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Chin. Phys. B  
  Chin. Phys. B--2020, Vol.29, No.2
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TOPICAL REVIEW—High-throughput screening and design of optoelectronic materials

Designing solar-cell absorber materials through computational high-throughput screening

Xiaowei Jiang, Wan-Jian Yin
Chin. Phys. B 2020, 29 (2): 028803;  doi: 10.1088/1674-1056/ab6655
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Although the efficiency of CH3NH3PbI3 has been refreshed to 25.2%, stability and toxicity remain the main challenges for its applications. The search for novel solar-cell absorbers that are highly stable, non-toxic, inexpensive, and highly efficient is now a viable research focus. In this review, we summarize our recent research into the high-throughput screening and materials design of solar-cell absorbers, including single perovskites, double perovskites, and materials beyond perovskites. BaZrS3 (single perovskite), Ba2BiNbS6 (double perovskite), HgAl2Se4 (spinel), and IrSb3 (skutterudite) were discovered to be potential candidates in terms of their high stabilities, appropriate bandgaps, small carrier effective masses, and strong optical absorption.
TOPICAL REVIEW—Optical field manipulation

Research progress of femtosecond surface plasmon polariton

Yulong Wang, Bo Zhao, Changjun Min, Yuquan Zhang, Jianjun Yang, Chunlei Guo, Xiaocong Yuan
Chin. Phys. B 2020, 29 (2): 027302;  doi: 10.1088/1674-1056/ab6717
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As the combination of surface plasmon polariton and femtosecond laser pulse, femtosecond surface plasmon polariton has both nanoscale spatial resolution and femtosecond temporal resolution, and thus provides promising methods for light field manipulation and light-matter interaction in extreme small spatiotemporal scales. Nowadays, the research on femtosecond surface plasmon polariton is mainly concentrated on two aspects: one is investigation and characterization of excitation, propagation, and dispersion properties of femtosecond surface plasmon polariton in different structures or materials; the other one is developing new applications based on its unique properties in the fields of nonlinear enhancement, pulse shaping, spatiotemporal super-resolved imaging, and others. Here, we introduce the research progress of properties and applications of femtosecond surface plasmon polariton, and prospect its future research trends. With the further development of femtosecond surface plasmon polariton research, it will have a profound impact on nano-optoelectronics, molecular dynamics, biomedicine and other fields.
TOPICAL REVIEW—Overcoming doping bottleneck in widegap semiconductors

Growth and doping of bulk GaN by hydride vapor phase epitaxy

Yu-Min Zhang, Jian-Feng Wang, De-Min Cai, Guo-Qiang Ren, Yu Xu, Ming-Yue Wang, Xiao-Jian Hu, Ke Xu
Chin. Phys. B 2020, 29 (2): 026104;  doi: 10.1088/1674-1056/ab65b9
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Doping is essential in the growth of bulk GaN substrates, which could help control the electrical properties to meet the requirements of various types of GaN-based devices. The progresses in the growth of undoped, Si-doped, Ge-doped, Fe-doped, and highly pure GaN by hydride vapor phase epitaxy (HVPE) are reviewed in this article. The growth technology and precursors of each type of doping are introduced. Besides, the influence of doping on the optical and electrical properties of GaN are presented in detail. Furthermore, the problems caused by doping, as well as the methods to solve them are also discussed. At last, highly pure GaN is briefly introduced, which points out a new way to realize high-purity semi-insulating (HPSI) GaN.
SPECIAL TOPIC—Recent advances in thermoelectric materials and devices

Oxide-aperture-dependent output characteristics of circularly symmetric VCSEL structure

Wen-Yuan Liao, Jian Li, Chuan-Chuan Li, Xiao-Feng Guo, Wen-Tao Guo, Wei-Hua Liu, Yang-Jie Zhang, Xin Wei, Man-Qing Tan
Chin. Phys. B 2020, 29 (2): 024201;  doi: 10.1088/1674-1056/ab5fbd
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The influence of oxidation aperture on the output characteristics of the circularly symmetric vertical-cavity-surface-emitting laser (VCSEL) structure is investigated. To do so, VCSELs with different oxide aperture sizes are simulated by the finite-difference time-domain (FDTD) method. The relationships among the field distribution of mode superposition, mode wavelength, output spectra, and far-field divergence with different oxide apertures are obtained. Further, VCSELs respectively with oxide aperture sizes of 2.7 μm, 4.4 μm, 5.9 μm, 7 μm, 8 μm, 9 μm, and 18.7 μm are fabricated and characterized. The maximum output power increases from 2.4 mW to 5.7 mW with oxide aperture increasing from 5.9 μm to 9 μm. Meanwhile, the wavelength tuning rate decreases from 0.93 nm/mA to 0.375 nm/mA when the oxide aperture increases from 2.7 μm to 9 μm. The thermal resistance decreases from 2.815 ℃/mW to 1.015 ℃/mW when the oxide aperture increases from 4.4 μm to 18.7 μm. It is demonstrated theoretically and experimentally that the wavelength spacing between adjacent modes increases with the augment of the injection current and the spacing becomes smaller with the oxide aperture increasing. Thus it can be reported that the aperture size can effectively reduce the mode overlaying but at the cost of the power decreasing and the wavelength tuning rate and thermal resistance increasing.

