MECN) through their collaboration and labor division is an effective guarantee for obtaining competitive advantages. To explore the topology and evolutionary process of MECN, in this paper we investigate an empirical MECN from the viewpoint of complex network theory, and construct an evolutionary model to reproduce the topological properties found in the empirical network. Firstly, large-size empirical data related to the automotive industry are collected to construct an MECN. Topological analysis indicates that the MECN is not a scale-free network, but a small-world network with disassortativity. Small-world property indicates that the enterprises can respond quickly to the market, but disassortativity shows the risk spreading is fast and the coordinated operation is difficult. Then, an evolutionary model based on fitness preferential attachment and entropy-TOPSIS is proposed to capture the features of MECN. Besides, the evolutionary model is compared with a degree-based model in which only node degree is taken into consideration. The simulation results show the proposed evolutionary model can reproduce a number of critical topological properties of empirical MECN, while the degree-based model does not, which validates the effectiveness of the proposed evolutionary model.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Zhao Yao-Peng, Wang Chong, Zheng Xue-Feng, Ma Xiao-Hua, Liu Kai, Li Ang, He Yun-Long, and Hao Yue

Two types of enhancement-mode (E-mode) AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs) with different gate insulators are fabricated on Si substrates. The HfO2 gate insulator and the Al2O3 gate insulator each with a thickness of 30 nm are grown by the plasma-enhanced atomic layer deposition (PEALD). The energy band diagrams of two types of dielectric MIS-HEMTs are compared. The breakdown voltage (VBR) of HfO2 dielectric layer and Al2O3 dielectric layer are 9.4 V and 15.9 V, respectively. With the same barrier thickness, the transconductance of MIS-HEMT with HfO2 is larger. The threshold voltage (Vth) of the HfO2 and Al2O3 MIS-HEMT are 2.0 V and 2.4 V, respectively, when the barrier layer thickness is 0 nm. The C–V characteristics are in good agreement with the Vth’s transfer characteristics. As the barrier layer becomes thinner, the drain current density decreases sharply. Due to the dielectric/AlGaN interface is very close to the channel, the scattering of interface states will lead the electron mobility to decrease. The current collapse and the Ron of Al2O3 MIS-HEMT are smaller at the maximum gate voltage. As Al2O3 has excellent thermal stability and chemical stability, the interface state density of Al2O3/AlGaN is less than that of HfO2/AlGaN.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Sun Chang Q

A sequential of concepts developed in the last decade has enabled a resolution to multiple anomalies of water ice and its low-dimensionality, particularly. Developed concepts include the coupled hydrogen bond (O:H–O) oscillator pair, segmental specific heat, three-body coupling potentials, quasisolidity, and supersolidity. Resolved anomalies include ice buoyancy, ice slipperiness, water skin toughness, supercooling and superheating at the nanoscale, etc. Evidence shows consistently that molecular undercoordination shortens the H–O bond and stiffens its phonon while undercoordination does the O:H nonbond contrastingly associated with strong lone pair “:” polarization, which endows the low-dimensional water ice with supersolidity. The supersolid phase is hydrophobic, less dense, viscoelastic, thermally more diffusive, and stable, having longer electron and phonon lifetime. The equal number of lone pairs and protons reserves the configuration and orientation of the coupled O:H–O bonds and restricts molecular rotation and proton hopping, which entitles water the simplest, ordered, tetrahedrally-coordinated, fluctuating molecular crystal covered with a supersolid skin. The O:H–O segmental cooperativity and specific-heat disparity form the soul dictate the extraordinary adaptivity, reactivity, recoverability, and sensitivity of water ice when subjecting to physical perturbation. It is recommended that the premise of “hydrogen bonding and electronic dynamics” would deepen the insight into the core physics and chemistry of water ice.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Liu Jie, Fan Jianzhong, Zhang Kai, Zhang Yuchen, Wang Chuan-Kui, and Lin Lili