A hybrid method of solving near-zone composite eletromagnetic scattering from targets and underlying rough surface

Xi-Min Li, Jing-Jing Li, Qian Gao, Peng-Cheng Gao
Chin. Phys. B 2020, 29 (2): 024202;  doi: 10.1088/1674-1056/ab5ef9
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For composite electromagnetic (EM) scattering from rough surface and target above it in near-field condition, modified shooting and bouncing ray (SBR) method and integral equation method (IEM), which are analytic methods combined with two-scale model for rough surface, are proposed to solve the composite near-field scattering problems. And the modified method is verified in effectiveness and accuracy by comparing the simulation results with measured results. Finally, the composite near-fielding scattering characteristics of a slanted plane and rough water surface below are obtained by using the proposed methods, and the dynamic tendency of composite scattering characteristics versus near-fielding distance is analyzed, which may have practical contribution to engineering programs in need of radar targets near-field characteristics under extra-low-altitude conditions.

Dynamically adjustable asymmetric transmission and polarization conversion for linearly polarized terahertz wave

Tong Li, Fang-Rong Hu, Yi-Xian Qian, Jing Xiao, Long-Hui Zhang, Wen-Tao Zhang, Jia-Guang Han
Chin. Phys. B 2020, 29 (2): 024203;  doi: 10.1088/1674-1056/ab5ef8
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The asymmetric transmission (AT) and polarization conversion of terahertz (THz) wave play a vital role in future THz communication, spectrum, and information processing. Generally, it is very difficult and complicated to actively control the AT of electromagnetic (EM) wave by using traditional devices. Here, we theoretically demonstrate a stereo-metamaterial (stereo-MM) consisting of a layer of metal structure and a layer of phase transition structure with a polyimide spacer in between. The performance of the device is simulated by using the finite-integration-technology (FIT). The results show that the AT and polarization conversion of linearly polarized wave can be dynamically controlled in a range of 1.0 THz-1.6 THz when the conductivity σ of vanadium dioxide (VO2) is changed under the external stimulation. This study provides an example of actively controlling of the AT and polarization conversion of the EM wave.

Compressed ghost imaging based on differential speckle patterns

Le Wang, Shengmei Zhao
Chin. Phys. B 2020, 29 (2): 024204;  doi: 10.1088/1674-1056/ab671a
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We propose a compressed ghost imaging scheme based on differential speckle patterns, named CGI-DSP. In the scheme, a series of bucket detector signals are acquired when a series of random speckle patterns are employed to illuminate an unknown object. Then the differential speckle patterns (differential bucket detector signals) are obtained by taking the difference between present random speckle patterns (present bucket detector signals) and previous random speckle patterns (previous bucket detector signals). Finally, the image of object can be obtained directly by performing the compressed sensing algorithm on the differential speckle patterns and differential bucket detector signals. The experimental and simulated results reveal that CGI-DSP can improve the imaging quality and reduce the number of measurements comparing with the traditional compressed ghost imaging schemes because our scheme can remove the environmental illuminations efficiently.

Enhancement effect of cumulative second-harmonic generation by closed propagation feature of circumferential guided waves

Guang-Jian Gao, Ming-Xi Deng, Ning Hu, Yan-Xun Xiang
Chin. Phys. B 2020, 29 (2): 024301;  doi: 10.1088/1674-1056/ab628d
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On the basis of second-order perturbation approximate and modal expansion approach, we investigate the enhancement effect of cumulative second-harmonic generation (SHG) of circumferential guided waves (CGWs) in a circular tube, which is inherently induced by the closed propagation feature of CGWs. An appropriate mode pair of primary- and double-frequency CGWs satisfying the phase velocity matching and nonzero energy flux is selected to ensure that the second harmonic generated by primary CGW propagation can accumulate along the circumference. Using a coherent superposition of multi-waves, a model of unidirectional CGW propagation is established for analyzing the enhancement effect of cumulative SHG of primary CGW mode selected. The theoretical analyses and numerical simulations performed directly demonstrate that the second harmonic generated does have a cumulative effect along the circumferential direction and the closed propagation feature of CGWs does enhance the magnitude of cumulative second harmonic generated. Potential applications of the enhancement effect of cumulative SHG of CGWs are considered and discussed. The theoretical analysis and numerical simulation perspective presented here yield an insight previously unavailable into the physical mechanism of the enhancement effect of cumulative SHG by closed propagation feature of CGWs in a circular tube.