To enhance the potential application of thermally activated delayed fluorescence (TADF) molecular materials, new functions are gradually cooperated to the TADF molecules. Aggregation induced emission can effectively solve the fluorescence quenching problem for TADF molecules in solid phase, thus aggregation-induced delayed fluorescence (AIDF) molecules were recently focused. Nevertheless, their luminescent mechanisms are not clear enough. In this work, excited state properties of an AIDF molecule DMF-BP-DMAC [reported in Chemistry–An Asian Journal14 828 (2019)] are theoretically studied in tetrahydrofuran (THF) and solid phase. For consideration of surrounding environment, the polarizable continuum method (PCM) and the combined quantum mechanics and molecular mechanics (QM/MM) method were applied for solvent and solid phase, respectively. Due to the increase of the transition dipole moment and decrease of the energy difference between the first single excited state (S1) and the ground state (S0), the radiative rate is increased by about 2 orders of magnitude in solid phase. The energy dissipation of the non-radiative process from S1 to S0 is mainly contributed by low-frequency vibrational modes in solvent, and they can be effectively suppressed in aggregation, which may lead to a slow non-radiation process in solid phase. Both factors would induce enhanced luminescence efficiency of DMF-BP-DMAC in solid phase. Meanwhile, the small energy gap between S1 and triplet excited states results in high reverse intersystem crossing (RISC) rates in both solvent and solid phase. Therefore, TADF is confirmed in both phases. Aggregation significantly influences both the ISC and RISC processes and more RISC channels are involved in solid state. The enhanced delayed fluorescence should be induced by both the enhanced fluorescent efficiency and ISC efficiency. Our calculation provides a reasonable explanation for experimental measurements and helps one to better understand the luminescence mechanism of AIDF molecules.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Liang Jinming, Lei Jiangtao, Wang Yun, Ding Yan, Shen Yun, and Deng Xiaohua

Graphene and black phosphorus have attracted tremendous attention in optics due to their support of localized plasmon resonance. In this paper, a structure consisted of graphene–black phosphorus heterostructure is proposed to realize terahertz anisotropic near-perfect absorption. We demonstrate that strong plasmonic resonances in graphene–black phosphorus heterostructure nanoribbons can both be provided along armchair and zigzag directions, and dominated by the distance between the graphene and black phosphorus ribbons. In particular, the maximum absorption of 99.6% at 10.2 THz along armchair direction can be reached. The proposed high performance anisotropic structure may have promising potential applications in photodetectors, biosensors, and terahertz imaging.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Liu Hongbo

The electrocaloric effect of ferroelectric ceramics has been studied extensively for solid-state caloric cooling. Generally, most ferroelectric ceramics are poor thermal conductors. In this work, the possibility of enhancing the thermal conduction of ferroelectric ceramics through the electrocaloric effect is studied. A multilayer ceramic structure is proposed and the proper sequential electric field is applied to each ceramic layer. The result shows that the thermal conduction of the multilayer structure is significantly enhanced because of the electrocaloric effect of the ferroelectric ceramics. As a result, the work finds an alternatively way of applying the electrocaloric effect, prompting thermal conduction.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Yang Xiao-Chen, and Xing Yan

The linear and nonlinear optical absorption coefficients (ACs) and refraction index changes (RICs) of 1s–1p, 1p–1d, and 1f–1d transitions are investigated in a wurtzite InxGa1 – xN/GaN core–shell quantum dot (CSQD) with donor impurity by using density matrix approach. The effects of built-in electric field (BEF), ternary mixed crystal (TMC), impurity, and CSQD size are studied in detail. The finite element method is used to calculate the ground and excited energy state energy and wave function. The results reveal that the BEF has a great influence on the linear, nonlinear, and total ACs and RICs. The presence of impurity leads the resonant peaks of the ACs and RICs to be blue-shifted for all transitions, especially for 1s–1p transition. It is also found that the resonant peaks of the ACs and RICs present a red shift with In-composition decreasing or core radius increasing. Moreover, the amplitudes of the ACs and RICs are strongly affected by the incident optical intensity. The absorption saturation is more sensitive without the impurity than with the impurity, and the appearance of absorption saturation requires a larger incident optical intensity when considering the BEF.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Su Guifeng, Li Xiaowen, Zhang Xiaobing, and Zhang Yi

The dynamics of zero-range processes on complex networks is expected to be influenced by the topological structure of underlying networks. A real space complete condensation phase transition in the stationary state may occur. We study the finite density effects of the condensation transition in both the stationary and dynamical zero-range processes on scale-free networks. By means of grand canonical ensemble method, we predict analytically the scaling laws of the average occupation number with respect to the finite density for the steady state. We further explore the relaxation dynamics of the condensation phase transition. By applying the hierarchical evolution and scaling ansatz, a scaling law for the relaxation dynamics is predicted. Monte Carlo simulations are performed and the predicted density scaling laws are nicely validated.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Li Li, Mak Kai-Yu, and Zhou Yan

We report a p24 (HIV disease biomarker) detection assay using an MgO-based magnetic tunnel junction (MTJ) sensor and 20-nm magnetic nanoparticles. The MTJ array sensor with sensing area of 890 × 890 μm2 possessing a sensitivity of 1.39 %/Oe was used to detect p24 antigens. It is demonstrated that the p24 antigens could be detected at a concentration of 0.01 μg/ml. The development of bio-detection systems based on magnetic tunnel junction sensors with high-sensitivity will greatly benefit the early diagnosis of HIV.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Zhang Jinbing, Wang Qiang, and Cao Zexian

Transport properties and the associated structural heterogeneity of room temperature aqueous ionic liquids and especially of super-concentrated electrolyte aqueous solutions have received increasing attention, due to their potential application in ionic battery. This paper briefly reviews the results reported mainly since 2010 about the liquid–liquid separation, aggregation of polar and apolar domains in neat RTILs, and solvent clusters and 3D networks chiefly constructed by anions in super-concentrated electrolyte solutions. At the same time, the dominating effect of desolvation process of metal ions at electrode/electrolyte interface upon the transport of metal ions is stressed. This paper also presents the current understanding of how water affects the anion–cation interaction, structural heterogeneities, the structure of primary coordination sheath of metal ions and consequently their transport properties in free water-poor electrolytes.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Li Peng-Cheng, and Chu Shih-I

We present the recent new developments of time-dependent Schr?dinger equation and time-dependent density-functional theory for accurate and efficient treatment of the electronic structure and time-dependent quantum dynamics of many-electron atomic and molecular systems in intense laser fields. We extend time-dependent generalized pseudospectral (TDGPS) numerical method developed for time-dependent wave equations in multielectron systems. The TDGPS method allows us to obtain highly accurate time-dependent wave functions with the use of only a modest number of spatial grid point for complex quantum dynamical calculations. The usefulness of these procedures is illustrated by a few case studies of atomic and molecular processes of current interests in intense laser fields, including multiphoton ionization, above-threshold ionization, high-order harmonic generation, attosecond pulse generation, and quantum dynamical processes related to multielectron effects. We conclude this paper with some open questions and perspectives of multiphoton quantum dynamics of many-electron atomic and molecular systems in intense laser fields.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Zeng Hong-Li, and Aurell Erik

As a problem in data science the inverse Ising (or Potts) problem is to infer the parameters of a Gibbs–Boltzmann distributions of an Ising (or Potts) model from samples drawn from that distribution. The algorithmic and computational interest stems from the fact that this inference task cannot be carried out efficiently by the maximum likelihood criterion, since the normalizing constant of the distribution (the partition function) cannot be calculated exactly and efficiently. The practical interest on the other hand flows from several outstanding applications, of which the most well known has been predicting spatial contacts in protein structures from tables of homologous protein sequences. Most applications to date have been to data that has been produced by a dynamical process which, as far as it is known, cannot be expected to satisfy detailed balance. There is therefore no a priori reason to expect the distribution to be of the Gibbs–Boltzmann type, and no a priori reason to expect that inverse Ising (or Potts) techniques should yield useful information. In this review we discuss two types of problems where progress nevertheless can be made. We find that depending on model parameters there are phases where, in fact, the distribution is close to Gibbs–Boltzmann distribution, a non-equilibrium nature of the under-lying dynamics notwithstanding. We also discuss the relation between inferred Ising model parameters and parameters of the underlying dynamics.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Wang Chen, and Xu Da-Zhi