Avalanching patterns of irregular sand particles in continual discrete flow

Ren Han, Yu-Feng Zhang, Ran Li, Quan Chen, Jing-Yu Feng, Ping Kong
Chin. Phys. B 2020, 29 (2): 024501;  doi: 10.1088/1674-1056/ab65b8
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We investigate the flow patterns of irregular sand particles under avalanching mode in a rotating drum by using the spatial filtering velocimetry technique. By exploring the variations of velocity distribution of granular flow, we find a type of avalanching pattern of irregular sand particles which is similar to that of spherical particles flow. Due to the fact that the initial position of avalanche in this pattern locates at the middle of the drum and the avalanche propagates toward the edge area gradually, we named it as mid-to-edge avalanching pattern. Furthermore, we find another avalanching pattern which slumps from the edge and propagates toward the opposite edge of the flow surface, named as edge-to-edge pattern. By analyzing the temporal and spatial characteristics of these two types of avalanching patterns, we discover that these two types of avalanche patterns are caused by that the avalanching particles constantly perturb the axial adjacent particles. Thus, the particles on the flow surface are involved in avalanching sequentially in order of the axial distance from the initial position.

Quantitative temperature imaging at elevated pressures and in a confined space with CH4/air laminar flames by filtered Rayleigh scattering

Bo Yan, Li Chen, Meng Li, Shuang Chen, Cheng Gong, Fu-Rong Yang, Yun-Gang Wu, Jiang-Ning Zhou, Jin-He Mu
Chin. Phys. B 2020, 29 (2): 024701;  doi: 10.1088/1674-1056/ab5f00
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Laminar methane/air premixed flames at different pressures in a newly developed high-pressure laminar burner are studied through Cantera simulation and filtered Rayleigh scattering (FRS). Different gas component fractions are obtained through the detailed numerical simulations. And this approach can be used to correct the FRS images of large variations in a Rayleigh cross section in different flame regimes. The temperature distribution above the flat burner is then presented without stray light interference from soot and wall reflection. Results also show that the extent of agreement with the single point measurement by the thermocouple is <6%. Finally, this study concludes that the relative uncertainty of the presented filtered Rayleigh scattering diagnostics is estimated to be below 10% in single-shot imaging.

Effects of square micro-pillar array porosity on the liquid motion of near surface layer

Xiaoxi Qiao, Xiangjun Zhang, Ping Chen, Yu Tian, Yonggang Meng
Chin. Phys. B 2020, 29 (2): 024702;  doi: 10.1088/1674-1056/ab5fba
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The influence rules of square micro-pillar array porosity on the liquid motion characteristics of the near-surface layer are investigated by quartz crystal microbalance (QCM). QCM is a powerful and promising technique in studying the interfacial behavior, which exhibits great advantages in investigating the effects of surface microstructure, roughness, and array. In our experiments, three different arrays with the same height of about 280 nm and center distance of 200 μm, but different diameters of about 78 μm, 139 μm, and 179 μm are investigated. The results indicate that when the surface array has a large porosity, its influence on the liquid motion of the near surface layer is slight, thus resulting in a small increase of half-bandwidth variation due to the additional friction energy dissipation. When the surface array has a small porosity, the array tends to make the liquid film trapped in the array oscillating with the substrate, then there may be a layer of liquid film behaving like rigid film, and it also will make the liquid motion near the array layer more complicated. Thus for the #3 surface with a small porosity, both the absolute values of frequency shift |Δf3| and half-bandwidth variation ΔΓ3 increase obviously. The experimental results show good consistence with the theoretical model of Daikhin and Urbakh. This study sheds light on understanding the influence mechanism of surface array porosity on the liquid motion of near-surface layer.

Shape reconstructions and morphing kinematics of an eagle during perching manoeuvres

Di Tang, Dawei Liu, Hai Zhu, Xipeng Huang, Zhongyong Fan, Mingxia Lei
Chin. Phys. B 2020, 29 (2): 024703;  doi: 10.1088/1674-1056/ab610a
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The key to high manoeuvre ability in bird flight lies in the combined morphing of wings and tail. The perching of a wild Haliaeetus Albicilla without running or wing flapping is recorded and investigated using a high-speed digital video. A shape reconstruction method is proposed to describe wing contours and tail contours during perching. The avian airfoil geometries of the Aquila Chrysaetos are extracted from noncontact surface measurements using a ROMBER 3D laser scanner. The wing planform, chord distribution and twist distribution are fitted in convenient analytical expressions to obtain a 3D wing geometry. A three-jointed arm model is proposed to associate with the 3D wing geometry, while a one-joint arm model is proposed to describe the kinematics of tail. Therefore, a 3D bird model is established. The perching sequences of the wild eagle are recaptured and regenerated with the proposed 3D bird model. A quasi-steady aerodynamic model is applied in the aerodynamic predictions, a four-step Adams-Bashforth method is used to calculate the ordinary differential equations, thus a BFGS based optimization method is established to predict the perching motions.

Dynamic evolution of vortex structures induced bytri-electrode plasma actuator

Bo-Rui Zheng, Ming Xue, Chang Ge
Chin. Phys. B 2020, 29 (2): 024704;  doi: 10.1088/1674-1056/ab671f
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Plasma flow control is a new type of active flow control approach based on plasma pneumatic actuation. Dielectric barrier discharge (DBD) actuators have become a focus of international aerodynamic research. However, the practical applications of typical DBDs are largely restricted due to their limited discharge area and low relative-induced velocity. The further improvement of performance will be beneficial for engineering applications. In this paper, high-speed schlieren and high-speed particle image velocimetry (PIV) are employed to study the flow field induced by three kinds of plasma actuations in a static atmosphere, and the differences in induced flow field structure among typical DBD, extended DBD (EX-DBD), and tri-electrode sliding discharge (TED) are compared. The analyzing of the dynamic evolution of the maximum horizontal velocity over time, the velocity profile at a fixed horizontal position, and the momentum and body force in a control volume reveals that the induced velocity peak value and profile velocity height of EX-DBD are higher than those of the other two types of actuation, suggesting that EX-DBD actuation has the strongest temporal aerodynamic effect among the three types of actuations. The TED actuation not only can enlarge the plasma extension but also has the longest duration in the entire pulsed period and the greatest influence on the height and width of the airflow near the wall surface. Thus, the TED actuation has the ability to continuously influencing a larger three-dimensional space above the surface of the plasma actuator.

Nonlinear simulation of multiple toroidal Alfvén eigenmodes in tokamak plasmas

Xiao-Long Zhu, Feng Wang, Zheng-Xiong Wang
Chin. Phys. B 2020, 29 (2): 025201;  doi: 10.1088/1674-1056/ab610e
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Nonlinear evolution of multiple toroidal Alfvén eigenmodes (TAEs) driven by fast ions is self-consistently investigated by kinetic simulations in toroidal plasmas. To clearly identify the effect of nonlinear coupling on the beam ion loss, simulations over single-n modes are also carried out and compared with those over multiple-n modes, and the wave-particle resonance and particle trajectory of lost ions in phase space are analyzed in detail. It is found that in the multiple-n case, the resonance overlap occurs so that the fast ion loss level is rather higher than the sum loss level that represents the summation of loss over all single-n modes in the single-n case. Moreover, increasing fast ion beta βh can not only significantly increase the loss level in the multiple-n case but also significantly increase the loss level increment between the single-n and multiple-n cases. For example, the loss level in the multiple-n case for βh=6.0% can even reach 13% of the beam ions and is 44% higher than the sum loss level calculated from all individual single-n modes in the single-n case. On the other hand, when the closely spaced resonance overlap occurs in the multiple-n case, the release of mode energy is increased so that the widely spaced resonances can also take place. In addition, phase space characterization is obtained in both single-n and multiple-n cases.

Discharge simulation and volt-second consumption analysis during ramp-up on the CFETR tokamak

Cheng-Yue Liu, Bin Wu, Jin-Ping Qian, Guo-Qiang Li, Ya-Wei Hou, Wei Wei, Mei-Xia Chen, Ming-Zhun Lei, Yong Guo
Chin. Phys. B 2020, 29 (2): 025202;  doi: 10.1088/1674-1056/ab610d
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The plasma current ramp-up is an important process for tokamak discharge, which directly affects the quality of the plasma and the system resources such as volt-second consumption and plasma current profile. The China Fusion Engineering Test Reactor (CFETR) ramp-up discharge is predicted with the tokamak simulation code (TSC). The main plasma parameters, the plasma configuration evolution and coil current evolution are given out. At the same time, the volt-second consumption during CFETR ramp-up is analyzed for different plasma shaping times and different plasma current ramp rates dIP/dt with/without assisted heating. The results show that the earlier shaping time and the faster plasma current ramp rate with auxiliary heating will enable the volt-second to save 5%-10%. At the same time, the system ability to provide the volt-second is probably 470 V·s. These simulations will give some reference to engineering design for CFETR to some degree.

Directional motion of dust particles at different gear structuresin a plasma

Chao-Xing Dai, Chao Song, Zhi-Xiang Zhou, Wen-Tao Sun, Zhi-Qiang Guo, Fu-Cheng Liu, Ya-Feng He
Chin. Phys. B 2020, 29 (2): 025203;  doi: 10.1088/1674-1056/ab6109
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Directional motion of dust particles in a dusty plasma ratchet is observed experimentally. The dusty plasma ratchet consists of two concentric gears with asymmetric sawtooth. It is found that the sawtooth number affects the directional motion of dust particles along the saw channel. With the increase of the sawtooth number, the particle velocity increases firstly and then decreases, and there is an optimum number of the sawtooth which could induce fast rotation of dust particles. The velocities of dust particles change as they are flowing along the saw channel. We also explore the force acting on the dust particle experimentally.