We investigate the quantum thermal transistor effect in nonequilibrium three-level systems by applying the polaron-transformed Redfield equation combined with full counting statistics. The steady state heat currents are obtained via this unified approach over a wide region of system–bath coupling, and can be analytically reduced to the Redfield and nonequilibrium noninteracting blip approximation results in the weak and strong coupling limits, respectively. A giant heat amplification phenomenon emerges in the strong system–bath coupling limit, where transitions mediated by the middle thermal bath are found to be crucial to unravel the underlying mechanism. Moreover, the heat amplification is also exhibited with moderate coupling strength, which can be properly explained within the polaron framework.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Wu Y Y, Zhu X L, Yang H Y, Wang Z G, Li Y H, and Wang B T

Sulfide nanocrystals and their composites have shown great potential in the thermoelectric (TE) field due to their extremely low thermal conductivity. Recently a solid and hollow metastable Au2S nanocrystalline has been successfully synthesized. Herein, we study the TE properties of this bulk Au2S by first-principles calculations and semiclassical Boltzmann transport theory, which provides the basis for its further experimental studies. Our results indicate that the highly twofold degeneracy of the bands appears at the Γ point in the Brillouin zone, resulting in a high Seebeck coefficient. Besides, Au2S exhibits an ultra-low lattice thermal conductivity (～ 0.88 W?m-1?K-1 at 700 K). At 700 K, the thermoelectric figure of merit of the optimal p-type doping is close to 1.76, which is higher than 0.8 of ZrSb at 700 K and 1.4 of PtTe at 750 K. Our work clearly demonstrates the advantages of Au2S as a TE material and would greatly inspire further experimental studies and verifications.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Han Zhanghua, Jiang Hui, Tan Zhiyong, Cao Juncheng, and Cai Yangjian

The advancement of terahertz technology in recent years and its applications in various fields lead to an urgent need for functional terahertz components, among which a terahertz switch is one example of the most importance because it provides an effective interface between terahertz signals and information in another physical quantity. To date many types of terahertz switches have been investigated mainly in the form of metamaterials made from metallic structures and optically-active medium. However, these reported terahertz switches usually suffer from an inferior performance, e.g., requiring a high pump laser power density due to a low quality factor of the metallic metamaterial resonances. In this paper, we report and numerically investigate a symmetry-broken silicon disk based terahertz resonator array which exhibits one resonance with ultrahigh quality factor for normal incidence of the terahertz radiations. This resonance, which can never be excited for regular circular Si disks, can help to realize a superior terahertz switch with which only an ultra-low optical pump power density is required to modify the free carrier concentration in Si and its refractive index in the terahertz band. Our findings demonstrate that to realize a high terahertz transmittance change from 0 to above 50%, the required optical pump power density is more than 3 orders of magnitude smaller than that required for a split-ring resonator (SRR) based terahertz switch reported in the literature.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Tan Zhi-Yong, Wan Wen-Jian, and Cao Jun-Cheng

As semiconductor devices, the terahertz quantum-cascade laser is a coherent source based on intersubband transitions of unipolar carriers while the terahertz quantum-well photodetector is a kind of detector which matches the laser frequency. They are solid-state, electrically operated, and can be easily integrated with other components. This paper reviews the state of the art for the design, working performance, and future directions of the two devices. Their applications in photoelectric characterization and imaging are also discussed.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
• Shao Ming-Zhe, Wang Yan-Ting, and Zhou Xin

It is very important to determine the phase transition temperature, such as the water/ice coexistence temperature in various water models, via molecular simulations. We show that a single individual direct simulation is sufficient to get the temperature with high accuracy and small computational cost based on the generalized canonical ensemble (GCE). Lennard–Jones fluids, the atomic water models, such as TIP4P/2005, TIP4P/ICE, and the mW water models are applied to illustrate the method. We start from the coexistent system of the two phases with a plane interface, then equilibrate the system under the GCE, which can stabilize the coexistence of the phases, to directly derive the phase transition temperature without sensitive dependence on the applied parameters of the GCE and the size of the simulation systems. The obtained result is in excellent agreement with that in literatures. These features make the GCE approach in determining the phase transition temperature of systems be robust, easy to use, and particularly good at working on computationally expensive systems.

Chinese Physics B
Aug. 01, 2020
• Vol. 29 Issue 8 (2020)
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