The E×B drift instability in Hall thruster using 1D PIC/MCC simulation

Zahra Asadi, Mehdi Sharifian, Mojtaba Hashemzadeh, Mahmood Borhani Zarandi, Hamidreza Ghomi Marzdashti
Chin. Phys. B 2020, 29 (2): 025204;  doi: 10.1088/1674-1056/ab6719
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The E×B drift instability is studied in Hall thruster using one-dimensional particle in cell (PIC) simulation method. By using the dispersion relation, it is found that unstable modes occur only in discrete bands in k space at cyclotron harmonics. The results indicate that the number of unstable modes increases by increasing the external electric field and decreases by increasing the radial magnetic field. The ion mass does not affect the instability wavelength. Furthermore, the results confirm that there is an instability with short wavelength and high frequency. Finally, it is shown that the electron and ion distribution functions deviate from the initial state and eventually the instability is saturated by ion trapping in the azimuthal direction. Also for light mass ion, the frequency and phase velocity are very high that could lead to high electron mobility in the axial direction.

Geant4 simulation of proton-induced single event upset in three-dimensional die-stacked SRAM device

Bing Ye, Li-Hua Mo, Tao Liu, Jie Luo, Dong-Qing Li, Pei-Xiong Zhao, Chang Cai, Ze He, You-Mei Sun, Ming-Dong Hou, Jie Liu
Chin. Phys. B 2020, 29 (2): 026101;  doi: 10.1088/1674-1056/ab5fc4
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Geant4 Monte Carlo simulation results of the single event upset (SEU) induced by protons with energy ranging from 0.3 MeV to 1 GeV are reported. The SEU cross section for planar and three-dimensional (3D) die-stacked SRAM are calculated. The results show that the SEU cross sections of the planar device and the 3D device are different from each other under low energy proton direct ionization mechanism, but almost the same for the high energy proton. Besides, the multi-bit upset (MBU) ratio and pattern are presented and analyzed. The results indicate that the MBU ratio of the 3D die-stacked device is higher than that of the planar device, and the MBU patterns are more complicated. Finally, the on-orbit upset rate for the 3D die-stacked device and the planar device are calculated by SPACE RADIATION software. The calculation results indicate that no matter what the orbital parameters and shielding conditions are, the on-orbit upset rate of planar device is higher than that of 3D die-stacked device.

Composition effect on elastic properties of model NiCo-based superalloys

Weijie Li, Chongyu Wang
Chin. Phys. B 2020, 29 (2): 026102;  doi: 10.1088/1674-1056/ab6204
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NiCo-based superalloys exhibit higher strength and creep resistance over conventional superalloys. Compositional effects on elastic properties of the γ and γ' phases in newly-developed NiCo-based superalloys were investigated by first-principles calculation combined with special quasi-random structures. The lattice constant, bulk modulus, and elastic constants vary linearly with the Co concentration in the NiCo solution. In the selected (Ni, Co)3(Al, W) and (Ni, Co)3(Al, Ti) model γ' phase, the lattice constant, and bulk modulus show a linear trend with alloying element concentrations. The addition of Co, Ti, and W can regulate lattice mismatch and increase the bulk modulus, simultaneously. W-addition shows excellent performance in strengthening the elastic properties in the γ' phase. Systems become unstable with higher W and Ni contents, e.g., (Ni0.75Co0.25)3(Al0.25 W0.75), and become brittle with higher W and Co addition, e.g., Co3(Al0.25 W0.75). Furthermore, Co, Ti, and W can increase the elastic constants on the whole, and such high elastic constants always correspond to a high elastic modulus. The anisotropy index always corresponds to the nature of Young's modulus in a specific direction.

Doping effects on the stacking fault energies of the γ' phase in Ni-based superalloys

Weijie Li, Chongyu Wang
Chin. Phys. B 2020, 29 (2): 026401;  doi: 10.1088/1674-1056/ab6203
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The doping effects on the stacking fault energies (SFEs), including the superlattice intrinsic stacking fault and superlattice extrinsic stacking fault, were studied by first principles calculation of the γ' phase in the Ni-based superalloys. The formation energy results show that the main alloying elements in Ni-based superalloys, such as Re, Cr, Mo, Ta, and W, prefer to occupy the Al-site in Ni3Al, Co shows a weak tendency to occupy the Ni-site, and Ru shows a weak tendency to occupy the Al-site. The SFE results show that Co and Ru could decrease the SFEs when added to fault planes, while other main elements increase SFEs. The double-packed superlattice intrinsic stacking fault energies are lower than superlattice extrinsic stacking fault energies when elements (except Co) occupy an Al-site. Furthermore, the SFEs show a symmetrical distribution with the location of the elements in the ternary model. A detailed electronic structure analysis of the Ru effects shows that SFEs correlated with not only the symmetry reduction of the charge accumulation but also the changes in structural energy.

High pressure and high temperature induced polymerization of C60 quantum dots

Shi-Hao Ruan, Chun-Miao Han, Fu-Lu Li, Bing Li, Bing-Bing Liu
Chin. Phys. B 2020, 29 (2): 026402;  doi: 10.1088/1674-1056/ab6657
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We synthesized C60 quantum dots (QDs) with a uniform size by a modified ultrasonic process and studied its polymerization under high pressure and high temperature (HPHT). Raman spectra showed that a phase assemblage of a dimer (D) phase (62 vol%) and a one-dimensional chain orthorhombic (O) phase (38 vol%) was obtained at 1.5 GPa and 300 °C. At 2.0 GPa and 430 °C, the proportion of the O phase increased to 46 vol%, while the corresponding D phase decreased to 54 vol%. Compared with bulk and nanosized C60, C60 QDs cannot easily form a high-dimensional polymeric structure. This fact is probably caused by the small particle size, orientation of the disordered structure of C60 QDs, and the barrier of oxide function groups between C60 molecules. Our studies enhance the understanding of the polymerization behavior of low-dimension C60 nanomaterials under HPHT conditions.

Triphenylene adsorption on Cu(111) and relevant graphene self-assembly Hot!

Qiao-Yue Chen, Jun-Jie Song, Liwei Jing, Kaikai Huang, Pimo He, Hanjie Zhang
Chin. Phys. B 2020, 29 (2): 026801;  doi: 10.1088/1674-1056/ab6583
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Investigations on adsorption behavior of triphenylene (TP) and subsequent graphene self-assembly on Cu(111) were carried out mainly by using scanning tunneling microscopy (STM). At monolayer coverage, TP molecules formed a long-range ordered adsorption structure on Cu(111) with an uniform orientation. Graphene self-assembly on the Cu(111) substrate with TP molecules as precursor was achieved by annealing the sample, and a large-scale graphene overlayer was successfully captured after the sample annealing up to 1000 K. Three different Moiré patterns generated from relative rotational disorders between the graphene overlayer and the Cu(111) substrate were observed, one with 4° rotation between the graphene overlayer and the Cu(111) substrate with a periodicity of 2.93 nm, another with 7° rotation and 2.15 nm of the size of the Moiré supercell, and the third with 10° rotation with a periodicity of 1.35 nm.

Molecular dynamics simulation of atomic hydrogen diffusion in strained amorphous silica

Fu-Jie Zhang, Bao-Hua Zhou, Xiao Liu, Yu Song, Xu Zuo
Chin. Phys. B 2020, 29 (2): 027101;  doi: 10.1088/1674-1056/ab5fc5
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Understanding hydrogen diffusion in amorphous SiO2 (a-SiO2), especially under strain, is of prominent importance for improving the reliability of semiconducting devices, such as metal-oxide-semiconductor field effect transistors. In this work, the diffusion of hydrogen atom in a-SiO2 under strain is simulated by using molecular dynamics (MD) with the ReaxFF force field. A defect-free a-SiO2 atomic model, of which the local structure parameters accord well with the experimental results, is established. Strain is applied by using the uniaxial tensile method, and the values of maximum strain, ultimate strength, and Young's modulus of the a-SiO2 model under different tensile rates are calculated. The diffusion of hydrogen atom is simulated by MD with the ReaxFF, and its pathway is identified to be a series of hops among local energy minima. Moreover, the calculated diffusivity and activation energy show their dependence on strain. The diffusivity is substantially enhanced by the tensile strain at a low temperature (below 500 K), but reduced at a high temperature (above 500 K). The activation energy decreases as strain increases. Our research shows that the tensile strain can have an influence on hydrogen transportation in a-SiO2, which may be utilized to improve the reliability of semiconducting devices.

Simulation of GaN micro-structured neutron detectors for improving electrical properties

Xin-Lei Geng, Xiao-Chuan Xia, Huo-Lin Huang, Zhong-Hao Sun, He-Qiu Zhang, Xing-Zhu Cui, Xiao-Hua Liang, Hong-Wei Liang
Chin. Phys. B 2020, 29 (2): 027201;  doi: 10.1088/1674-1056/ab671e
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Nowadays, the superior detection performance of semiconductor neutron detectors is a challenging task. In this paper, we deal with a novel GaN micro-structured neutron detector (GaN-MSND) and compare three different methods such as the method of modulating the trench depth, the method of introducing dielectric layer and p-type inversion region to improve the width of depletion region (W). It is observed that the intensity of electric field can be modulated by scaling the trench depth. On the other hand, the electron blocking region is formed in the detector enveloped with a dielectric layer. Furthermore, the introducing of p-type inversion region produces new p/n junction, which not only promotes the further expansion of the depletion region but also reduces the intensity of electric field produced by main junction. It can be realized that all these methods can considerably enhance the working voltage as well as W. Of them, the improvement on W of GaN-MSND with the p-type inversion region is the most significant and the value of W could reach 12.8 μm when the carrier concentration of p-type inversion region is 1017 cm-3. Consequently, the value of W is observed to improve 200% for the designed GaN-MSND as compared with that without additional design. This work ensures to the researchers and scientific community the fabrication of GaN-MSND having superior detection limit in the field of intense radiation.

Breakdown voltage enhancement in GaN channel and AlGaN channel HEMTs using large gate metal height Hot!

Zhong-Xu Wang, Lin Du, Jun-Wei Liu, Ying Wang, Yun Jiang, Si-Wei Ji, Shi-Wei Dong, Wei-Wei Chen, Xiao-Hong Tan, Jin-Long Li, Xiao-Jun Li, Sheng-Lei Zhao, Jin-Cheng Zhang, Yue Hao
Chin. Phys. B 2020, 29 (2): 027301;  doi: 10.1088/1674-1056/ab5fb9
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A large gate metal height technique is proposed to enhance breakdown voltage in GaN channel and AlGaN channel high-electron-mobility-transistors (HEMTs). For GaN channel HEMTs with gate-drain spacing LGD=2.5 μm, the breakdown voltage VBR increases from 518 V to 582 V by increasing gate metal height h from 0.2 μm to 0.4 μm. For GaN channel HEMTs with LGD=7 μm, VBR increases from 953 V to 1310 V by increasing h from 0.8 μm to 1.6 μm. The breakdown voltage enhancement results from the increase of the gate sidewall capacitance and depletion region extension. For Al0.4Ga0.6N channel HEMT with LGD=7 μm, VBR increases from 1535 V to 1763 V by increasing h from 0.8 μm to 1.6 μm, resulting in a high average breakdown electric field of 2.51 MV/cm. Simulation and analysis indicate that the high gate metal height is an effective method to enhance breakdown voltage in GaN-based HEMTs, and this method can be utilized in all the lateral semiconductor devices.

A simple tight-binding approach to topological superconductivity in monolayer MoS2

H Simchi
Chin. Phys. B 2020, 29 (2): 027401;  doi: 10.1088/1674-1056/ab6552
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Monolayer molybdenum disulfide (MoS2) has a honeycomb crystal structure. Here, with considering the triangular sublattice of molybdenum atoms, a simple tight-binding Hamiltonian is introduced (derived) for studying the phase transition and topological superconductivity in MoS2 under uniaxial strain. It is shown that spin-singlet p+ip wave phase is a topological superconducting phase with nonzero Chern numbers. When the chemical potential is greater (smaller) than the spin-orbit coupling (SOC) strength, the Chern number is equal to four (two) and otherwise it is equal to zero. Also, the results show that, if the superconductivity energy gap is smaller than the SOC strength and the chemical potential is greater than the SOC strength, the zero energy Majorana states exist. Finally, we show that the topological superconducting phase is preserved under uniaxial strain.

Time-dependent photothermal characterization on damage of fused silica induced by pulsed 355-nm laser with high repetition rate

Chun-Yan Yan, Bao-An Liu, Xiang-Cao Li, Chang Liu, Xin Ju
Chin. Phys. B 2020, 29 (2): 027901;  doi: 10.1088/1674-1056/ab671d
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Time-dependent damage to fused silica induced by high frequency ultraviolet laser is investigated. Photothermal spectroscopy (PTS) and optical microscopy (OM) are utilized to characterize the evolution of damage pits with irradiation time. Experimental results describe that in the pre-damage stage of fused silica sample irradiated by 355-nm laser, the photothermal spectrum signal undergoes a process from scratch to metamorphism due to the absorption of laser energy by defects. During the visible damage stage of fused silica sample, the photothermal spectrum signal decreases gradually from the maximum value because of the aggravation of the damage and the splashing of the material. This method can be used to estimate the operation lifetime of optical elements in engineering.

Atomically flat surface preparation for surface-sensitive technologies

Cen-Yao Tang, Zhi-Cheng Rao, Qian-Qian Yuan, Shang-Jie Tian, Hang Li, Yao-Bo Huang, He-Chang Lei, Shao-Chun Li, Tian Qian, Yu-Jie Sun, Hong Ding
Chin. Phys. B 2020, 29 (2): 028101;  doi: 10.1088/1674-1056/ab6586
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Surface-sensitive measurements are crucial to many types of researches in condensed matter physics. However, it is difficult to obtain atomically flat surfaces of many single crystals by the commonly used mechanical cleavage. We demonstrate that the grind-polish-sputter-anneal method can be used to obtain atomically flat surfaces on topological materials. Three types of surface-sensitive measurements are performed on CoSi (001) surface with dramatically improved quality of data. This method extends the research area of surface-sensitive measurements to hard-to-cleave alloys, and can be applied to irregular single crystals with selective crystalline planes. It may become a routine process of preparing atomically flat surfaces for surface-sensitive technologies.

High sensitive pressure sensors based on multiple coating technique

Rizwan Zahoor, Chang Liu, Muhammad Rizwan Anwar, Fu-Yan Lin, An-Qi Hu, Xia Guo
Chin. Phys. B 2020, 29 (2): 028102;  doi: 10.1088/1674-1056/ab6721
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A multi-coating technique of reduced graphene oxide (RGO) was proposed to increase the sensitivity of paper-based pressure sensors. The maximum sensitivity of 17.6 kPa-1 under the 1.4 kPa was achieved. The electrical sensing mechanism is attributed to the percolation effect. Such paper pressure sensors were applied to monitor the motor vibration, which indicates the potential of mechanical flaw detection by analyzing the waveform difference.

A numerical study on pattern selection in crystal growth by using anisotropic lattice Boltzmann-phase field method

Zhaodong Zhang, Yuting Cao, Dongke Sun, Hui Xing, Jincheng Wang, Zhonghua Ni
Chin. Phys. B 2020, 29 (2): 028103;  doi: 10.1088/1674-1056/ab6718
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Pattern selection during crystal growth is studied by using the anisotropic lattice Boltzmann-phase field model. In the model, the phase transition, melt flows, and heat transfer are coupled and mathematically described by using the lattice Boltzmann (LB) scheme. The anisotropic streaming-relaxation operation fitting into the LB framework is implemented to model interface advancing with various preferred orientations. Crystal pattern evolutions are then numerically investigated in the conditions of with and without melt flows. It is found that melt flows can significantly influence heat transfer, crystal growth behavior, and phase distributions. The crystal morphological transition from dendrite, seaweed to cauliflower-like patterns occurs with the increase of undercoolings. The interface normal angles and curvature distributions are proposed to quantitatively characterize crystal patterns. The results demonstrate that the distributions are corresponding to crystal morphological features, and they can be therefore used to describe the evolution of crystal patterns in a quantitative way.

Effects of buried oxide layer on working speed of SiGe heterojunction photo-transistor

Xian-Cheng Liu, Jia-Jun Ma, Hong-Yun Xie, Pei Ma, Liang Chen, Min Guo, Wan-Rong Zhang
Chin. Phys. B 2020, 29 (2): 028501;  doi: 10.1088/1674-1056/ab5f01
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The effects of buried oxide (BOX) layer on the capacitance of SiGe heterojunction photo-transistor (HPT), including the collector-substrate capacitance, the base-collector capacitance, and the base-emitter capacitance, are studied by using a silicon-on-insulator (SOI) substrate as compared with the devices on native Si substrates. By introducing the BOX layer into Si-based SiGe HPT, the maximum photo-characteristic frequency ft, opt of SOI-based SiGe HPT reaches up to 24.51 GHz, which is 1.5 times higher than the value obtained from Si-based SiGe HPT. In addition, the maximum optical cut-off frequency fβ, opt, namely its 3-dB bandwidth, reaches up to 1.13 GHz, improved by 1.18 times. However, with the increase of optical power or collector current, this improvement on the frequency characteristic from BOX layer becomes less dominant as confirmed by reducing the 3-dB bandwidth of SOI-based SiGe HPT which approaches to the 3-dB bandwidth of Si-based SiGe HPT at higher injection conditions.

Memristor-based vector neural network architecture

Hai-Jun Liu, Chang-Lin Chen, Xi Zhu, Sheng-Yang Sun, Qing-Jiang Li, Zhi-Wei Li
Chin. Phys. B 2020, 29 (2): 028502;  doi: 10.1088/1674-1056/ab65b5
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Vector neural network (VNN) is one of the most important methods to process interval data. However, the VNN, which contains a great number of multiply-accumulate (MAC) operations, often adopts pure numerical calculation method, and thus is difficult to be miniaturized for the embedded applications. In this paper, we propose a memristor based vector-type backpropagation (MVTBP) architecture which utilizes memristive arrays to accelerate the MAC operations of interval data. Owing to the unique brain-like synaptic characteristics of memristive devices, e.g., small size, low power consumption, and high integration density, the proposed architecture can be implemented with low area and power consumption cost and easily applied to embedded systems. The simulation results indicate that the proposed architecture has better identification performance and noise tolerance. When the device precision is 6 bits and the error deviation level (EDL) is 20%, the proposed architecture can achieve an identification rate, which is about 92% higher than that for interval-value testing sample and 81% higher than that for scalar-value testing sample.
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